Marine Biological Laboratory Library Woods Hole, Massachusetts Gift of Dr. Bostwick H. Ketchum - 1976 GO AOS TO0 TOEO oO AMI IOHM/T8IN =) THE BIOLOGY OF MARINE ANIMALS ~ by J.A.COLIN NICOL Zoologist at the Plymouth Laboratory of the Marine Biological Association MARINE BIOLOG!CAL LABORATORY LIBRARY WOODS HOLE, Mass. W. H. O. |. INTERSCIENCE PUBLISHERS. ING. New York First published 1960 Published in Great Britain by Sir Isaac Pitman & Sons, Ltd., London © J. A. COLIN NICOL 1960 PRINTED AND BOUND IN ENGLAND BY HAZELL WATSON AND VINEY LTD AYLESBURY AND SLOUGH F.0(T.695) To the memory of my Father i PREFACE EACH YEAR many young biologists make an excursion to the sea, not on idle pleasure bent but for the purpose of studying and investigating marine organisms. For some of these visitors it is perhaps their first actual experi- ence of marine animals in their natural environment, and they will be captivated but bewildered by the multiplicity of life and by the diversity of biological adaptations which they will encounter. It is for these young men and women and for undergraduates specializing in marine zoology that this book has been written, but it is hoped that lecturers and investiga- tors in other fields may find some diversion, if not actual instruction, in the following chapters. This book is concerned with the comparative physiology of marine animals, and a knowledge of comparative morphology and general biology has been presumed on the part of the reader. There are limited excursions into allied fields of animal behaviour and ecology, when these can be related to the main theme. In the pages which follow I have tried to show some of the manifold ways in which marine animals, from all kinds of © environments, have been able to maintain themselves in the face of hostile physical conditions and severe biotic competition. Whenever possible, at the risk of prolixity, I have cited specific examples in the belief that the functioning and adjustments of each animal deserve particular considera- tion, as revealing how it has managed to solve certain problems of existence. Only animals belonging to the marine environment are considered. Probably no apology is needed for this, since the ocean forms a remarkably stable and uniformly graded environment, with far less range of variation than that encountered on land or in fresh water. I have given some con- sideration to problems of littoral and estuarine ecology at the transition between sea, land and fresh water, and of animal associations, when these present features of particular interest to the marine zoologist. Of course, the great majority of examples are taken from littoral and inshore species, for these are the ones most readily available and therefore most thoroughly investigated. This book has been in preparation for six years, and during that time many aspects have received extensive treatment elsewhere. Full biblio- graphies of earlier work in comparative physiology are now available and I have therefore confined myself principally to quoting works of the past two decades, including sources in which extensive reference lists may be found. Whenever possible I have attempted to use the presently accepted scientific name for each species mentioned, but there are undoubtedly many instances in which this goal has not been achieved. All measurements are given in the c.g.s. system. Vil Vlil PREFACE During the preparation of this book I have received much assistance and advice from friends and colleagues. I am grateful to the following for reading certain chapters and for critical comments: Dr. W. R. G. Atkins, D. F. S. Russell, Professor C. M. Yonge, Dr. J. D: Robertson; Dra Davenport, Dr. G. Y. Kennedy, Dr. H. W. Harvey, Mr. F. A. J. Arm- strong and Dr. R. D. Keynes. Their criticisms and suggestions have done much to give the book any merit it deserves, but for the errors and egregi- ous blunders which remain I reserve for myself full credit. Finally, | wish to acknowledge the great debt I owe to my wife for her encouragement and for assistance in preparing the text. She is also responsible for prepar- ing many of the illustrations, which have been redrawn from the original sources. J. A. COLIN NICOL Plymouth, 1956 ACKNOWLEDGEMENTS THE author wishes to thank the Editorial Boards and the Proprietors of the following scientific periodicals, and the authors concerned, for permission to reproduce the following figures: Acta Physiologica Scandinavica, Fig. 3.13 Allgemeine Zoologie und Abstammungslehre, Figs. 8.4, 8.5 American Journal of Physiology, Fig. 9.21 Annales de I’ Institut Océanographique, Figs. 14.11, 15.2, 15.4 Archiv fiir Anatomie und Physiologie, Fig. 3.14 (a) Archiv fiir Protistenkunde, Fig. 9.1 Archives Néerlandaises de Physiologie de ’ Homme et des Animaux, Fig. 4.23 Archives de Zoologie Expérimentale et Générale, Fig. 14.14 (c) Archives des Sciences Physiologiques, Figs. 9.19, 9.20 Biological Bulletin, Figs. 3.15, 3.16, 3.19 (b), 4.17, 5.3, 6.8, 8.6, 8.9, 8.10, 8.17, 9.9 (b), 11.6 (6), 12.7 Biological Reviews of the Cambridge Philosophical Society, Figs. 2.8, 4.11, 5.11, 6.5, 8.13,-8.26 (a), 10.8, 10.9,.14.22, 14.23 British Museum Economic Series No. 10, Figs. 5.13, 15.15, 15.16 British Museum Guide to the Fish Gallery, Fig. 15.17 Bulletin No. 19 of the Cranbrook Institute of Science, Fig. 8.12 Bulletin of the U.S. Bureau of Fisheries, Figs. 6.4, 11.6 (a), 11.9 Bulletin of the U.S. Fisheries Commission, Fig. 15.3 Cold Spring Harbour Symposia on Quantitative Biology, Figs. 8.20 (b), 10.13 Commonwealth of Australia Council for Scientific and Industrial Research Bulletin 159, Fig. 15.18 Comptes Rendus des Travaux du Laboratoire Carlsberg, Fig. 4.8 Discovery Reports of the National Institute of Oceanography, Fig. 8.15 Ergebnisse der Biologie, Fig. 3.12 Experimental Cell Research, Fig. 9.7 Fauna e Flora del Golfo di Napoli, Fig. 14.14 The Feeding Mechanisms of a Deep-sea Fish Choliodus sloani, Fig. 5.28 Handbuch der vergleichenden Anatomie der Wirbeltiere, Figs. 8.1, 8.31 Harvey Lectures, Fig. 8.19 Hyalradets Skrifter, No. 22, Fig. 4.15 Journal of Biological Chemistry, Figs. 6.7, 6.10 Journal of Comparative Neurology, Figs. 8.27, 10.17 Journal of Cellular and Comparative Physiology, Figs. 2.3, 3.14 (b), 3.18, 4.9, 4.25, S22 .9 51 5, 9:16;-10.6, 13:26 Journal of Experimental Biology, Figs. 2.4, 2.6, 2.8, 2.9, 2.10, 2.11, 2.12, 4.5, 4.10, a eos 20, 621, 6.2, 6:11, 8:8. 8:17, 8.23, 9.4, 9.6, 9.10, 9.14, 9:12, 10:5; LONOT1O2 1 O22. 12-11 Journal of General Physiology, Figs. 8.17, 8.18, 8.25, 13.24 Journal of the Linnean Society of London, Fig. 5.10 (b) Journal of Morphology, Figs. 5.9, 5.12 (a), 8.3, 12.6 1X x ACKNOWLEDGEMENTS Journal of Neurophysiology, Fig. 10.19 Journal of Physiology, Figs. 3.7, 8.30, 9.8, 9.14, 9.15, 10.3, 10.14 (a), 12.4 National Geographic Magazine, Fig. 13.18 (b) Oceanic Birds of South America, Fig. 5.23 Pacific Science, Fig. 1.1 Le Parasitisme (Librairie de l'Université Lausanne), Fig. 14.12 Physiologia Comparata et Oecologia, Figs. 3.11, 9.13 Physiological Zoology, Figs. 3.21, 4.12, 4.13, 12.14 Proceedings of the Cambridge Philosophical Society: Biological Sciences, Fig. 6.5 Proceedings of the National Academy of Sciences, Fig. 12.13 Proceedings of the Royal Society: Series B, Figs. 4.21, 5.27, 8.11, 8.28, 8.33, 9.18, 10:2,-10.4, 10.8, 10:12, 10416, 10.18, 10:25, 11.1,.11.3, 11-7 (c): ABS eet Proceedings of the Royal Society of Edinburgh, Fig. 5.16 Proceedings of the Zoological Society of London, Figs. 4.1, 11.12, 14.3 Quarterly Journal of Microscopical Science, Figs. 3.17, 5.6, 5.7, 5.13 (6), 5.14, 5.19 (be 7.1, 722,-8.2, 8.24, 9.3, 10:14), 14.19, 4.21. 13525 Rapports et Procés-Verbaux des Réunions: Conseil Permanent International pour l’ Exploration de la Mer, Fig. 1.7 Schriften des Naturwissenshaftlichen Vereins fiir Schleswig-Holstein, Fig. 5.18 Scientific Reports of the Great Barrier Reef Expedition, 1928-29, Fig. 15.14 Transactions of the Illuminating Engineering Society, Fig. 1.8 Transactions of the Royal Society of Edinburgh, Figs. 5.5, 5.21, 6.6, 6.9, 15.12 Verhandlungen der deutschen zoologischen Gesellschaft, Figs. 5.25, 13.19, 13.20 Zeitschrift fiir vergleichende Physiologie, Figs. 3.2, 8.14, 12.1 Zeitschrift fiir Zellforschung, Fig. 10.20 Zoologia, Figs. 3.4, 3.5 Zoologica, Fig. 3.18 (a), (c), (d) Zoologischer Anzeiger, Fig. 5.1 Acknowledgement is made also to the following publishers for permission to reproduce illustrations from their publications: Cambridge University Press: Frontispiece from Plant-animals: a Study in Symbiosis, F. Keeble Gustav Fischer Verlag: Grundziige einen Lehre vom Licht- und Farbensinn, Frolich Methuen and Co., Ltd.: Fig. 72 from Adaptive Coloration in Animals, H. B. Cott Prentice Hall, Inc.: Fig. 20 from The Oceans, their Physics, Chemistry, and General Biology, H. U. Sverdrup et al. Volharding: Figs. 20 and 21 from Dissertation on the Utilization of Oxygen and Regulation of Breathing in Some Aquatic Animals, L. van Dam CONTENTS Preface Acknowledgements CHAP. 1 INTRODUCTORY . 2 WATER, SALTS AND MINERALS 3. Bopy FLUIDS AND CIRCULATION 4 RESPIRATION 5 NUTRITION AND FEEDING MECHANISMS 6 DIGESTION. i EXCRETION 8 SENSORY ORGANS AND RECEPTION 9 EFFECTOR MECHANISMS 10 NERVOUS SYSTEM AND BEHAVIOUR 11 PIGMENTS AND COLOURS 12 COLOUR CHANGES 13. LUMINESCENCE 14 ASSOCIATIONS: COMMENSALISM, PARASITISM AND SYMBIOSIS 15 SKELETONS, SHELTERS AND SPECIAL’ DEFENCES Appendix: Saline media Index x1 GHAPTER 1 INTRODUCTORY The sea, therefore, we may safely infer, has its offices and duties to perform; so, may we infer, have its currents, and so, too, its inhabi- tants; consequently, he who undertakes to study its phenomena must cease to regard it as a waste of waters. He must look upon it as a part of that exquisite machinery by which the harmonies of nature are pre- served, and then he will begin to perceive the developments of order and the evidences of design; these make it a most beautiful and interest- ing subject for contemplation. LreuT. F. M. Maury, 1883 THE REALM OF MARINE LIFE THE oceans in their vast expanses cover seven-tenths of the earth’s surface and in their deepest reaches extend downwards into the earth’s crust to some 10,000 metres below sea level. The mean depth of the ocean has been estimated at about 4,000 metres, which is considerably greater than the mean height of land above sea level, namely some 850 metres. All this tremendous expanse and depth are inhabited by living things; animals have been secured from beneath the polar ice sheets, and from the ocean deeps more than 10 kilometres beneath the surface. These abyssal forms are normal inhabitants of that world and know no other, and similarly at intermediate depths there are other animals which tend to remain at defi- nite levels. The ocean is not evenly populated throughout its extent. The density and total volume of living organisms are greatest in coastal waters and at the surface, and decrease rapidly with depth in the waters of the open ocean. The food of all animals in the sea is ultimately derived from marine plants, phytoplankton and, to a small extent, seaweeds. Since the energy for their synthetic activities is provided by sunlight, plants can thrive only in shallow or surface waters within the range of adequate light penetration. In these regions the herbivores graze upon the plants, but at deeper levels, extending down to the deepest waters of the abyss, the animals are depen- dent upon the remains of dead or dying surface organisms which slowly shower upon them from above, and to some extent upon the spatial organization of food chains in vertical series. VARIETY OF MARINE ANIMALS Of the nineteen or so phyla recognized by zoologists all except one or two are found in the sea. Of these, four are exclusively and four are predomi- nantly marine in habit, while many of the remaining phyla are well represented. The entirely marine phyla are the Brachiopoda, Chaetognatha, M.A.— | l 2 THE BIOLOGY OF MARINE ANIMALS Phoronidea and Echinodermata. Of the three most highly evolved groups of animals now extant, the molluscs, arthropods and vertebrates, all have marine representatives. The cephalopods are without doubt the most highly developed of marine invertebrates, and approach the vertebrates in the complexity of their sensori-neural organization and behaviour. Arthropods reached their evolutionary peak in the insects, few of which have returned to the sea. But more remarkable, from the historical view- point, has been the repeated re-invasion of the seas by most of the major groups of vertebrates. So successful was this colonization that the teleosts were able to exploit all marine environments from the tidal zone to the ocean abyss, and occupy a dominant position in the oceanic fauna. The maritime birds and mammals, partly through their acquisition of homoio- thermism, have been able to spread through the surface waters of all the oceans from arctic to antarctic ice, and in speed and agility they even out- class the fish and squid in their own element. On the invertebrate animals hitherto occupying the seas the effects of these evolutionary changes have been tremendous. From our point of view they enter into a consideration of the morphological and functional adaptations which have permitted the exploitation of a marine environment. MARINE HABITATS On the basis of their distribution and habits marine animals are generally classified as plankton, nekton and benthos. The first-named comprises all those small drifting organisms, both plants and animals, which have only feeble powers of locomotion and are carried helplessly at the mercy of currents and tides. Nekton refers to strong swimming animals, such as squid, fish and whales, whose movements are powerful enough to make them independent of water movements to a considerable degree. And thirdly, the benthos embraces all those bottom-living organisms which crawl over the substratum, burrow into it, or are sedentary in habit and remain fixed to one spot, for example, starfish, bivalves and sponges. In addition to this classification of marine animals on the basis of habitat it is usual to recognize certain well-defined environments in the sea, each with special characteristics of its own. These are the littoral or inter-tidal region, the continental shelf and slope, the pelagic zone and the abyssal region. Subdivision of these various regions is often necessary for oceanographical and ecological purposes, and is briefly described in the following paragraphs. More extended treatment will be found in works on marine natural history and oceanography, such as Sverdrup ef a/. (37), Coker (11), Colman (12) and Marshall (31). The Inter-tidal Zone At the junction between sea and land lies the shore or littoral zone, subject to tidal ebb and flow. This is the region bounded by extreme high- and low-water levels of spring tides. Its vertical range depends on the extent of the tides, and the area involved is also governed by the slope of INTRODUCTORY 3 the shore. Conditions of life in the littoral zone are quite dissimilar from those occurring elsewhere. When the tide is in the inhabitants are bathed by sea water, a relatively constant medium, but during tidal ebb they are periodically uncovered and exposed to the rigours of aerial climate. Quantitatively the fauna of the shore is very rich, but the vicissitudes of existence associated with this environment have led to a high degree of specialization. Consequently the population of the shore is peculiar in many respects and contains a high proportion of animals not found else- where. Because the littoral zone is more readily accessible to the zoologist than the waters offshore, and the animals living there can be observed directly, this region has received much attention. In the tidal zone several environmental levels can be distinguished, determined by the degree of atmospheric exposure to which they are sub- jected. On the lower shore, lying below low-water neaps, there are long periods during neap tides when the shore is not exposed. Here live many sublittoral animals which can tolerate only limited exposure to the air. Other sublittoral animals invade the inter-tidal region during tidal flow only, or come inshore on occasion to spawn. Lying between the limits of low- and high-water neaps is a region which is covered twice daily by the sea, and which contains a rich fauna of typical inter-tidal species. Above high-water neaps there are long periods when the shore is exposed for days on end. The fauna of this region contains fewer species than lower levels, and many of these are restricted to higher regions of the shore. Consequently, zonation of animals is a conspicuous feature of the shore, and is illustrated in Fig. 1.1 depicting conditions on a rocky shore in New Zealand (30, 36, 43, 44). The environmental variables which adult animals encounter in the inter- tidal zone, and which they endeavour to counter by morphological, physiological and behavioural means, are manifold and complex. No other region in the ocean presents such diversity of habitats and range of physical conditions as the inter-tidal zone. The variables with which we are con- cerned may be considered as follows. Water Movements. In the littoral region water movements result from waves, swell and tidal action. In sheltered bays and estuaries such move- ments may be slight and gentle, but on rocky coast lines facing the open ocean the mechanical force of the waves is tremendous. Animals living on wave-swept shores resist the destructive effects of wave action by suitable structural devices, and by modifications of form, or they actively seek shelter and cover. Acorn barnacles live cemented to rocks, chitons and limpets adhere firmly with their broad feet, and mussels attach themselves by strong byssus threads. Certain shore-fish, for example Lepadogaster, have their pelvic fins modified into strong suckers. Finally, the depressed or conical shape of littoral chitons and limpets offers minimal resistance to water movements. Emergence. As the result of tidal movements animals on the shore are periodically exposed to air, either daily, or for longer intervals if they live C(O) MOT[PC JoIpY) ‘sniunuy ‘oydisavuiny) ‘sopoeusreg ‘snd ‘vassoxvg ‘vyjasjo, ‘aydvavjaW “SOSNTJOY, “VjjaudaFy “vyjiiag ‘soyoeydkog ‘“uinpydydodivy ‘pduajnod “MINIPOD “dUY]DAOD “wunipuysvnjdgy “pAlsowsoyy ‘snuupyjojargs ‘visfioy ‘umipyay ‘pisung ‘wnuojsoziyYy “bydsowosajugq ‘x14yjojoy ‘se3;y ‘doy ye suustuesiQ ‘ssurids I9}CM-MO| SU9I}X9 “SMTA :Sdvou 10}eM-MOT JO [OAI] ISOYSIY ‘NAA TH ‘Sdvou 39}eM-YSIY JO [9A] SOMO] “NMHT :Ssurids Joyem-ysry owWex9 ‘SAHA 319] 94} UO UMOYS S[OAZ] SPL, GNV1VdZ MAN NI (dad XOIN MOWAVN) AYOHS AMIOY AALOALOUd V NO SINVTg GNV STVWINY TVYOLLIT 4O NOLLVNOZ ‘["] ‘O14 auoys auoys Pee IIOYS 2/PPILW uaMoy £ € . Z Ll ownyeg Wn yg odu 29 = — a5 .= ! 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Such animals may possess thick integuments which reduce the water lost through evaporation when the animal is in air (e.g. shore crabs) or hard shells which can be clamped firmly together during the period of exposure (e.g. shore barnacles and mussels). Still other forms obtain protection by burrowing into the sub- stratum, creeping under rocks or into crevices, or among the fronds of shore-wrack, e.g. inter-tidal polychaetes and teleosts. The amount of exposure to the atmosphere which an animal can tolerate, and the pattern of its behaviour when the tide ebbs, are important factors governing its distribution on the shore. Temperature. The inter-tidal zone experiences to a great degree the vicissitudes of terrestrial climate, and at low water animals are subject to the full range of aerial temperatures. Shore temperatures show great variation with latitude and season. On the shores of temperate regions ground temperatures may fall below 0°C in winter and reach 40°C in the heat of the summer sun, and higher temperatures will be encountered on tropical shores. Rock pools vary greatly in temperature, depending on their volume, on the air temperature and the length of time they are separated from the sea. Littoral animals must of necessity be eurythermal in order to survive, and this is illustrated in investigations dealing with the heat tolerance of littoral gastropods. Broekhuysen (5) determined the lethal temperatures and the survival times at high temperatures for a series of shore gastropods at False Bay (South Africa), and he discovered that these factors were graded according to the zonal sequence of the species on the shore. Evans (20) has investigated the thermal death-points of littoral gastropods in Cardigan Bay. Mean lethal temperatures varied from 46-3°C for Littorina neritoides which lives above mean high-water neaps, to 36:2°C for Gibbula cineraria which occurs in damp shaded positions below mean low-water neaps. The highest temperatures recorded on the shore were 40-5°C for sun- baked rocks and 30°C in tidal pools. Both workers concluded that the degree of heat tolerance shown by these various snails is related to the temper- ature range which they encounter in nature, and the safety factor is sufficiently high so that they rarely, if ever, are exposed on the shore to temperatures that are lethal. Abnormal temperatures may occasionally exceed the extremes tolerated by the species and result in considerable mortality. Few cold-blooded invertebrates have any means of opposing thermal fluctuations apart from seeking shelter. An interesting exception is the littoral isopod Ligia which has a temperature significantly below that of the environment in full sun- light, owing to the effect of evaporation of water vapour from the surface of the body. It has been suggested that such animals may migrate from enclosed humid niches in which the air is still, upwards into the open, where convection and evaporation can reduce the body temperature (17, 18). Also, there are some shore crabs which blanch when illuminated, and thereby reflect more of the incident light and heat. When these crabs 6 THE BIOLOGY OF MARINE ANIMALS are exposed on the shore this mechanism would tend to reduce the liability to heating in sunshine. Salinity. Large variations in salinity occur in the inter-tidal region and in estuaries. During low tide the shore may be washed by rain water and those animals that are active and remain in the open may be surrounded by water which is almost fresh until the return of the sea. Freshwater seepage on the shore forms areas of reduced salinity, and these are favoured by certain species. Tidal pools are subject to considerable fluctuations in Distance from the Mouth (kilometres) Bristol Channel Severn Estuary 60 40 20 0 20 40 60 80 COTA Salinity (g/l) No. of species Fic. 1.2. SALINITY CHANGES IN AN ESTUARY (SEVERN), AND THE DECREASE IN THE NUMBER OF MARINE INTER-TIDAL ANIMALS AWAY FROM THE MOUTH (Above) tidal and seasonal changes in salinity in the Bristol Channel and Severn Estuary (1940). (Below) number of species of marine inter-tidal animals recorded at various stations. (After Bassindale (4).) salinity, particularly those pools high on the shore which are cut off from the sea for some days during the period of neap tides. Wide environmental variations in salinity demand tolerance or functional methods of regula- tion against osmotic stress, and these will be considered in the next chapter. In estuaries the salinity conditions are peculiar owing to the effects of tidal oscillations and river discharge, leading to variable admixture of fresh and salt waters. Only a restricted number of marine animals can exist under such conditions of reduced and variable salinity, and it is found that the number of marine species decreases as the estuary is ascended (Fig. 1.2). The distance which any given species can penetrate up the estuary depends on the lowest salinity it can tolerate for a given INTRODUCTORY q | time. For fixed or sedentary animals such as molluscs, this is determined by the lowest salinity at springs during the season of maximal run-off. Active and migratory animals such as shrimps, however, are able to execute seasonal movements up and down the estuary, moving towards the mouth in winter and upstream during the spring and summer (4, 23, 29, 44), The Sea Floor Below low-tide mark the sea floor slopes gently at first across the conti- nental shelf to the continental edge at about 200 metres. The gradient then increases and the floor falls off rapidly down to a depth of around 4,000 metres. This region of sharp descent is the continental slope below which the floor tends to level out again as the abyssal plain. The continental shelf has an average width of about 50 kilometres, but the actual extent varies widely in different parts of the world. In certain areas, such as the Grand Banks of Newfoundland and the North Sea, the shelf extends several hundred kilometres offshore, whereas off steep coasts, such as Spain and Chile, it is only a few kilometres wide. The abyssal plain is far from even, and presents great depressions and ridges; the greatest depres- sions, called deeps, extend down to 10,500 metres. Pelagic Zone The pelagic zone comprises the waters of the open ocean, and because of its volume, expanse and the density of its population, it forms the major oceanic environment which the biologist has to consider. It is sometimes divided into three horizontal regions on the basis of light penetration in the following manner. 1. An upper photosynthetic zone in which the light intensity is sufficient to provide plants with energy for growth. This will vary in depth according to the amount of light falling upon the water, and with the transparency of the water. In clear tropical oceanic waters the photosynthetic zone will extend down to some 100 metres, but will be less elsewhere. This region is often rich in plankton and herbivorous animals. 2. A twilight zone extending below the photosynthetic zone to the limit of light penetration. 3. An aphotic zone extending from the limit of light penetration to the sea bottom. This region is dark, without living plant life and is populated solely by carnivorous animals and detritus feeders. Planktonic Organisms. The most important members of the phyto- plankton, upon whichall pelagic animals are ultimately dependent for food, are the diatoms, followed by dinoflagellates. These plants can flourish only in the photosynthetic layer and, correspondingly, zooplankton is densest near the surface and diminishes with depth. The character of the plankton also changes qualitatively as well as quantitatively in hauls from deeper waters, surface species giving way to mesopelagic and bathypelagic forms. The quantity of zooplankton in surface waters also shows great 8 THE BIOLOGY OF MARINE ANIMALS geographical variation, becoming more abundant over the continental shelf and in higher latitudes. In general there is a tendency for a few species of planktonic animals to predominate in catches made in far northern and southern waters, but these occur in immense numbers. In tropical waters, on the other hand, the population density is low but there is great diversity and richness of species. In temperate and sub-polar waters there is a rich seasonal growth of nutritive phytoplankton which in turn regulates the abundance of plank- tonic animals. Around the British Isles the seasonal increase in plankton first becomes noticeable between February and March as a rapid bloom- ing of the diatom pasturage. This is followed in April by the hatching of 4,000 8000 "E a E s 3000 6,000 « v s Q = m) S 3 E E & 2,000 4,000°= NS c s a $ 2 1,000 2,000 S > Nov. Dec. Fic. 1.3. GRAPHS SHOWING THE ANNUAL VARIATION OF PHYTOPLANKTON AND ZOOPLANKTON IN THE ENGLISH CHANNEL (1934) Continuous line, phytoplankton; broken line, zooplankton. (From Harvey, Cooper, Lebour, and Russell (28).) vast hordes of planktonic animals which batten upon the phytoplankton. Owing to the depletion of nutrient salts and the grazing effect of planktonic animals, the plants decline in abundance. A second minor outburst of phytoplankton may follow in the autumn (Fig. 1.3). During the winter months the plankton content of the surface waters sinks to a low ebb. In arctic and antarctic waters there is only a single annual outburst of plank- tonic life, in the summer. The permanent members of the zooplankton are animals such as fora- minifers, copepods, euphausiids, siphonophores, ctenophores, salps, chaetognaths, pteropods and so on, which spend all their life adrift. These are the holopelagic forms. In coastal waters another element comes into prominence, namely the temporary plankton. This embraces the drifting larval stages of numerous littoral and benthic species such as polychaetes, decapod crustacea, echinoderms and molluscs. With these should be INTRODUCTORY 9 included young stages of fish and cephalopods. The temporary members of the plankton attain great abundance during the spring and summer, only to disappear after metamorphosing, settling on the bottom or perishing. In addition there are certain temporary planktonic forms which are characteristic of the open ocean, such as the Phyllosoma larva of the rock lobster Scy//arus, and the Leptocephalus larva of the eel. CHEMICAL AND PHYSICAL PROPERTIES OF SEA WATER Physical and chemical conditions in the oceans have been investigated actively since the voyage of the Challenger in 1872-6, and extensive data are at hand for forming an appreciation of sea water as an environmental medium for animal life. The environmental variables which are of immedi- ate interest to the biologist are temperature, density, viscosity, pressure, light, salinity, suspended matter and dissolved gases. These are briefly reviewed in the following pages and are related, in so far as is practicable, to their biological effects. Chemical Composition of Sea Water Sea water contains a characteristic assemblage of dissolved solids and gases, and a variable amount of suspended inorganic and organic material. As first shown by Dittmar on the Challenger expedition samples, the main constituents of sea water, with minor exceptions, show remarkable uni- formity throughout the oceans of the world. This fact is of fundamental TABLE; 1.1 AVERAGE COMPOSITION OF SEA WATER (CHLORINITY 199/99, SALINITY 34:325°/o9) Composition Concentration . Concentration Ion toy & g/kg of eee g/l. at 20°C sea salt sea water MS (S.G. 1-024) Nat 30-61 10-556 459-02 10-809 Kr 1-10 0-380 9-72 0-389 Mgt* 3:69 1272 52-30 1-303 Cart 1-16 0-400 9-98 0-410 Sr 0-04 0-0085 0-15 0-013 HBO; 0-07 0-026 0-42 0-027 @l= 55-04 18-980 535-30 19-435 SO.s 7:68 2-649 DAS 2-713 HCO, * 0-41 0-140 2:29 0-143 Br 0-19 0-065 0-81 0-067 a 0-004 0-001 0-05 0-001 * Bicarbonate and carbonate will vary according to the pH of the sea water. importance in oceanography and marine biology, since it ensures that the results of studies on the physical properties of sea water in any part of the world are of general and universal significance. Table 1.1 shows mean values for the principal dissolved substances in sea water of chlorinity 19%, (salinity 34-325%,). 1 See p. 10 for explanation of 9/99. M.A.—1* 10 THE BIOLOGY OF MARINE ANIMALS Since the major constituents of sea water retain the same relative pro- portions wherever the sample is taken, it is possible, by determining the concentration of any one of them, to estimate the concentrations of the others. Because of this constant composition the relations between chlorin- ity, salinity, density and temperature are fixed, and interconversion of values is readily carried out. Finally, the biologist recognizes, in the cons- tant ionic composition of sea water, a stable environmental factor of the utmost importance in the physiology of marine animals. The constancy in composition of sea water is due to the system of oceanic circulation and to the continual mixing which occurs. This soon equalizes any local variations resulting from the discharge of rivers, the activity of living organisms, formation and melting of sea ice, the inter- action of suspended material with dissolved substances and exchange with bottom deposits. There are, however, restricted areas, such as the Black and Baltic Seas and the mouths of large rivers, where dilution and pecu- liarities of circulation bring about changes in the relative concentrations of dissolved substances. In such regions modifications of the chlorinity ratios with respect to sodium, potassium, calcium and sulphate ions may be encountered. Salinity. The salt concentration of sea water is known as salinity. Salinities are always expressed as grammes per kilogramme of sea water (parts per mille, %,) and, in practice, are usually obtained by measuring the chlorinity, using argentometric titration. The reader will find a simpli- fied procedure in Harvey (25). The relationship between the two quantities, salinity and chlorinity, is given by the expression— Salinity (%,) = 0-03 + 1-8050 x chlorinity (%, For most biological purposes, the concentrations of substances in solution are usually expressed on a volume basis as percentages or grammes per litre. It is convenient, therefore, to have corresponding values for chlorine content, and this is available in the use of the term chlorosity, which is the equivalent of chlorinity expressed as grammes per litre at 20°C. The calculations involved are— (vol AgNO) (molarity AgNO,) vol sample c.c. chlorosity = 35-5 (molarity-C]) chlorosity se density of sample at 20°C’ molarity of Cl in sample = chlorinity The chlorinity may be read from the graph shown in Fig. 1.4 relating it to chlorosity, or may be calculated with greater accuracy from the data given in Knudsen’s hydrographical tables. The salinity range in the open oceans is rather small, and usually lies between 33%, and 37%, in surface waters, with a mean of nearly 35%,. Marked deviations from these values are due to peculiar conditions. In regions where there is much dilution by heavy rainfall, discharge of rivers INTRODUCTORY 11 or melting ice the surface salinity may be much less, for example in semi- enclosed areas such as the Baltic Sea and Gulf of Bothnia, where surface salinities may fall to 5%, or less. On the other hand, in partially isolated regions such as the Red Sea, where temperatures are high and evaporation excessive, salinities may exceed 41%. Certain general features of salinity distributions may be noted. Surface salinities, on the average, reach a maximum around latitudes 20°N. and 20°S. Salinities tend to be low in high latitudes due to low temperatures and little evaporation. The range is less for intermediate and deep waters 22 Chlorosity (g/l) () 2 a 6 8 10 12 14 16 18 20 22 Chlorinity (Too) Fic. 1.4. CURVE RELATING CHLORINITY TO CHLOROSITY than at the surface, which is more directly affected by evaporation and precipitation. Values for deep waters generally lie between 34:5%, and 35%; exceptions are the Red Sea and Mediterranean where deep waters of high salinity are found. Charts showing surface salinities and lines of isohalinity over the oceans of the world may be found in standard text- books of oceanography. Minor Constituents. Besides the major constituents shown in Table 1.1, sea water contains small amounts of some forty other elements, apart from dissolved gases (22, 35a). The more important of these are silicon, phosphorus, nitrogen, iron, manganese, copper and vanadium. The con- centrations are very low, 7 mg/kg or less (Table 1.2). Some of these minor constituents, despite their low concentrations, are necessary for the continued existence of all organisms. Phosphates and 12 THE BIOLOGY OF MARINE ANIMALS TABLE 1.2 A PARTIAL LIST OF THE MINOR ELEMENTS IN SEA WATER (CHLORINITY 19°/,9) (From various sources) Element mg/kg Boron 5-0 Silicon 0-01-7-0 Nitrogen (as NH,, NO;, NOj) 0-001-0-7 Phosphorus 0-001-—0-17 Iron 0-001-0-29 Manganese 0-001—0-01 Copper 0-01-0-024 Zinc 0-005—0-014 Molybdenum 0-0003-0:016 Vanadium 0-0002-0-007 Chromium 0-001—0-003 Cobalt 0-0001 nitrogen-containing salts are essential nutrient substances for the growth of marine plants. An evaluation of this subject, which is outside the scope of the present work, is available elsewhere (26, 27). Phosphorus 1s an essential element in the composition of animals, and as inorganic and organic phosphates it occurs in the skeleton, body and cellular fluids. Iron, copper and vanadium are found in blood pigments, and iron is also an essential element in certain intracellular enzymes; silicon is utilized in the coverings and skeletons of diatoms, radiolarians, sponges, etc. Organic Matter. Sea water contains a small quantity of dissolved and suspended organic matter, which is derived from the excreta of living organisms and from the decomposition of their tissues when dead. The amounts present are very small, and have been estimated at 1-2-2 mg of carbon per litre. There appears to be more organic matter in inshore than offshore waters, and some decrease during the winter months. A part of the organic matter in solution is utilized by bacteria, but it does not appear to contribute directly to the nourishment of marine animals (25, 37). On the other hand, the presence of minute quantities of organic substances in solution may well exert physiological effects out of all proportion to their concentration. It has been demonstrated that the development of certain echinoderm and polychaete larvae shows differing degrees of success in samples of sea water from different localities, owing to the presence of unknown factors. Studies of this kind may point the way towards deter- mining the more elusive biological differences which exist between different waters (42). Hydrogen Ion Concentration (pH). Sea water is normally alkaline in INTRODUCTORY 13 reaction and shows a range rarely exceeding pH 8-0 to pH 8-4 in surface waters, although values above and below these limits are sometimes met with under extraordinary conditions. Thus, in tidal pools, the pH some- times rises to 9-6 owing to the activity of plants; and in isolated basins, where decomposition of organic matter is taking place and H,S is given off, it may fall to 7:0. Sea water which is in equilibrium with the atmo- sphere has a hydrogen ion concentration of nearly pH 8-1, which can be taken as a normal value. Sea water is alkaline as a consequence of excess of cations over anions derived from strong acids. The excess base is equivalent to ions of bicarbon- ate, carbonate and borate, and in consequence, sea water possesses limited buffering power. The removal or addition of 1:25 ml of CO, per litre will produce alterations in the hydrogen ion concentration of pH + 0:1. The resistance of sea water to changes in hydrogen ion concentration is a factor of some importance to marine animals, many of which are very sensitive to changes in the pH of the medium. Dissolved Gases. In sea water the dissolved gases which are of particular biological interest are oxygen and carbon dioxide. The partial pressures of these two gases in the atmosphere are 20-99 and 0-03 vols % (partial pressures of 159-52 and 0:23 mm Hg). The amount of gas present in aqueous solution is proportional to the partial pressure exerted by the gas. The solubilities of gases and therefore the amounts held in solution decrease with rise of temperature and increase of salinity. Solubility values are given in Table 1.3 for oxygen in sea water of different temperatures and salinities. The dissolved oxygen content of ocean waters varies from 0-8-5 ml/I. It is greater in the surface layers, where free exchange with the atmosphere can take place, than in subsurface waters, which obtain their oxygen through mixing, wind action, etc. The oxygen content of sea water is significantly influenced by marine organisms. Both animals and plants consume oxygen, but the latter also release oxygen as the result of photo- synthetic activity. Deeper water masses have derived their oxygen origi- nally from the atmosphere prior to submergence, and this is depleted by the respiratory needs of animals and the oxidation of organic material. In the Atlantic Ocean, for example, maximal values of 8-2 ml O, per litre are found in surface layers; central water near the equator shows minimal values of 0-5 ml; and intermediate and deep waters have oxygen contents of 4-5-6 ml/I. Although oxygen varies irregularly in distribution, the amounts present are usually adequate for the existence of animal life at all levels. Excep- tions are certain enclosed seas, basins and fjords, in which there is deficient circulation, with the consequence that the bottom layers become stagnant and deficient in oxygen. Carbon dioxide varies in concentration from 34-56 ml/I. in sea water, and is not a limiting factor in animal life. Part is dissolved as free CO, and as H,CO;, but most is present as carbonate and bicarbonate in 14 THE BIOLOGY OF MARINE ANIMALS TABLE 1.3 SOLUBILITY OF OXYGEN IN SEA WATER (ml/I.) (From Fox (21)) Salinity °/o9 27-11 28-91 30-72 32-52 34-33 36°13 Chlorinity °/o4 15 16 17 18 19 20 Temperature (°C) —2 | 9-01 8-89 8:76 8-64 8-52 8:39 == | 8-78 8-66 8-54 8-42 8-30 8-18 ) 2 EB -55 8-43 8-32 8:20 8-08 7:97 1 ! iveStaa 8:22 8-11 8-00 7°88 7:77 2 8-12 8:02 7-91 7:80 7:69 7°58 3 7:93 7:82 7T-12 7°61 7°51 7:40 4 974 7-64 7-53 7-43 738 7-23 5 7:56 7:46 7:36 7:26 7:16 7:07 6 7°39 7:29 7:20 7:10 7:01 6-91 y fi Pe 7-13 7:04 6°95 6°85 6°76 8 7:06 6:97 6-89 6-80 6:71 6°62 9 6:91 6°83 6:74 6°66 6:57 6:48 10 6:77 6:69 6:60 6°52 6:44 6°35 11 6°63 6°55 6:47 6°39 6°31 6-23 i2 6°50 6:43): -:15 46:35 6:27 6°19 6-11 13 6°38 6°31 ees eae 6:15 6:08 6:00 14 6:26 6:19 | 6-11 6:04 5:97 5-89 15 6°14 6:07 6:00 5-93 5-86 5-79 16 6:03 5:96 5-89 5-82 5:76 5-69 17 5-93 5-86 5:79 5:72 5-66 5-59 18 5-83 5:76 5-69 5-63 5-56 5-49 19 ae S43 5:66 5:60 5-53 5-47 5-40 20 5-63 5:56 5:50 5-44 5:38 5:31 21 5°53 5:47 5-41 5°35 5:29 5-22 oe 5-44 5-38 5:32 5:26 5-20 5-13 23 5:55 5:29 5:23 Soh ch) ESedt 5-04 24 5:26 5:20 5-14 5-097 | 5-03 4-95 25 5°17 5-12 5:06 5:00 | 4:95 4-86 combination with the excess base, which allows sea water to hold larger amounts of CO, than distilled water at the same partial pressure. Laboratory Sea Water The sea water which is used in the aquaria of marine laboratories often differs substantially from natural sea water for various reasons. Different laboratories have their own methods of maintaining a sea-water circulation. The majority use sea water pumped from the sea and circulated in a closed system for variable periods. This, of course, is subject to evaporation and liable to concentration. Owing to the death and decomposition of animals, especially in warm weather, there may be an increase in the amount of organic matter in circulation. Oxygen levels in tanks may fall well below saturation, depending on total respiratory exchanges of aquarium animals. The hydrogen ion concentration tends to rise owing to the absence of plants, and some laboratories raise the pH by liming, or by the addition of sodium bicarbonate. In the Plymouth Laboratory, for example, water of salinity 38%, and pH 7-9 had a calcium content of 0-62 g/l., compared INTRODUCTORY 15 with 0-39 g/l. in outside sea water. The experimental worker should take these factors into consideration when planning experiments which involve the use of aquarium water (1, 13, 41). Physical Properties of Sea Water These are outlined below under the headings of temperature, viscosity and density, light and pressure. Temperature. The range of temperatures encountered in the sea is not- ably small when compared with conditions over the land surface. This is true whether diurnal variations, seasonal variations or changes with lati- tude are considered. The narrow temperature range of oceanic water results from the steady system of oceanic circulation and the high specific heat of water. The coldest waters are those of antarctic seas, where tem- peratures of — 1-9°C are encountered at the edge of the ice pack. The warmest waters are found in partially enclosed areas north of the equator —the Red Sea, Gulf of Oman and the Persian Gulf—where temperatures of 35°C are attained, while the temperatures in small tropical lagoons and tidal pools may reach 40°C. In the open oceans, however, temperatures are much lower and rarely reach 30°C. The annual and latitudinal range of temperatures throughout all temperate and sub-tropical seas lies between 0 and 28°C, and at no place in the open ocean Is the annual range of temperature more than 10°C. These are surface temperatures, and it is found that with increasing depth not only does the temperature drop, but seasonal variations become negligible and disappear below depths of 200 metres. Thus in tropical waters, when the surface temperature is say, 25°C, the temperature at 200 metres is 20°C, at 1,200 metres is 5°C, below which the temperature continues to fall to minimal values of 1—2°C in abyssal regions (37). Average surface temperatures for all the oceans range from about 27°C near the equator to — 1°C in arctic and antarctic regions. In the northern hemisphere the mean surface temperature is about 3°C higher than in the southern, and there are also differences between the average temperatures of the several oceans. It is apparent, therefore, that the oceans furnish a relatively stable thermal environment for the animals occurring therein. Some marine organisms are very sensitive to temperature changes, but the relative uni- formity of water temperatures over vast areas minimizes the effects of this environmental variable. Temperature affects animals in several ways; extremes of temperature establish lethal parameters which can restrict the distribution of a species. By its effect on metabolism, temperature regu- lates spawning and affects development and growth. Because of the inverse relationship between viscosity and temperature, it also influences the rate of sinking of small planktonic organisms, as described in the next section. Few animals have a universal distribution, a notable exception being the ctenophore Beroé cucumis. This species ranges from arctic to tropical regions, but there are probably physiological differences between animals 16 THE BIOLOGY OF MARINE ANIMALS from warm and cold waters, which are either due to acclimatization or have a genotypic basis. The widely distributed common solitary sea-squirt Ciona intestinalis has been shown to comprise several distinct physiological races which have different temperature optima for breeding. In addition, some species, which are found at the surface in colder seas, extend across the equator, but in deeper and colder waters around 400 metres or more (19). Temperature is a major environmental factor determining the range of a species, and many examples of this effect may be found among benthic as well as pelagic animals. The reef-building madreporarians are unable to tolerate temperatures below 20°C and are restricted to shallow inter- tropical waters between latitudes 25°N. and 25°S. The restrictive effect of temperature may be operative on somatic metabolism, or on reproductive activity. Thus the native British oyster Ostrea edulis breeds when the temperature rises above 15°C whereas the Portuguese oyster Crassostrea angulata requires a sea temperature of 20°C before it will commence spawning, and hence cannot reproduce itself in British waters where the temperature in inshore waters does not exceed 16°C. The rate of metabolism is greatly increased in poikilothermic animals by rise of temperature. According to van’t Hoff’s rule the increase in metabolic rate for each 10°C rise in temperature (Q,,) is two- to threefold. External temperatures, therefore, will profoundly affect all the vital activities of the organism, development, growth, reproduction, digestion, etc. In species inhabiting temperate and boreal waters, vital activities occur at a lower level than in corresponding forms from tropical regions, and will be further depressed during the colder winter months of the year. Similar conditions may be expected in bathypelagic and bathybenthic animals (vide Chapter 4). Viscosity and Density. The viscosity and density of sea water are proper- ties of great biological significance in relation to movement and suspension of marine organisms. The viscosity of sea water is slightly greater than that of fresh water, and increases gradually with rise in salinity, and to a much greater extent with fall in temperature. At a salinity of 35%,, for example, the increase of viscosity is almost twofold for a temperature drop from 25-0°C. The viscosity of sea water is high compared with that of air, and offers much frictional resistance to the passage of bodies through it. Neverthe- less, some active nektonic animals are able to swim at surprisingly high speeds, e.g. whales, porpoises, penguins and scombrid fishes. We usually find that active swimmers are streamlined in some manner as an adapta- tion for securing higher locomotory efficiency. Streamlining is rather a loose term, but it refers usually to the possession of a smooth tapering shape with a minimal amount of projecting surface that could offer resistance to progression. Some animals with large rounded heads and tapering trunks are almost ideally streamlined, for example sperm whales. Besides laminar viscosity, which is concerned with the movement of thin INTRODUCTORY We uniform layers of fluid gliding smoothly over one another, progression nearly always involves some degree of eddy viscosity and turbulence as well. A streamlined form produces less turbulence and drag, and the animal accordingly encounters less resistance to its progress through the water. The density of sea water is correlated with salinity and temperature. At atmospheric pressure and 0°C the specific gravity of sea water of salinity 35%, is about 1-028. The specific gravity decreases with rise in temperature, and is increased slightly by high pressures (Fig. 1.5). The cells and tissues of marine animals have nearly the same specific gravity as sea water, which accordingly forms a circumambient medium supporting their bodies. The density and viscosity of sea water are also important statically in the flotation of planktonic organisms. To maintain themselves in the surface waters, or at particular levels, the organisms concerned must either be no heavier than the water, if they are quiescent, or expend energy actively in order to counteract the pull of gravity and maintain their position. There are relatively few animals that have an overall specific gravity less than or equal to sea water. This is achieved by siphonophores which have gas-filled floats (pneumatophores), such as the Portuguese man-o’-war Physalia and the by-the-wind sailor Vele/la. Those teleosts with swim-bladders are able to achieve the same result by controlling the volume of gases in the air-bladder, which acts as a buoyancy organ. The pelagic cephalopods Nautilus and Spirula have a chambered shell containing air, and the planktonic snail G/aucus is said to contain intestinal gases which fulfil a similar role. Other adaptations serve to lower the specific gravity of marine organ- isms relative to sea water, and thus reduce the sinking factor. Pelagic animals frequently have the skeleton reduced compared with benthic forms, or have lost it altogether, e.g. the pelagic holothurian Pelagothuria, heteropods, pteropods, the pelagic lamellibranch Planktomya, pelagic crustaceans, cephalopods and fishes. A relative decrease in weight is also achieved by incorporating large amounts of water in the body tissues. This phenomenon is widespread in pelagic animals, many of which have soft transparent tissues of a jelly-like consistency (coelenterates, pelagic annelids, chaetognaths, pelagic cephalopods, salps and fish). Light. Light is rapidly absorbed when passing through the surface waters of the sea and the intensity falls off with depth. Transmission of light through sea water is of great biological importance from several aspects. Since daylight provides the energy for photosynthetic activity it is one factor regulating the growth of plants, upon which animals ulti- mately depend for foodstuffs. The majority of animals are sensitive to light which acts as an environmental stimulus. Some of the more complex biological phenomena that are governed by changes in light intensity are phototactic responses, the diurnal migrations of planktonic animals, the incidence of reproductive activity, colour-responses and alterations in pigment-density. The absence of daylight in the deeper waters of the ocean has resulted in peculiar morphological and ethological specializations, and Chlorinity 10 Oz +35 1-035 30 1-030 25 1:025 20 020 > * = % < © 18) 15 1-015 = 18) ev Q ce) 10 1-010 5 1-005 0 1-000 -5 0-995 Salinity (Yo) Fic. 1.5. GRAPHS SHOWING THE RELATIONSHIP BETWEEN THE SPECIFIC GRAVITY OF SEA WATER AT SELECTED TEMPERATURES, AND SALINITY AND CHLORINITY INTRODUCTORY 19 many marine animals have developed luminescent organs for use at night or in the ocean depths. When light passes through sea water it suffers diminution in intensity owing to the absorptive power of water and solutes and the scattering effects of water molecules and suspended particles. Pure water allows maximal penetration of radiant energy in the visible portion of the spec- trum from 400 mu to 580 my (violet to yellow), less of light waves from 580 mu to 700 mu (orange and red), while far ultra-violet and infra-red are heavily absorbed. In oceanographical work the rate of decrease of light with depth is given by the extinction coefficient i, = 2:30 (los,, DP, — lofi psa where p, and py, are the percentage illuminations at two points differing in depth by d metres. The extinction coefficient is a measure of the true absorption by sea water, absorption by coloured substances in solution if any, and scattering of light by suspended particles (34). The amount of light which penetrates into the sea depends on several factors, namely surface intensity and the transparency of the water. There are obvious diurnal, seasonal and latitudinal changes in the intensity of incident light. A variable amount of the light that falls on the surface of the sea is reflected back, the amount being minimal when the sun is at the zenith. Oblique rays, on entering the water, must travel farther than verti- cal rays to reach the same depth, and are quickly absorbed before they penetrate far beneath the surface (37). Studies in different regions have shown that the transmission of light in the sea varies widely with locality. Light penetration is maximal in the open ocean in the tropics, where turbidity and plankton density are low and the sun’s rays at noon fall vertically on the surface, and is reduced at higher latitudes. In general, absorption is much greater in coastal than in oceanic waters because of the greater turbidity of the former (Fig. 1.6). Oceanic waters off the coast of Washington, for example, show minimal extinction coefficients twice that of pure water, and maximal values up to ten times as great. In coastal waters (Strait of Juan de Fuca), minimal and maximal values are sixteen times and thirty-four times as great as those for pure water. Absorption changes across the visible spectrum according to the charac- ter of the water. In clear oceanic water, with a minimal content of sus- pended matter and organisms, penetration is greatest in the blue and least in the red region of the spectrum, whereas in coastal waters containing more suspended material, maximal penetration shifts to the green. This is due to the differential scattering effects of particulate matter on light of different wave-lengths, the blue end of the spectrum being affected most. Utterback (38), who has investigated light penetration off the coast of Washington, found that clear oceanic water had maximal transparency at wave-length 480 mu, at which 97-5 % of the radiation penetrated | metre; and in coastal waters, maximal transparency occurred at 530 my, with 20 THE BIOLOGY OF MARINE ANIMALS 84:5°% penetration. Similar results have been obtained elsewhere, and contrasting data for the Sargasso Sea and the English Channel are shown in Figs. 1.7 and 1.8. Very turbid coastal and estuarine waters show much lower values than these, and maximal penetration shifts towards the red end of the spectrum under such conditions (2, 3, 9, 14, 15). Earlier estimates of illumination levels in deep waters have been superseded by photo-electric measurements, and data are now available showing how light intensities change during the day at various depths in the water column. As the daylight disappears with depth, the light produced by animals becomes more pronounced. At certain stations in CG °2sta/ Mean Extinction Coefficient per metre Oceanic Mean Pure Water 450 500 550 600 650 Wave-l/ength (mp) Fic. 1.6. SMOOTHED GRAPHS SHOWING THE EXTINCTION COEFFICENTS FOR LIGHT OF DIFFERENT WAVE-LENGTHS IN PURE WATER, AND IN DIFFERENT KINDS OF SEA WATER (After Sverdrup, Johnson, and Fleming (37) from Utterback’s data) the West Atlantic, background light remained constant or increased at depths greater than 300 metres, and individual flashes at 600 metres were as much as 1,000 times brighter than background illumination (9, 10, 28a). There is little information available for the photo-sensitivity of marine animals in weak illumination, but what there is suggests that some species possess a sensitivity at least equal to that of man, about 1 x 10°-1° «W/cm? receptor surface, and special adaptations in deep-sea fishes may result in greater sensitivity (see Chapter 8). The deepest records hitherto ob- tained at about 600 metres show light intensities of at least 1 x 10°° uW/cm? (10). Light and Pelagic Animals. Many plankton species in upper pelagic waters exhibit vertical migrations of great magnitude. These are responses INTRODUCTORY 21 to changes in light intensity and are discussed in Chapter 8. Other phen- omena of great interest concern differences in the pigmentation, eyes and luminescent organs of pelagic animals from different depths. In the upper waters many animals are transparent, or are tinted with blue. Below the photosynthetic zone, where the light becomes weak, silvery fishes form a conspicuous element of the fauna, and there is an increase in the number of reddish or dark-coloured species. At greater depths, where daylight fails, uniform dark colours, black, violet or red, prevail. Since the longer wave-lengths of visible light are rapidly absorbed in the open sea, reddish- Depth (metres) 6 0-01 0-05 0-1 05 #1 5 10 50 100 Percentage of Surface Light Fic. 1.7. PENETRATION OF DAYLIGHT INTO OCEANIC WATER (SARGASSO SEA) The numbers on the graphs refer to the median of the spectral ranges which are as follows: red, 600-700; blue, 346-526; violet, 310-450; green, 490-620 my. (From Clarke (9).) coloured crustaceans in the twilight zone will reflect little incident light and in fact appear black at levels below about 25 metres. The eyes also exhibit great transformations, and those of mesopelagic animals are sometimes large and specialized in shape and structure. Degeneration of eyes is uncommon among abyssal cephalopods and fish, but in some genera—e.g. Macrurus (Teleostei)—the species occurring below the level of light penetration tend to have smaller eyes than meso- pelagic forms inhabiting the twilight zone. Certain bathypelagic species of crustaceans, squid and fish, however, have completely degenerate eyes and are blind. Some groups also show a correlation between the size of the eyes and the presence or absence of photophores. There are luminescent animals at all levels in the sea, but it is estimated that luminescence is most 22 THE BIOLOGY OF MARINE ANIMALS common among crustaceans, cephalopods and fishes of the dimly lighted mesopelagic region. Complex photophores are characteristic of animals in this region, but many light-producing organisms are also found at greater depths, either in abyssal pelagic waters or on the bottom (31, 39, 40). Pressure. Pressure alterations with depth form a major environmental variable in the bionomics of marine animals. Pressure increases by about Depth (metres) be i) 40 Yi a S 50 G O LA e 60 O°] 05 ] 5 10 50 100 Percentage of Sub-surface Illumination Fic. 1.8. PENETRATION OF DAYLIGHT INTO COASTAL WATER (ENGLISH CHANNEL) The graphs refer to spectral ranges as follows: red 700+; red €00+ ; ultra-violet, 330-420; yellow, 550-80; blue, 330-480; blue-green, 455-80; green, 480-580 mw. (After Atkins (2).) 1 atmosphere for each 10 metres increase in depth, and shows a range from zero at the surface to some 1,000 atmospheres in the ocean deeps. Great pressures such as occur in the ocean depths are known to have profound effects on certain physiological and biochemical processes, but relatively little is known about how they influence marine animals. It has long been recognized that life exists in the abyssal regions, and the organ- isms occurring there are certainly physiologically adjusted to the great pressures which they encounter. Specimens have been obtained from the Philippine trench at over 10,400 metres, namely actinians, lamellibranchs, holothurians and echinoids, all of which were dead on reaching the surface (7). INTRODUCTORY 23 The biological effects of pressure are certain to be complex. The volume of water changes only slightly under compression: this amounts to about 0-46% at 1,000 m, 3-30% at 8,000 m, and 5-01 % at 10,000 m (O°C). At any given depth the pressures inside and outside the animal will be the same, and even if the animal changes its level, mechanical changes due to alterations in volume will be very small and will be equally distributed so long as air is absent. It is conjectural whether pressure limits the vertical range of motile species, since other factors may intervene before they exceed the pressure change which they can tolerate. Some species carry out vertical migrations of up to 400 metres (equivalent to 40 atmospheres), and can be designated eurybathic, or tolerant of pressure change. There are other benthic species which are known to have extraordinary vertical ranges, e.g. Henricia sanguinolenta, an asteroid, has been reported from inshore waters to depths of some 2,500 metres, equivalent to a pressure range of 0-250 atmospheres. Conditions in vertebrates are somewhat different, since many pelagic teleosts have an air-filled hydrostatic organ, and cetaceans dive with inflated lungs. Teleosts are often brought to the surface with inflated sounds as the result of the sudden pressure change, but this gives no indication of their powers of accommodation to pressure alterations which take place more gradually. The reader is referred to p. 401 for a discussion of the physiology of the teleostean air- bladder, and to p. 172 for a discussion of problems of respiration in cetaceans. The ability of marine organisms to survive great changes in pressure has been investigated experimentally to a limited extent. Particularly interesting data have been obtained for bacteria, which may be expected to throw some light on the effect of pressure changes on biochemical pro- cesses. Marine bacteria are abundant in nearly all bottom deposits, and Zobell (45) has obtained samples from the Philippine trench at depths exceeding 10,000 metres. When cultured, many of the bacteria from these abyssal depths survived at a temperature of 30°C and at atmospheric pressure. Significantly larger bacterial counts, however, were obtained in cultures incubated at 2:5°C and under pressures of 1,000 atmospheres, than at 30°C and at atmospheric pressure (46). Some experiments have been carried out on the ability of marine animals to withstand great changes in pressure. Beebe attached a lobster (Panulirus argus?) to his bathysphere during one of his descents, and recorded that the animal survived a dive to 671 metres (pressure of about 67 atmospheres). The effect of hydrostatic pressures up to 1,000 atmo- spheres has been tested by Regnard (35) on a wide variety of organisms. Experiments of this sort have shown that it is always possible to kill shallow-water animals by exposure to a sufficiently high pressure, some- where between 400 and 1,000 atmospheres. Molluscs (Cardium), annelids (Nereis), crustaceans (Eupagurus) and tunicates are inactivated by an exposure of one hour to 400-600 atmospheres. Some other forms are more resistant and survive an hour’s exposure to 1,000 atmospheres, but con- 24 THE BIOLOGY OF MARINE ANIMALS siderable swelling takes place and normal activity returns only after some hours (8, 35). Animals are usually dead when brought to the surface from great depths. In this process various factors besides pressure changes are also operative, such as mechanical disturbance and temperature alterations, and these may be lethal in themselves, quite apart from changes in pres- sure per se. The effect of these pressure changes on the functioning of tissues is still imperfectly understood. It has been shown that the compressibility of muscle (rabbit) is approximately 88% of that of pure water, and at 500 atmospheres the volume of the muscle is decreased by almost 2%. Some interesting data are available about the physiological effect of pressure on various tissues and extracts, but all derived from shallow-water or ter- restrial animals. Pressure inhibits gelation of protoplasm, solation results, and the cell loses its power of contraction. Disappearance of movement and contraction following rise of pressure has been observed in Amoeba, dividing egg cells (Arbacia) and chromatophores (Fundulus). Effective pressures for producing these effects lie between about 350-550 atmo- spheres. Pressure also reduces the beat of the embryonic fish heart (Fundulus), affects ciliary movement (Mytilus), abolishes muscle contrac- tion (striated muscle, frog) and blocks nerve coaduction (frog), the effec- tive absolute pressures varying with the tissues under investigation (3.325393): Moderate hydrostatic pressures (300-500 atmospheres) are known to affect many biochemical processes. On luminous bacteria the effect of pressure is to alter light intensity at temperatures departing from the optimum. When highly purified preparations of luciferin and luciferase (luminescent substrate and activating enzyme) are tested, it is found that pressure reversibly increases light intensity over a wide temperature range once the reaction is under way. Effects of this kind are explained on the grounds that some oxidative enzyme reactions proceed with a large volume increase, which is opposed by pressure and results in change of reaction rate and displacement of equilibrium. Each reaction displays its own peculiarities and must be interpreted separately. For example, the positive pressure effect shown by the luminescent reaction of ostracod extracts is due to a shift in equilibrium of a non-luminous oxidation of substrate, which proceeds with volume increase and is opposed by pressure. As a result, the concentration of luciferin substrate available for the lumi- nescent oxidation reaction is increased (6, 24). It is generally believed that the most significant effect of pressure on biological systems lies in the volume changes which it brings about. At high pressures protein molecules are compressed, denatured and altered in structure and chemical activity. In the ocean depths two factors, low temperatures and high pressures, both of which affect the rate of bio- logical processes, are acting concomitantly, and the animals of the abyss must be genotypically modified to withstand the conditions obtaining there. INTRODUCTORY 25 RESEARCH ON MARINE ANIMALS From earliest times man has made casual observations on the animals found on the shore but it is only in comparatively recent times that marine animals have been classified and their morphology determined in sufficient detail to permit accurate observations of their habits, activities and physiol- ogy to be made. The observations gradually accumulated by naturalists have revealed many complex structural and functional adaptations to different modes of life which display great intricacy and beauty. During the past half-century by far the greater volume of research on the physiology, behaviour and development of marine animals has been carried out ashore at the marine laboratories. This is because animals can be collected easily and regularly in littoral and inshore habitats, laboratory facilities on the shore are commodious and intricate enough for any demands, and the animals can be observed alive in their natural habitats or in aquaria. The work that can be carried out on animals of the inshore fauna is of general significance and will go far towards solving many of the problems that arise in trying to reach an understanding of how animals live and maintain themselves in the sea. But there is still a host of unsolved bio- logical problems revolving around the conditions of existence in pelagic and benthic animals, far beyond the reach of the normal shore laboratory. All these animals are delicate and fragile, and rarely live any time in captivity. To study their behaviour and physiology it is necessary to examine them at sea, and in the future we may expect more work of this character to be carried out. REFERENCES 1. ATKINS, W. R. G., ““Note on the condition of the water in a marine aquar- ium,” J. Mar. Biol. Ass. U.K., 17, 479 (1931). 2. ATKINS, W. R. G., “Daylight and its penetration into the sea,” Trans. Illum. Engng. Soc., 10 (7), 12 pp. (1945). 3. ATKINS, W. R. G. and Poo.e, H. H., “The photo-electric measurement of the penetration of light of various wave-lengths into the sea and the physiological bearing of the results,” Phi/. Trans. Roy. Soc. B., 222, 129 (1933). 4. BASSINDALE, R., “Studies on the biology of the Bristol Channel. 11,” J. Eco/., Sil (1943): 5. BROEKHUYSEN, G. J., ““A preliminary investigation of the importance of desiccation, temperature and salinity as factors controlling the vertical distribution of certain intertidal marine gastropods in False Bay, South Attica. Trans. Roy. Soc. 8: Afrs, 28,255 (1941). 6. BRONK, J. R., HARVEY, E. N. and JOHNSON, F. H., “The effects of hydro- static pressure on luminescent extracts of the ostracod crustacean, Cypri- dina,” J. Cell. Comp. Physiol., 40, 347 (1952). 7. BRUNN, A. F., “The Philippine trench and its bottom fauna,” Nature, 168, 692 (1951). 26 28a. THE BIOLOGY OF MARINE ANIMALS CATTELL, M., “The physiological effects of pressure,” Biol. Rev., 11, 441 (1936). CLARKE, G. L. and Backus, R. H., ‘““Measurements of light penetration in relation to vertical migration and records of luminescence of deep-sea animals,” Deep-Sea Research, 4, 1 (1956). CLARKE, G. L. and WERTHEIM, G. K., ““Measurements of illumination at great depths and at night in the Atlantic Ocean by means of a new bathy- photometer,’ Deep-Sea Research, 3, 189 (1956). Coker, R. E., This Great and Wide Sea (Chapel Hill, Univ. N.C. Press, 1947). CoLMAN, J. S., The Sea and its Mysteries (London, Bell, 1950). Cooper, L. H. N., “‘On the effect of long-continued additions of lime to aquarium sea water,” J. Mar. Biol. Ass. U.K., 18, 201 (1932). Cooper, L. H. N. and MILNE, A., “The ecology of the Tamar estuary. 2. Under-water illumination,” ibid., 22, 509 (1938). Cooper, L. H. N. and MILNE, A., ““The ecology of the Tamar estuary. 5. Under-water illumination. Revision of data for red light,” ibid., 23, 391 (1939). DELLow, V., “‘Inter-tidal ecology at Narrow Neck Reef, New Zealand. (Studies in inter-tidal zonation. 3), Pacif. Sci., 4, 355 (1950). Epnery, E. B., ““The body temperature of woodlice,” J. Exp. Biol., 28, 271 (1951). EpDnEY, E. B., ““Body temperature of arthropods,” Nature, 170, 586 (1952). EKMAN, S., Zoogeography of the Sea (London, Sidgwick and Jackson, 1953). Evans, R. G., “The lethal temperatures of some common British littoral molluscs,” J. Anim. Ecol., 17, 165 (1948). Fox, C. J. J., ““On the coefficients of absorption of the atmospheric gases in distilled water and sea water. 1. Nitrogen and oxygen,” Publ. Circ. Cons. Explor. Mer., No. 41, 23 pp. (1907). GOLDSCHMIDT, V. M., Geochemistry, Ed. Muir, A. (Oxford, Clarendon Press, 1954). GUNTER, G., “Seasonal population changes and distributions as related to salinity, of certain invertebrates of the Texas coast,” Publ. Inst. Mar. Sci. Univ. Tex: 1 (2), 7.950). Harvey, E. N., Bioluminescence (New York, Academic Press, 1952). Harvey, H. W., Recent Advances in the Chemistry and Biology of Sea Water (Cambridge Univ. Press, 1945). Harvey, H. W., “On the production of living matter in the sea off Ply- mouth,” J. Mar. Biol. Ass. U.K., 29, 97 (1950). Harvey, H. W., The Chemistry and Fertility of Sea Waters (Cambridge Univ. Press, 1955). Harvey, H. W., Cooper, L. H. N., LEBour, M. V. and RUSSELL, F. S., “Plankton production and its control,” J. Mar. Biol. Ass. U.K., 20, 407 (1935). Kampa, E. M. and BopEn, B. P., “‘Light generation in a sonic-scattering layer,’ Deep-Sea Research, 4, 73 (1957). Lioyp, A. J. and YONGE, C. M., “The biology of Crangon vulgaris L. in the Bristol Channel and Severn Estuary,” ibid., 26, 626 (1947). MacGinitigE, G. E. and MacGinitie, N., Natural History of Marine Animals (London, McGraw-Hill, 1949). aL: 32. Spe 34. 35. 35a. 36. aie 38. INTRODUCTORY 27 MARSHALL, N. B., Aspects of Deep Sea Biology (London, Hutchinson, 1954). MARSLAND, D., ““Protoplasmic contractility: pressure experiments on the motility of living cells,” Sci. Monthly, 67, 193 (1948). Pease, D. C. and KITCHING, J. A., “The influence of hydrostatic pressure upon ciliary frequency,” J. Ce//. Comp. Physiol., 14, 135 (1939). PooLe, H. H. and ATKINs, W. R. G., “The penetration into the sea of light of various wave-lengths as measured by emission or rectifier photo-electric celis® Proc: Roy. Soc: B.. 123: 151 (1937). REGNARD, P., Recherches Expérimentales sur les Conditions Physiques de la Vie dans les Eaux (Paris, Masson, 1891). RICHARDS, F. A., ““Some current aspects of chemical oceanography,” in Progress in Physics and Chemistry of the Earth (London, Pergamon, 1957). RIckeTTs, E. F. and CALVIN, J., Between Pacific Tides, 3rd ed., revised by Hedgpeth, J. W. (California, Stanford Univ. Press, 1952). SVERDRUP, H. U., JOHNSON, M. W. and FLEMING, R. H., The Oceans, their Physics, Chemistry and General Biology (New York, Prentice Hall, 1942). UTTERBACK, C. L., “Spectral bands of submarine solar radiation in the North Pacific and adjacent inshore waters,’ Rapp. Cons. Explor. Mer, 101 Oy 4 15 pp., 1936); WELSH, J. H.( and CHASE, F. A. JR., “Eyes of deep-sea crustaceans. 1. Acanthephyridae,”’ Biol. Bull., 72, 57 (1937). WELSH, J. H. and CHASE, F. A. JR., “Eyes of deep-sea crustaceans. 2. Sergestidae,” ibid., 74, 364 (1938). WILSON, D. P., “The aquarium and sea-water circulation system at the Plymouth Laboratory,” J. Mar. Biol. Ass. U.K., 31, 193 (1952). WILSON, D. P. and ARMSTRONG, F. A. J., “Biological differences between sea waters: experiments in 1953,” ibid., 33, 347 (1954). YONGE, C. M., A Year on the Great Barrier Reef (London, Putnam, 1930). YONGE, C. M., The Sea Shore (London, Collins, 1949). ZOBELL, C. E., “Bacterial life at the bottom of the Philippine trench,” Science, 115, 507 (1952). ZOBELL, C. E. and JOHNSON, F. H., ““The influence of hydrostatic pressure on the growth and viability of terrestrial and marine bacteria,” J. Bact., 57, 179 (1949). CHAP TER: 2 WATER, SALTS AND MINERALS The stability of the milieu intérieur is the primary condition for freedom and independence of existence; the mechanism which allows of this is that which insures in the milieu intérieur the maintenance of all the conditions necessary to the life of the (tissue) elements. CLAUDE BERNARD THE total amount of water contained in the body varies greatly from species to species, but for each kind of animal there is usually a charac- teristic value, or range of values for different functional states. The water content is high in many transparent pelagic forms, amounting to about 96% in the common jellyfish Aurelia aurita. Generally the water content of animals lies between 70 and 85% (Table 2.1). In any given animal there are differences in the amounts of water contained in the various tissues. Nevertheless, the osmotic concentrations throughout an organism are approximately equal and animals with greatly different water contents may have similar osmotic concentrations. This is explained by the fact that only a proportion of the substances composing an organism are osmotically active; bone, connective tissue and chitin, for example, may be considered as osmotically inert (14). The osmotically active substances found in animals comprise, broadly, electrolytes, organic non-electrolytes and colloids (proteins). Within the organism the cells are in osmotic equilibrium with their surroundings— blood, lymph and tissue fluids. The concentrations of individual ions within the cells, however, are markedly different from the concentrations existing in the circumambient tissue fluids, and the presence of selectively permeable cellular membranes bars the passage of certain ions, or retains diffusible organic molecules at relatively high levels (48). OSMOTIC ADJUSTMENT AND REGULATION In the normal environment characteristic of the species, animals tend to maintain their water content at a constant level referable to some basic relationship such as water/protein ratio. In the maintenance of a steady and optimal water content, osmotic adjustments and regulations are fundamentally concerned. Marine animals, vertebrates excepted, generally are in osmotic equilibrium with their environment. This is true of most members of predominantly marine groups (e.g. Scyphomedusae, Poly- chaeta, Mollusca, Crustacea, Echinodermata, Ascidiacea). Marine animals have repeatedly invaded fresh waters and the land by various routes, through estuaries, up the shore or across brackish reaches of 28 WATER, SALTS AND MINERALS 29 meadows and swamps, and many instances of such transitions may be found on the margins of the oceans at the present time. But the sea has also been reinvaded from estuaries and the land, as witness the existence of marine pulmonates, insects and vertebrates. In these transitional reaches between different environments the osmotic conditions present special difficulties to organisms, and these have been solved in various ways. On entering aqueous media of different salt concentration, or emerging into the atmosphere where there is risk of desiccation, organisms are subjected to osmotic stress. Under the former conditions some animals adjust themselves osmotically by passive alteration of the concentration of their body fluids towards that of the external medium, and are said to be poikilosmotic. Others possess powers of osmotic homoeostasis to greater or lesser degree, and are able to regulate the osmotic level of their internal fluids within certain limits independently of the environment, and these organisms are termed homoiosmotic. Other terms in common use are stenohaline referring to animals which can tolerate only limited changes in salinity, and euryhaline referring to animals which can stand wide fluctua- tions in salinity. Krogh (77), Prosser (107) and Robertson (114a) have reviewed the field of osmotic relations in animals, and have presented extensive bibliographies. Physico-chemical Relations In experiments dealing with osmoregulation the osmotic pressures of the body fluids are usually expressed as depressions of the freezing point, although chloride concentrations sometimes are given as a rough measure of osmotic pressure. The salt concentrations of sea water are expressed variously as salinities (%,), chlorinities (%,), grammes per litre, or as per- centages (grammes per 100 c.c.). Sea water of chlorinity 19%, (salinity 34-325%,) has a freezing-point depression of — 1-872°C. This is equivalent osmotically to 0-56 molal NaCl (32-15 g/l). The depression of the freezing point of sea-water samples can be calculated from the chlorinity by means of the equation A = — 0:0966 Cl — 0-0000052 CIs Values of A for sea water of different salinities are plotted in Fig. 2.1, and the graphs in Fig. 1.5 show the variation in specific gravity of sea water at five selected temperatures as a function of salinity and chlorinity. Some workers, on occasion, have expressed salt concentrations in terms of osmotically equivalent solutions of NaCl. For the convenience of the reader concentrations of NaCl and sucrose are related to freezing-point depressions in the graphs of Fig. 2.2 (for further details see Appendix). Osmotic Adjustment in Poikilosmotic Animals Most marine invertebrates are isosmotic with their environment and some of the poikilosmotic forms can endure a certain amount of dilution of the medium and still function effectively. When the outer coating is permeable, water flows in or out of the animal according to the external — ee Hl tl el 3! vl el vl Cr DL ‘Ol 6 201NO0S (9) LO (9) 0-7 (ETC (9)6:0 (aS: 1 (9) v1 (9) S6:1 (9) 0-1 (a) 1S-OT (9) €L-€ (9) 6P-€1 (9) 61-€ (9) 9€-€-SE-7 CQ) CE (V) Z-€ (9) OL-7 (9) USV (Y) WS is 3 Lg ae cs ony vl 7a. [-] an Dt = vL:l = eS: ] = (@te-O = (8¢°0 = (Lb:C-9T: 1 09-1 , high Molality sane One water) Os’ 0-6 jee 14 1-6 , 0-2 me Pore : ere / ihaeeeae ime sau ae a ae eee AAA COC IN At PEA A COA eu Na Cl | ay 7am eA | | | ey Aas | Se | ia a2 i ‘aaa Wi es ceaGuae rH air: ie Cl re eee A Bs i il z i a, 2-0 = i 2 1:5 is i eee Ze oe y Wi, [- Age F ee = = & PZ i VA ae ae ee eee an, [i/ 7 eee ae pan B/S. WAeiale ee O- Fic. 2.2. CURVES FOR SOLUTIONS OF SUCROSE AND SODIUM CHLORIDE, RELATING DEPRESSION OF FREEZING POINT TO CONCENTRATION M.A.—2 34 THE BIOLOGY OF MARINE ANIMALS acidity, etc.), but it has not yet been possible to relate these to the flotation mechanism with any certainty (24). Among Metazoa osmotic adjustment has been studied in several species with interesting results. Sipunculid worms such as Go/fingia (= Phascolo- soma), Sipunculus and Dendrostoma tolerate a certain degree of osmotic stress, adjusting to anisosmotic media. Go/fingia gouldii survives exposure to sea- water concentrations of 55°% and 160%, and Dendrostoma recovers after loss of 43 % of body weight by desiccation. Superficially these animals behave like osmometers, and on transferring to dilute or concentrated B S Percentage Body Weight SS 100 0) ] 2 3 a 5 6 7 8 Time (hours) Fic. 2.3. CHANGES IN BoDY WEIGHT OF Golfingia gouldii WHEN INJECTED WITH DILUTE AND CONCENTRATED SEA WATER Worms were injected at 0-2 hour with 12-17% body weight of sea water having the concentrations shown, after which they were immersed in normal sea water. Gradual decrease in weight about four hours after injecting concentrated sea water indicates salt loss. (From Adolph, 1936.) sea water they swell or shrink rapidly, reaching equilibrium in a day or less. On returning the animals to normal sea water they tend to regain original volume. Again, on injecting sea water of different concentrations, animals subsequently alter in volume to a degree corresponding to the concentrations of the fluids injected, i.e. when injected with hypertonic solutions they swell and when injected with hypotonic solutions they shrink, owing to osmotic transfer of water (Fig. 2.3). The integument of sipunculids (Golfingia, Dendrostoma) is semi- permeable, permitting osmotic transfer of water in anisosmotic media. In consequence, the body fluids remain isosmotic with the external medium when the concentration of the latter is altered, i.e. osmotic adjustment WATER, SALTS AND MINERALS 35 takes place. Nevertheless, there is some regulation of volume. Specimens of Dendrostoma immersed in slightly dilute sea water (90°/) swell, but after 24 hours they lose weight and return to normal (Fig. 2.4). Volume control is also shown in concentrated media (120% sea water). Critical studies reveal that the body wall of Dendrostoma is highly permeable to water, but only slightly permeable to salts; permeability to water and salts is greater inwards than outwards. Other loci, gut or nephridiopores, how- ever, are more permeable to salt. The increase in volume which occurs 125 (% original) a Body Weight 95 Time (days) Fic. 2.4. VOLUME CONTROL EXHIBITED BY Golfingia gouldii IN MODERATELY DILUTE SEA WATER (75°% AND 90%) Loss of weight of worms (controls) in normal sea water shown for comparison (Simplified from a diagram of Gross (41).) when worms are placed in hypo-osmotic media is due to passive diffusion of water across the semipermeable body wall ; subsequent volume regula- tion is achieved by loss of salt through nephridiopores and outward diffu- sion of water. In addition to these processes, there is some release of osmotically active particles into the blood from the body wall, tending to counterbalance the loss of salts to the exterior (41). Other poikilosmotic marine invertebrates which take up water and swell in dilute sea water are Nereis pelagica, Perinereis cultrifera, Arenicola marina and Sabella pavonina (Polychaeta); Mytilus edulis and ‘‘Doris”’ (Mollusca); and Caudina chilensis (Holothuria) (Fig. 2.5) (28). Some of 36 THE BIOLOGY OF MARINE ANIMALS these animals, e.g. Caudina, suffer salt loss which reduces the degree of swelling. Experiments with the sea hare Aplysia punctata in dilute sea water show that these animals swell rapidly during the first two hours as water is absorbed, but the weight then falls off as salt is lost. On returning to normal sea water there is a further loss in weight since the external medium is now hyperosmotic to the animal (Fig. 2.5). A converse experiment in which Ap/ysia was placed in a solution containing part sea water and part sugar caused the animal to shrink, since the body wall is impermeable to sugar whereas salt diffuses out. Similarly, starfish and echinoids suffer GN: ES ae < mee - ris 0140 p° > ‘i 3 7 aS mMytil es 50 /o S22 ae vl20- 2 To 100% s.w. in 4 rr Nereis pelagica —c§€->22—————-- -.. & | 75°/o S.w. a hia} »% me) N OO eee eas as OL Soe a s % iS) Py _ a) Ee ES ee eet he 0 2 4 6 8 10 12 14 16 18 24 «= «48 Time (hours) Fic. 2.5. OSMoTIC ADAPTATION IN DIFFERENT MARINE ANIMALS Percentage change in body weight of animals placed in dilute sea water. (From Bethe, 1934, and Maloeuf, 1937.) some weight increase in dilute media, but their hard skeletons restrict volume changes and considerable salt loss is believed to take place (77, 107). Some of these animals are partly estuarine in habit or occur in enclosed waters of reduced salinity. Thus Arenicola marina is found in nature in concentrations down to 23% sea water, and its tissues are capable of functioning in sea water diluted to that extent (Fig. 2.6) (144). Some freezing-point values for this species in natural and experimental media are shown in Table 2.2 (cf. Fig. 2.7). Species such as A. marina which can tolerate considerable dilutions of their body fluids are able to live in brackish water of reduced salinities (8%, and less), but they are unable to tolerate fresh water. To invade rivers and lakes, animals have needed some powers of osmoregulation, together with an integument which resists flooding by water and prevents salt loss. During short periods some animals are able to fend off unfavourable FiG. 2.6. ADAPTATION OF EXTROVERT OF Arenicola marina TO DEPRESSION OF SALINITY At beginning of experiment preparation was in normal sea water. First arrow indi- cates start of progressive dilution; second arrow, termination of dilution two hours later, when concentration had fallen to 28% sea water. Second record shows accom- modation to diluted medium, at low amplitude, six hours later. Time scale in minutes. (From Wells and Ledingham (144).) 2:0 1.5 1-0 A Internal Medium 0°5 O 0-5 1-0 1:5 ZO A External Medium FIG. 2.7. OSMOTIC RELATIONS IN POLYCHAETES The curves show variation in concentration of internal with external medium. Nereis diversicolor regulates at low concentrations; N. pelagica and Arenicola marina remain isosmotic with the environment. (Based on data of Schlieper’s, 1929.) 38 THE BIOLOGY OF MARINE ANIMALS conditions by shutting themselves off from the outside world, e.g. oysters, mussels and barnacles. In dilute media lamellibranchs such as Mytilus and Scrobicularia retract their siphons or close their shells, thus protecting the blood from extreme changes in concentration (34a). The acorn barn- acle Balanus improvisus is reported to be markedly euryhaline; specimens TABLE 2.2 Arenicola marina Medium A Internal External Heligoland 6 A PP 1-70 1-72 Kiel 149/00 0:76-0:77 0:75 Experimental aaa 0:28-0:30 0:29 26 hours of B. balanoides and B. tintinnabulum can survive for many days in fresh water, or the atmosphere, shutting their valves except for a minute breathing aperture (2a). These, of course, are behavioural mechanisms for avoiding inclement conditions. Osmoregulation A number of mechanisms are available to euryhaline animals which attempt to keep their internal medium constant in the face of changing external salt concentrations, and these have been utilized in variable degree. Some animals show only very limited powers of osmotic regulation, and from this level there range all degrees of regulatory ability to that possessed by truly homoiosmotic species which are able to maintain their internal fluids relatively constant, irrespective of wide external fluctuations in salinity. In waters of lowered salt concentration, such as estuaries, animals can passively oppose reduced salinities by means of membranes having reduced permeability to waters and salts. They can also react functionally by actively pumping out the water which tends to flow into the organism, and by absorbing salts from the surrounding medium so as to keep up the internal salt level in the face of various conditions tending to cause internal dilu- tion. Waters of greatly raised salt content are not often encountered by marine animals and are most liable to be found in tidal pools high on the shore which are subject to evaporation during neap tides. Mechanisms utilized against high salinities involve membranes with lowered perme- ability to water and salts, active absorption of water against a salinity gradient, and secretion of excess salts. All processes involving active trans- fer of water and salts against osmotic gradients necessitate the expenditure of energy by the organism in the form of osmotic work, and theoretically, should be capable of detection in the form of a corresponding increased level of oxygen consumption. WATER, SALTS AND MINERALS 39 PROTOZOA. Processes of osmoregulation among protozoans have been linked with activity of the contractile vacuole. This organ is especially characteristic of freshwater Protozoa, but it also occurs in many marine flagellates and ciliates and in freshwater sponges (Table 2.3). In its simplest TABLE, 2.3 SYSTEMATIC OCCURRENCE OF CONTRACTILE VACUOLES IN RELATION TO HABITAT Class Freshwater Marine Endoparasitic Rhizopoda — Present Absent from most Absent Mastigophora Present Present in many Absent from nearly all Ciliophora Present Present in most Present in many Sporozoa ~~ | — Absent from all form the contractile vacuole consists of a vesicle which contracts rhythmic- ally and discharges its fluid contents through a temporary pore in the body surface. Vacuolar activity is less in marine than in freshwater forms, and there is general agreement that it operates in osmoregulation by pumping out water. From collected data Kitching (64) has shown that the normal frequency of vacuolar contraction, expressed as vacuolar duration, varies from a few seconds to several minutes in freshwater Protozoa, but in marine Protozoa it is considerably longer (Table 2.4). When marine Protozoa possessing TABLE 2.4 VACUOLAR OUTPUT FOR SEVERAL FRESHWATER, ESTUARINE AND MARINE CILIATES (selected data) . . Vacuolar Rate of vacuolar Species Medium duration output j1?/sec Vorticella convallaria FW Lieses 34-7 Rhabdostyla brevires FW 20-100 sec 4-1-17-2 Zoothamnium sp. FW 6-39 sec 7:8-17°9 Frontonia marina SW 6:99/o9 45-82 sec 330-560 Zoothamnium hiketes SW 14:5%oo | 45-125 sec 2:5-4-6 Cothurnia curvula SW. ca: 35°/o0 | 0:-7-20 min 0:18-1:24 Vorticella marina SW ca 35°/o9 13-32 min 1-1-3-4 contractile vacuoles are placed in dilute sea water, vacuolar frequency and output increase (Fig. 2.8). It has also been observed that contractile vacuoles appear in certain marine protozoans normally lacking them, when they are kept in fresh water. When marine peritrichs are subjected to dilute sea water below 75% vacuolar output is raised and body volume increases (Fig. 2.9). Cyanide (a respiratory inhibitor) interferes with this vacuolar activity so that individuals in diluted sea water continue to swell (Fig. 2.8). Removal of the cyanide leads to recovery of vacuolar activity, and the vacuole pumps at a faster rate than normal, with consequent decrease in body volume. 40 THE BIOLOGY OF MARINE ANIMALS It appears, therefore, that contractile vacuoles are not indispensable to marine Protozoa, since they are absent in most rhizopods and some ciliates, and they function at a much lower rate in marine peritrichs than in comparable freshwater species. It may be that in marine forms they serve to remove excess water taken in with the food and metabolic water, and counteract the osmotic pressure of cellular colloids; the possibility of ionic regulation by contractile vacuoles has also been raised. The rate of vacuolar output in Podophrya (freshwater suctorian) increases tenfold dur- 3 30 x 30 WwW 3 i c : % au S S$ 60 S25 8 1S) x S 59 : = Qw ic 8 Se) 40 2:0 ¢ wee : E 20 O Cie S : a) % 10 i8) ag C) 10 100 80 60 40 20 0 100 80 60 40 20 0 Concentration of sea water in medium (%) Fic. 2.8. OSMOREGULATION IN MARINE PERITRICHS (Left) relation of rate of vacuolar output to concentration of sea water in Zoothamnium marinum (solid circle) and Cothurnia curvula (open circle). (Right) relation of body volume to concentration of the medium in the presence and absence of cyanide in C. curvula. (From Kitching, 1936, 1938.) ing feeding. This increased vacuolar activity reduces hydration, resulting from ingestion of food, and reduces body volume (65, 67). In dilute media, such as that encountered by estuarine ciliates, vacuolar output is increased. The cell membrane is considered to be semipermeable and to resist the passage of salts. Consequently, when the organism is in a hypotonic environment, water is drawn into the cell by osmosis and the increased activity of the contractile vacuole serves to bale out water as it pours into the cell. There are instances, however, of Protozoa lacking hard coverings and contractile vacuoles, which tolerate transfer to fresh water for some time, e.g. the marine variety of Actinophrys sol which has been acclimatized to fresh water (64, 66). There are probably species differences in salt permeability and volume regulation among different protozoans. In marine ciliates studied by Kitching and others, swelling of the cytosome persists in dilute media, and WATER, SALTS AND MINERALS 4] vacuolar output remains at a high level. In the marine rhizopod Amoeba mira there is no contractile vacuole, but hyaline vacuoles appear during feeding, and the fluid in these vacuoles is eventually discharged. In dilute sea water the rate of elimination of fluid during feeding is inversely pro- portional to the osmotic concentration of the culture fluid. There is also an initial increase in cell volume, which gradually returns to normal. Amoeba lacerta, a freshwater species, possesses contractile vacuoles. Vacuolar output also varies inversely as the concentration of the medium. In concentrated media the Amoeba shrinks, then returns to normal size. These protozoans display volume regulation and seem to be permeable to m 140 | ! Body volume | 8 120 : | x x 100 . pod (0) | | % i oe -| Rate of output | ; a 40 free ot Se ‘arose = my ; ; hee - oi 100% | x = S.W. | 40% S.W. , 100%. S.w. 40% ad 100 Yo S.w oC elegy a oo a — — =< OE ee Es oe eo, ee a 60 120 180 240 300 Time (minutes) Fic. 2.9. EFFECT oF HYPOTONIC SEA WATER ON THE BODY VOLUME AND VACUOLAR OUTPUT OF A MARINE PERITRICH, Zoothamnium marinum. (From Kitching, 1934) salts: in anisosmotic media they soon approach the concentration of the external medium (50, 89). METAZOA. Forms with limited powers of osmoregulation are described below. Turbellaria. Many lower metazoans inhabiting brackish and fresh waters obviously possess some osmoregulatory capacity. Among flatworms the nephridial system of Gyratrix hermaphroditus has been implicated in osmoregulation on morphological evidence. In freshwater varieties of Gyratrix there is a well-developed protonephridial system, with flame cells, ampullae and bladders. The nephridial system is reduced in brackish- water varieties, and absent as far as we can tell from salt-water forms. In dilute sea water and fresh water the nephridial system appears to function in a manner analogous to the contractile vacuoles of Protozoa, by rhyth- mically pumping water out of the organism (78). In the triclad Procerodes (= Gunda) ulvae, the role of the nephridial system is doubtful. This species lives in estuarine reaches of small streams where salinity conditions fluctuate from salt to fresh water according to the state of the tide. When placed in dilute sea water Procerodes under- goes swelling but equilibrium is soon reached with some subsequent decrease in weight. During this process there is loss of salt to the external as 42 THE BIOLOGY OF MARINE ANIMALS medium. In distilled water or in certain stream waters the animals die and disintegrate, and it has been demonstrated that this is due to absence of calcium in the external medium (Fig. 2.10). In nature Procerodes has been found in streams with minimal calcium levels of 0-5 mg/I., where they are exposed to fresh water for as long as 5 days during neap tides (100). Much of the water that is taken up osmotically by Procerodes, when in dilute sea water, passes through the parenchyma to the endoderm where it ‘is stored in large vesicles. Contrary to expectation, these vacuoles do not discharge into the gut. In dilute sea water oxygen consumption is increased once the worms have attained a steady state, and swelling proceeds beyond normal when the animals are subjected to oxygen lack, or are poisoned with cyanide (100). Procerodes is probably isosmotic with sea water, and in dilute media the tissues at first become flooded with water. Calcium is believed to act by 120 i 2 © Y Gy Jeo Y S38 - “ 5 5 0 30 8% wo W ‘09S o— oi © ny Ss RS GS is © Q 20 40 60 80 100 120 Time (minutes) Fic. 2.10. BEHAVIOUR OF Procerodes ulvae IN DISTILLED WATER Upper curve shows increase in volume; lower curve, loss of salts. Arrows indicate dissolution of ectoderm. (Somewhat diagrammatic. Smoothed curves based on a figure of Pantin’s (100).) lowering the permeability of the body wall to water and salts, thereby reducing the inflow until regulatory mechanisms become effective, while excess water is stored in endodermal vesicles. In summary, active osmo- regulatory processes in Procerodes appear to involve: the secretion of water, by nephridia or through the body wall, against an osmotic gradient; maintenance of depressed ectodermal permeability; retention of water in endodermal vesicles having a low salt content. Polychaeta. As indicated above, the sublittoral nereid worms Nereis pelagica and Perinereis cultrifera adjust to low salt concentrations by dilution of the body fluids and succumb when the external medium falls much below 8%,. N. diversicolor, on the other hand, possesses limited powers of osmoregulation and is able to tolerate brackish waters with salinities as low as 0:5%,. The internal fluids of the latter species are isosmotic with normal sea water, but in dilute media they maintain some degree of hypertonicity (Fig. 2.7). WATER, SALTS AND MINERALS 43 In hypotonic media, water absorption and salt loss both take place (Figs. 2.11 and 2.12). On transferring N. diversicolor from full strength to 20% sea water, the initial increase in weight is followed by a slow decline, and on returning the animals to normal sea water they suffer a further loss in weight. This is due to a reduction of internal salt concentration which now leaves the animal hypotonic to its normal environment. Direct measurements of ionic influx into the worm have been made with the aid of #4Na and **Cl. In normal sea water the uptake of sodium by Nereis diversicolor is about 260 ug/g/hour wet weight. At high dilutions (9%,) per- meability to water is reduced, and there is active uptake of NaCl against the concentration gradient. Worms which have been exposed to dilute media and then returned to normal sea water show increased uptake of sodium, which compensates for the loss of salt at lower salinities. ght D S Percentage of Initial Wes KO <3) 100 20 40 60 80 100 120 Time (hours) Fic. 2.11. REGULATION OF VOLUME IN Nereis diversicolor WHEN TRANSFERRED FROM NORMAL TO DILUTE (20%) SEA WATER A, Roscoff worms; B, Bangor worms. (From Ellis, 1937.) Calcium is also necessary for osmotic regulation, and in a dilute medium lacking this ion the animal remains swollen. The maintenance of hyper- tonicity in waters of lowered salt concentration must involve the expendi- ture of energy. In N. diversicolor and in Neanthes virens, another eury- haline species, oxygen consumption goes up in dilute sea water, and cyanide prevents the operation of regulatory processes with the consequence that the weight curve continues to rise (3, 27, 35, 55a, 130, 130a, 137). Nereis diversicolor thus adjusts itself to a large extent when subjected to a hypotonic environment: first, by absorbing water and swelling; second, by regulating its volume through salt loss. Calcium aids in diminish- ing the permeability of the integument to water but the ability of the animal to keep its internal fluids hypertonic to the environment shows that active regulatory processes come into operation. These involve the active uptake of ions (Nat and Cl-) against a concentration gradient and possibly 44 THE BIOLOGY OF MARINE ANIMALS the secretion of water. It is interesting, in this regard, that the nephridial canal of N. diversicolor is longer and more convoluted than that of P. cultrifera, suggesting nephridial participation in osmoregulation. Isolated tissues of N. diversicolor continue to function in low dilutions, and display spontaneous activity in 5-10% sea water. In P. cultrifera, on the other hand, the lower limits of salinity that will still permit muscular activity lie between 20-25 % sea water (144). A seemingly racial difference has been described in the osmoregulatory performance of N. diversicolor. Animals at Bangor and Plymouth require more than 100 hours to complete their weight regulation in 20% Chloride Output (mg/g anima/) 0 5 10 15 20 25 Time (hours) Fic. 2.12. Osmotic ADJUSTMENT IN Nereis diversicolor Salt loss accompanying water intake in dilute sea water (concentrations shown against each curve). (From Ellis, 1937.) sea water, whereas worms at Roscoff take only half that time (Fig. 2.11). Racial differences of this kind might be expected to develop in animals inhabiting waters of low salinity, as adaptations to new or altered environ- mental conditions. Freshwater molluscs have well-developed osmoregulatory powers, and this rather suggests that brackish-water forms may be able to regulate their internal media. The bivalve Mercenaria mercenaria shows some hyper- tonicity in dilute sea water (blood 374 mm Cl, in sea water 319 mm Cl). Potamopyrgus jenkinsi, a brackish-water gastropod, has invaded fresh water in recent historic times. On the other hand, certain freshwater pulmonates have penetrated into brackish waters, e.g. Limnaea pereger, and several species are found near high-tide mark on the shore. Littoral prosobranchs WATER, SALTS AND MINERALS 4S show well-marked zonation from permanently submerged to splash levels and species inhabiting the upper shore must be subject to considerable osmotic stress. In a South African investigation it was found that littoral snails could be graded in sequence according to their tolerance of desicca- tion and changes of salinity; this sequence accords with their vertical distribution and with gradation of like conditions in their environment (he 22.251). Osmoregulatory Powers in Crustacea. The majority of Crustacea are wholly marine, but various groups have representatives which have pene- trated into brackish or even fresh waters, or have invaded the land and become terrestrial. As a class the Crustacea are characterized by a hard exoskeleton which may be strengthened by lime deposits. Among other functions the exoskeleton reduces permeability. The strictly oceanic Crustacea found in the lower littoral zone or in sublittoral habitats are stenohaline and have body fluids isosmotic with sea water, e.g. Maia, Portunus and others. The blood concentrations of these animals follow closely that of sea water over the range A 2:5 to 1-5 or somewhat lower (Fig. 2.13). The spider crab Maia is unable to survive for more than a few hours in sea water diluted below one-fifth. In 80% sea water it quickly swells, but in less than a day its weight returns to normal as salts and water are lost, and its body fluids regain osmotic equilibrium with the environmental medium (Fig. 2.14). A slight tendency to maintain the blood hypertonic to the external medium at concentrations below J 1-5 is shown by the edible crab Cancer pagurus. Cancer swells to a smaller extent in dilute media than Maia, its rate of swelling is less and volume regulation is also slower (Fig. 2.14). Cancer is less permeable to water and salts than Maia, and manifests a slight amount of osmoregulatory ability in hypotonic media. By the use of iodide, which can be detected easily, estimations have been obtained of the permeability of the integument of various stenohaline and euryhaline crabs. In solutions containing this ion it has been found that the stenohaline brachyurans Hyas araneus and Portunus depurator are much more permeable than the relatively euryhaline Cancer pagurus and Carcinus maenas. Penetration takes place mostly through the gills but also proceeds at a slow rate through other external surfaces. The common shore crab Carcinus maenas shows well-developed powers of osmoregulation in hypo-osmotic media and can survive dilutions down to 40-6. In normal sea water the blood is isosmotic with the external medium, but in increasing dilutions it develops hypertonicity until at an external concentration of A 0-6 the internal concentration is maintained at twice that value; lower external concentrations are lethal (Fig. 2.13). Carcinus shows little swelling in dilute sea water when compared with other brachyurans previously considered (Fig. 2.14). Permeability to water and salts is relatively low in Carcinus, but nevertheless some transfer still occurs across the gills in hypo-osmotic media, water being absorbed and salts lost. The water continually flowing in is eliminated by the kidneys, CIP6l ‘ieyylueg S [pl ‘souor 56761 ‘Jodolpyos S¢7ZS] “TeaAnd wos 2} EC) ‘WINIPSL [vUI9}xX9 9Y} JO UONeUIOUOS UI sUOTIeJO}Te YIM Ydurs;ouSRY Jo dIMssoid SIJOWSO JO UOTVIIVA SNVAOVLSNUD GOdVOAd FAAILVLNASIAdAY AO SNOILV TAY OILOWSO “ETT “SOI wnipapy /eus4a1xXI V O-f- 0:7- 0:[- 0 Of - (OKA S-0 0l- SUPII PA Gl - sadissPuso OZ= snsdeughyred S:C- O1- sepuaew Snulgue) 9) ~ { ° re | Go:7- Umnipsy Jeusazuy 7 WATER, SALTS AND MINERALS 47 which produce a greater volume of urine under these conditions (cf. Ee 5. D299). In all concentrations of the external medium the urine tends to be isotonic with the blood with some variation in either direction. The kid- neys, therefore, play no part in salt conservation and the hypertonic level of the blood is maintained by active salt absorption across the gill mem- branes. This involves osmotic work, necessitating a higher rate of metabol- ism. Increased respiratory activity has been detected in prawns (Palae- monetes) and crabs (Carcinus, Ocypode) that have been transferred to anisosmotic media. It has been estimated that the osmotic work performed at the body surface and excretory organs of brackish and freshwater Maia verrucoSa Portunus depurator Cancer pagurus Percentage Change in Weight Carcinus maenas 0 12 24 3E 48 Time (hours) Fic. 2.14. CHANGES IN WEIGHT OF BRACHYURAN CRABS WHEN TRANSFERRED TO DILUTE SEA WATER Time, in hours, after placing in experimental medium. Cancer pagurus and Portunus depurator in 67 % sea water (from Hukuda, 1932). Maia verrucosa and Carcinus maenas in 80°% sea water (from Schwabe, 1933.) animals amounts to about 1°% of total metabolic energy (32, 57, 77, 79a, 103, 106a, 119, 142). Many decapods are able to regulate the concentration of their blood in hyperosmotic as well as hypo-osmotic media, and some prawns and grapsoid crabs are actually hypo-osmotic to normal sea water under natural conditions (Fig. 2.13). The Australian rock crab Leptograpsus variegatus, which lives on the shore, displays definite hypotonicity to normal sea water (blood 4 1-97, in sea water A 2:14) and similar condi- tions are found in Heloecius cordiformis, a crab inhabiting mangrove swamps where it is uncovered at low tide. The American shore crabs Uca crenulata and Pachygrapsus crassipes have blood concentration curves that reveal a high degree of osmotic regulation, the blood becoming hypotonic when the external medium exceeds 31:2%,, and remaining hypertonic when the external medium falls below this value (57, 109). Some of these animals are normal inhabitants of brackish and estuarine waters where their osmoregulatory ability is of adaptive value. Blue crabs 48 THE BIOLOGY OF MARINE ANIMALS (Callinectes sapidus) from different environments show osmotic acclima- tion: those from low salinities (Cl about 6%,) survive in extreme dilution (Cl < 2:5%,) longer than crabs from high salinities (Cl about 25%,). Other examples of hyperosmotic regulating decapods are brackish-water crabs such as Sesarma erythrodactyla, and certain penaeid and palaemonid prawns such as Metapenaeus monoceros and Palaemon serratus. Some prawns even penetrate into fresh water, but the female returns to salt water to breed (98, 102, 108). The final stage in euryhalinity, the penetration of fresh water, is depen- dent on the ability of the animal to regulate its blood concentration in highly dilute media, below 8%,. The blood of many euryhaline species becomes swamped at these low concentrations but in the Chinese crab Eriocheir sinensis the blood is maintained at a steady level under such conditions. This crab is an Asiatic species which was first noticed in Europe in 1912, and has since extended its range widely. Individuals grow to maturity in fresh water, but return to the sea to breed. The factors permitting Eriocheir to live in fresh water are low permeability to water and salts, and the ability of the gill membranes to absorb salt against an osmotic gradient (77). During the moulting process of crabs there is a striking increase in size and weight which is correlated with osmotic changes in the blood. Prior to the moult water is lost, the osmotic pressure of the blood rises and the animal shrinks within its shell. Shortly after moulting sea water is absorbed osmotically and the osmotic pressure falls. In Carcinus maenas, an animal weighing about 50 g absorbs about 35 g during the moult. As a result the animal increases considerably in size and fills out its new exo- skeleton (77, 110). In the inter-tidal zone crabs occurring at higher levels may be exposed to the atmosphere for several hours during tidal ebb and remain active. Carcinus maenas has been shown experimentally to live up to 8 days in air, during which time its blood concentration increased from about 610 mM to 815 mm. Jones (57) has determined the effect of air exposure on a series of crabs (Hemigrapsus, Pachygrapsus, Uca) during ebb tide and has found that the osmotic pressure of the blood increases, on the average, A 0-18 during this period. A semi-terrestrial crab Ocypode albicans, which dwells near high-tide level, displays well-marked osmoregulatory capacity m air and in anisosmotic solutions. Ocypode is normally hypo-osmotic and possesses a blood-chloride level of 378 mm when the external chloride is 480 mm. Internal chloride levels are maintained constant for 24 hours in air, or in solutions ranging from 120-600 mm (32). In warmer regions of the world some crabs (anomurans and brachyu- rans) have taken to a terrestrial existence, although they must return to the sea for reproduction. These animals are protected to a considerable degree against water loss by their relatively impermeable exoskeleton. Littoral and terrestrial crabs show much variation in their ability to with- stand desiccation on exposure to air, but there is some tendency for crabs WATER, SALTS AND MINERALS 49 occurring near or above high-tide mark to survive longer in the atmosphere than more aquatic species. The terrestrial hermit crab Coenobita diogenes and the land crab Gecarcinus lateralis live in air for 4 days or more whereas the oceanic crabs Portunus sulcatus and P. spinimanus die in an hour. Some determinations of blood concentrations of littoral and terrestrial decapods made in Florida by Pearse appear in Table 2.5. The majority of terrestrial and semi-terrestial crabs are hypo-osmotic to sea water, and are able to regulate in dilute and concentrated media. Hyperosmotic regulation in these terrestrial crabs is an expression of their ability to conserve water and discharge salts, both processes of adaptive value in dry environments. When desiccated, Pachygrapsus is able to take up water against a salinity gradient. The coco-nut crab Birgus latro shows TABLE 2:5 BLOOD CONCENTRATIONS OF DECAPOD CRUSTACEANS FROM DIFFERENT HABITATS Species A Blood (mean) Habits Brachyura 1. Gecarcinus lateralis — 1-65 Terrestrial: in burrows 2. Cardisoma guanhumi 1-66 Terrestrial: in burrows 3. Ocypode albicans 1-70 Burrows near sea 4. Grapsus grapsus 1-92 Wave-swept rocky shore 5. Mithrax verrucosus ZY Coral reefs near low-tide mark Anomura 6. Coenobita clypeatus 2-09 Land hermit crab 7. Petrochirus bahamensis 2:09 Low-tide mark to sublittoral zone Reptantia 8. Panulirus argus 2:20 Shallow water: sublittoral Sea water 36:05°/ 5, 2:04 specific behavioural adaptations for life on land: it can drink water from small puddles, moisten its respiratory membranes with external water, and control the concentration of its blood by selecting water of the appropriate salinity to drink (42, 57, 104). The shore isopod Ligia oceanica is also osmoregulatory to a notable degree. The blood of this animal shows a value of A 2-15 in normal sea water. The blood concentration is held fairly steady when the sea water is varied in concentration from 100-50%. Outside this range the blood concentration rises or falls, but osmoregulation still holds the blood hypo- or hyperosmotic to the external medium, when this deviates from accus- tomed values (101). Amphipods are very common inhabitants of brackish and estuarine water. Various species of gammarids show well-marked temperature and salinity preferenda (Fig. 2.15). The typically marine species such as Marinogammarus marinus, and Gammarus locusta can withstand sea water diluted down to 25%, under which conditions they maintain the blood markedly hyperosmotic. The brackish-water species G. duebeni shows a wide tolerance of concentrations from full strength to 2% sea water, and 50 THE BIOLOGY OF MARINE ANIMALS keeps its blood hyperosmotic to the external medium when the concentra- tion of the latter drops. In G. pulex the blood concentration is relatively low but is maintained above that of the external medium. The nephridium is larger and more complicated in G. pu/ex (fresh water) than in G. /ocusta (marine) and this suggests a functional role in salt resorption in the fresh- water species (4, 5, 132). VERTEBRATES. Among cyclostomes the myxinoids are exclusively salt- water forms. The blood is about isosmotic with sea water, and the osmotic pressure is due largely to inorganic ions. In dilute or concentrated sea water (salinity, 25—40%,), blood chloride of Polistotrema follows that of the medium. Some lampreys are anadromous in habit, ascending rivers —Fresh Nath Brackish water ——_——>|=«—Sea water—— Tidehead Springs Mouth Neaps IGammarus locustal Gammarus Zaddachi salinus | Gammarus z.zaddachi | lGammarus chevreuxi (W. Britain) Channel species a Gammarus pulex Gammarus duebeni brackish water [iat] Intertidal Sheltered Marinogammaru, — Il Marinogammarus 'ma SSS species | S Salinity 22 ae ee | sradient Fic. 2.15. DIAGRAM REPRESENTING THE RELATIVE DISTRIBUTIONS OF SOME RELATED GAMMARIDS IN ESTUARINE AND CONTIGUOUS REGIONS OF RIVERS (Suggested by a figure of Bassindale’s, 1942) from salt water to spawn. In the sea they are markedly hypo-osmotic (blood of Petromyzon marinus from the Mediterranean, A 0-586°C). In fresh water, lampreys regulate by mechanisms similar to those of teleosts (see later section). Fresh-run lampreys (Lampetra fluviatilis) can maintain their plasma chloride constant only in media more dilute than half sea water. Presumably, some change takes place in their capacity to osmo- regulate during up-stream migration. Chloride excretory cells, resembling those of teleosts, occur in the gills of lampreys (see p. 54). It is these cells which are responsible for the excretion of salt across the gills when the animals are in sea water (22, 29, 9la, 91b, 114, 146a). There is evidence for believing that both major groups of extant fishes, the elasmobranchs and teleosts, are fresh-water in origin and during their successful invasion of the sea they have had to accommodate themselves to a hypertonic medium. Fishes are homoiosmotic to a considerable degree WATER, SALTS AND MINERALS a1 and their osmoregulatory mechanisms have been investigated extensively (Fig. 2.16). The blood of marine elasmobranchs is nearly isosmotic with sea water and has a higher osmotic pressure than that of marine teleosts (Table 2.6). The salt concentration of the blood is somewhat higher in elasmobranchs (about 240 mm Cl) than in teleosts (about 180 mm C)l). The greater osmotic pressure of elasmobranch blood is mainly due to its =o A Blood Conger conger (marine) esh-water) Anguilla anguilla (fr 0 0-5 2-0 =9.& 1-0 15 ZA External Medium Fic. 2.16. VARIATION IN BLOOD CONCENTRATION OF THREE FISHES IN WATERS OF DIFFERENT CONCENTRATIONS A marine elasmobranch (dogfish), marine teleost (conger eel) and freshwater eel (euryhaline teleost). (Data from Duval, 1925.) high urea content, reaching 1-5°%% (250 mm) and responsible for about a third of the total osmotic pressure (127). Urea is retained by elasmobranchs as a useful metabolite, much of the urea in the glomerular filtrate being absorbed in the kidney tubules. More- over, the oral membrane, gills and integument are relatively impermeable to urea, which is thereby conserved. As the result of the osmotic gradient maintained by this high urea content water tends to flow into the blood from the surrounding medium. The water content of the tissues is main- tained at a steady level by the excretion of a hypotonic urine at a relatively constant rate. Some data for urea levels in elasmobranchs are given in 52 THE BIOLOGY OF MARINE ANIMALS TABLE 2.6 OSMOTIC CONCENTRATIONS OF THE BLOOD OF SOME MARINE FISH A Blood A External medium Elasmobranchs Mustelus mustelus — 2:36 — 2-29 Squalus acanthias 1-62 1-33 Scyliorhinus canicula 2:22 2°15 Carcharias taurus 2:03 1-83 Raja laevis 1-93 1-86 Torpedo marmorata | 2:20 2715 Holocephali | Callorhynchus milii | 1-76 1:50-1:85 Chimaera monstrosa | 1-99 SW Teleosts | Conger conger 0-77 2:14 Arna gigas | 1-03 | 2°29 Charax puntazzo | 1-04 2:29 Pleuronectes platessa | 0-79-0-90 1-90 Anguilla anguilla 0-63 | 1-90 Gadus callarias 0:72 1:67 Cyclopterus lumpus 0-66 1-89 Lophius piscatorius 0-63 92 Mola mola 0-80 2°15 - Scorpaena scrofa 0-71 OMS (Various sources) Table 2.7. There is some extrarenal excretion of urea and of salts (Nat, K* and CI-) as in teleosts. The urea is diffused throughout the body, and appears to be osmotically and functionally neutral, as far as the tissues are concerned. In addition to this substance the blood contains relatively large amounts of trimethylamine oxide, in concentrations of 0-5-0-:9% TABLE -2.7 UREA AND CHLORIDE CONTENT OF THE BLOOD OF SOME CARTILAGINOUS FISHES . Chloride Urea Species Fluid m/] mm/I Squalus acanthias Blood plasma 234 248 Mustelus canis Blood plasma 230-7 165-86 Carcharias taurus Blood plasma 228-41 165-80 Raja laevis Blood plasma 230-73 200-335 R. erinacea Blood plasma 230-85 254-384 Callorhynchus milii Blood 228 400-76 Chimaera monstrosa Serum 287-97 _— (66-120 mm) (Table 7.3). Like urea, it is resorbed in the kidney tubules and conserved by the fish, and is responsible for some 6—-12°% of the osmotic pressure of the blood (44, 59, 60, 126). A few elasmobranchs have reinvaded estuaries and fresh waters from the sea. These animals have retained the same urea mechanisms as salt- water species, but the urea, and to a lesser extent the chloride, content of WATER, SALTS AND MINERALS 53 the blood are diminished. Threatened hydration of the tissues is to that extent reduced and the water balance is conserved by the production of a very dilute urine. The chimaeroids also contain large quantities of urea, and appear to have an osmoregulatory system similar to that of selachians (127). Teleosts. Unlike the great majority of marine animals the blood of marine teleosts is hypo-osmotic to sea water and the animals are con- fronted with the functional problem of avoiding osmotic dehydration in an aqueous medium. Blood osmotic pressures in these animals generally lie between A 0-7 and A 1-0, well below that of the external medium (Figs. 2.16 and 2.17). Marine fish drink large quantities of sea water; water : AN Animal Group A Blood or body fluids Bele. Delphinus dolphin : terrestrial mammal , Fulmarus marine terrestrial bird ears marine turtle Emys freshwater turtle Gadus marine cod Cyprinus freshwater carp Paes marine dogfish ! ! ‘ 1 i ' ! ' ' { 1 ' Pristis freshwater saw-fish H ' i en marine Petromyzon freshwater eee marine Anodonta freshwater ' Maia Marine crab Potamon freshwater crab 0 0-4 o8 1-2 «16 A Freezing point depression Fic. 2.17. HistOGRAMS ILLUSTRATING OSMOTIC PRESSURES OF BLOOD AND BoDy FLUIDS OF REPRESENTATIVE MARINE AND FRESHWATER ANIMALS FROM DIFFERENT GROUPS together with sodium, potassium and chloride ions is absorbed through the gut wall, while most of the ionic magnesium and sulphate is rejected (Anguilla, Myoxocephalus, Lophius). Water conservation is aided by the production of only small quantities of urine, which is always isotonic or slightly hypotonic to the blood and low in ionic sodium, potassium and chloride. The constant salt uptake, which is a consequence of these pro- cesses, is compensated by the secretion of salts across the gills (13, 126). In estuarine and freshwater environments the salt content of the blood is lowered, but the blood is still strongly hyperosmotic to fresh water (A 0-45-A 0-60, see Table 2:6). Water is absorbed by endosmosis through the exposed gill and oral membranes, but the rest of the body is relatively impermeable because of the covering of scales and mucus. Fish living in brackish or fresh water drink little water, and they get rid of the excess quantities absorbed by excreting copious amounts of a hypotonic urine. There is some loss of salts in the urine and in the faeces, which are parti- 54 THE BIOLOGY OF MARINE ANIMALS ally made up in the food. In addition, as shown by Krogh, the gills of several species of freshwater fish are able to absorb chloride actively against a concentration gradient (Sa/mo, Gasterosteus, etc.). It is therefore concluded that the ability of teleost fishes to regulate the osmotic concentration of their blood and thereby maintain a steady water content is due to two complementary mechanisms. In a hyperosmotic environment, sea water is absorbed through the alimentary canal and the excess salt is eliminated through the gills, while the kidneys produce minimal quantities of urine. In hypo-osmotic media, however, the gills resist the penetration of water, which is eliminated through the kidneys, and the necessary salts are obtained partly in the food but to a large extent in many species by absorption through mucous membranes in the buccal and branchial regions (77, 126). The cells responsible for salt transfer have been identified as columnar acidophilic elements located in the gill filaments and elsewhere in the oral and pharyngeal region (Conger, Fundulus, Pleuronectes, etc.). In the eury- haline species Fundulus heteroclitus, cytological changes appear in these cells when the fish are transferred from salt to fresh water, and fresh to salt, and it is probable that salt transfer—absorption and excretion—is performed by the same cell under altered conditions (19, 23, 37). A number of marine fish have become morphologically adapted to a regime of diminished urine production by reduction or complete atrophy of kidney glomeruli, which are essentially filtration devices. Aglomerular kidneys occur in toadfishes (Haplodoci), anglers (Pediculati) and pipe- fishes (Lophobranchii). Teleosts vary greatly in their ability to tolerate salinity fluctuations, and the majority of strictly marine and freshwater species are stenohaline. Tolerance of osmotic changes is important in estuarine teleosts and in euryhaline species which migrate to and from the sea for spawning pur- poses. Examples are estuarine flounder Platichthys flesus; anadromous salmon; catadromous eels. The eel (Anguilla anguilla) tolerates an abrupt change from fresh to salt water, and after an initial loss in weight due to exosmosis, it re-establishes equilibrium in about 48 hours. Adjustment of the killifish Fundulus heteroclitus to both fresh and sea water has been investigated from several aspects. On transferring to fresh water from salt there is a temporary increase in weight, which returns to normal after 18 hours, and a loss of chloride amounting to 60% in 4 days. Adaptation to fresh water is complete after 24 hours. When returned to sea water they regain their normal chloride content and density within 6 hours. The presence of calcium in fresh water reduces chloride loss and water uptake under experimental conditions and is probably a factor influencing the distribution of euryhaline species, and permitting the colonization of fresh waters by marine species (11, 12, 46). In the littoral zone some species must be able to resist desiccation when the tide is out. For example, the mud skipper Periophthalmus hops about actively on the mud in the heat of the sun during tidal ebb. Waters where WATER, SALTS AND MINERALS a the salinity rises above 35%, present an analogous problem in the added strain imposed in resisting dehydration. In the Bitter Lakes and Lake Timsah of the Suez Canal Zone, salinities up to 53%, have been reported. Normal inhabitants of these waters are sole Solea solea and grey mullet Mugil cephalus. In the desert brine pools bordering the Bitter Lakes a cyprinodont Aphanius dispar and a gastropod Pirenella conica live in waters where the salt concentration reaches 140m (about 6M NaCl). Osmotic stresses of this magnitude are most unusual (33). REPTILES, BIRDS AND MAMMALS. The higher vertebrates which have re- invaded the sea have to some extent been functionally pre-adapted for a maritime life since their terrestrial ancestors have had to solve the problem of water conservation. Marine reptiles, birds and mammals are all pro- vided with an impermeable integument which prevents the passage of water and salts. Since these animals are predominantly air-breathers, the respiratory surfaces are shielded from sea water. Reptiles. The chief marine reptiles are turtles (Sphargidae and Chelonii- dae), Galapagos marine lizard (Amblyrhynchus), estuarine and salt water crocodiles (Crocodylus), and sea snakes (Hydrophiidae). Some of these animals, on occasion, have access to fresh water, but others, such as the leathery turtle Dermochelys coriacea and the pelagic sea snake Pelamis platurus, are wholly marine (120, 129). In reptiles and birds the terminal portion of the cloaca serves to recover water from the faeces and kidney excreta. The main end-product of nitro- gen metabolism (in birds and many reptiles) is uric acid, which possesses low solubility and is excreted in a nearly solid state. This is not the case in aquatic turtles, however, which excrete mainly urea and ammonia. The Osmotic concentration of the blood of marine turtles is somewhat higher than that of freshwater forms (Caretta, marine, A 0:76; Emys, fresh water, A 0-44; see Fig. 2.17). The urine is reported to be hypotonic to the blood. As far as we know, marine turtles are largely dependent upon food and metabolic sources for water (62, 92, 126, 135). Birds. Inshore species of birds such as gulls, cormorants and steamer ducks may periodically resort to fresh water, but many pelagic species (petrels, albatrosses, penguins) never taste fresh water throughout their lives. Maritime birds drink salt water and also derive water from their food and metabolic processes. A considerable amount of water is lost by evaporation from the lungs and air sacs (76, 77, 93, 147). It is a common physiological condition among vertebrates for the marine representatives to have more concentrated blood than allied fresh- water species (Fig. 2.17). There is some evidence that birds can produce a slightly hypertonic urine. Water is conserved by absorption in the kidney tubules and hind gut, and excess salt is excreted in the urine. The more concentrated blood of marine species, by lowering the osmotic gradient, reduces the amount of osmotic work that must be performed in the secre- tion of salt and the production of a hypertonic urine (105). A peculiarity of petrels and their allies is the presence of a greatly 56 THE BIOLOGY OF MARINE ANIMALS enlarged proventriculus, responsible for secreting large amounts of lipoids. It appears that the secretion of stomach oil is confined to the breeding season and that it is used in feeding the young. Adult fulmars also show a preference for fatty foods. A highly adaptive lipoid metabolism is charac- teristic of animals which are exposed to water shortage. The metabolic oxidation of fats yields far more water than other foodstuffs, and probably is an important factor in maintenance of water balance among pelagic birds (76, 90). Mammals. Osmotic relations in marine mammals are similar to those in pelagic birds. The osmotic pressure of the blood is slightly higher than that of terrestrial species, but still well below sea water (Fig. 2.17), and active processes of osmoregulation are necessary to maintain the salt and water balance of the animal. Water is obtained largely from the food. Both birds and mammals that feed upon marine vertebrates obtain a food of relatively low salt content (about 1 % NaCl), not markedly different from the concentration of their own body fluids (fish-eating birds, seals, killer whales). Those species that feed upon marine invertebrates (some birds, crab-eating seals, walruses, whalebone whales) are in a rather different category since the salt concentration of their prey is practically equivalent to sea water, and the margin of water left for their functional needs is reduced. Water is lost by evaporation at the respiratory surfaces, in the faeces and in the urine, but loss through the latter two routes is reduced by resorption in the rectum and production of a hypertonic urine. A further loss of water occurs during lactation and the production of a concentrated milk (rich in fat), in whales and seals, is related to the necessity for water economy (26, 77, 91, 124). In the mammalian kidney the urine is concentrated by resorption of water in a special segment of the distal convoluted tubule. Measurements of osmotic pressures and chloride concentrations in the urine of marine animals show values of A 0:73-4:50 and 10-413 mm Cl for seals and A 1-83-3-41 and 75-820 mm Cl for whales. The majority of values lie within the range encountered in terrestrial species. In the harbour seal Phoca vitulina it has been found that the rate of urine formation is low between meals but increases markedly after a meal of fish. Loss of water in the urine is thus curtailed between meals, and the kidneys are enabled to function more effectively after feeding when additional water is avail- able for renal excretion. Marine mammals drink little sea water: seals are able to swallow their prey while submerged without taking in salt water, and whalebone whales use their huge fleshy tongues to press out the water from the crustaceans trapped in the baleen plates. It appears, then, that marine mammals keep down their salt content by avoiding the ingestion of salt water, by excreting minimal amounts of water and by the produc- tion of a hypertonic urine (47, 55, 79, 128). The data derived from seals which have been fed upon fish (herring) indicate that there is sufficient water available from this source to provide WATER, SALTS AND MINERALS mW for the normal physiological requirements of the animal. Marine mam- mals which feed largely or exclusively upon invertebrates are subject to an additional osmotic strain. The salt content of the urine is generally less than 32 g/l., and accurate experiments on the dolphin Tursiops truncatus have shown that individuals of this species fail to regain their original state in 9 hours after administering 2 1. of 0-5 M NaCl. It is still uncertain how the water requirements of marine mammals which feed largely upon inverte- brates are met. Some workers have suggested that water can be absorbed by the mucous membranes of mouth and gullet. The fat content of the food of these animals (copepods, herring, etc.) is high (Table 2.1), and could provide a rich source of metabolic water (30, 31, 77). OSMOTIC RELATIONS AMONG PARASITES. Parasitic animals are sometimes subjected to peculiar osmotic conditions as the result of their parasitic mode of life. These arise in relation to the external medium of the host (ectoparasites and free-living stages), the internal medium of the host (endoparasites) and the use of the host’s fluids as a source of food (blood- sucking ectoparasites). Osmotic relations have been investigated in only a few species and information is limited, but from what is known of their life-histories parasites may be expected to display peculiar conditions of adaptation and regulation in relation to their biotic environments. Highly specialized parasites, which have external phases and utilize a succession of hosts, may be exposed to widely different salt concentrations at different stages of their life-history. Examples are marine trematodes passing from marine invertebrates to vertebrate hosts. These soft-bodied animals appear to be poikilosmotic and osmolabile. Some marine cercariae will withstand sea water diluted to 50-25%. At least one species, Spelo- trema, 1s restricted in distribution by salinities below 17%, (7). Cestodes from fish appear to be isotonic or slightly hypotonic to the hosts’ fluids. Plerocercoid larvae of Schistocephalus from Gasterosteus are poikilosmotic, and shrink or swell when the external medium is made more concentrated or dilute (above or below 0:75% NaCl). Another cestode, Bothriocephalus, from the spiral valve of Scyliorhinus, is slightly hypotonic to the surrounding intestinal fluid (4 2 against A 2-08—2-65). This is an interesting problem since a large part of the osmotic pressure of selachian body (and intestinal) fluids is due to urea (p. 51). Evidence exists that intestinal nematodes possess some powers of osmotic and ionic regulation (49, 131). Some parasites have become adapted to host animals which migrate into estuaries, fresh water or even upon land. Species of fish-lice Argulus infect both fresh- and salt-water fishes and will tolerate immediate transfer from one medium to the other. Some bopyrid and rhizocephalan parasites accompany their crustacean hosts into brackish or fresh water. In contrast are caligid copepods which are parasitic upon sea-run trout and salmon: these parasites are stenohaline, and are killed when the host returns to fresh water (2, 146). Ectoparasites, which infect terrestrial crabs, face a somewhat different (PET) EUS ‘SI $(8b) JOQOH ‘PI S(E1T) Wosueqoy “ET! 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An example is Cancricola, a harpacticid copepod found on the gills of littoral and land crabs. These parasites feed on blood and mucus, and benefit from the fact that the host must keep its gill surfaces moist. External parasites which feed on the blood of marine vertebrates take in a fluid of much lower osmotic pressure than the surrounding medium, and tend to become flooded until the surplus water is eliminated. A peculiar degree of osmolability is displayed by the parasitic copepod Lernaeocera branchialis from the cod. Lernaeocera is normally isotonic with sea water but becomes hypotonic when living upon a fish and feeding upon the latter’s blood (99). MINERAL SALTS AND IONIC REGULATION Osmotic relations are a function of the total concentrations of solutes in the media. Although osmotic pressures of external and internal media are usually equal in marine invertebrates, the concentrations of individual ions are frequently dissimilar. In this regard it is necessary to distinguish between intracellular and extracellular fluids. All cells appear to differ in inorganic composition from the surrounding media, whether tissue fluids or sea water (Table 2.8). The cell sap of the large unicellular alga Va/onia differs greatly in ionic composition from surrounding sea water. Eggs of the sea urchin Paracentrotus show high levels of potassium and relatively low concentrations of sodium, chloride and sulphate ions. Muscles gener- ally contain large amounts of potassium and low levels of sodium and chloride ions. Calcium and magnesium ions are present in greater (Ap/ysia) or lesser amounts (Pecten) than in sea water. Blood corpuscles of Caudina concentrate ionic potassium and sulphate, but reduce sodium, magnesium and chloride. Axoplasm contains particularly large amounts of potassium and low levels of sodium and chloride ions, in conjunction with special excitability characteristics of nerve fibres (Chapter 10). Cells both within the body and freely exposed to sea water can regulate individual ions at concentration-levels differing from sea water or tissue fluids. There is a general tendency for cells to concentrate K* and, to a lesser extent Ca**, and to reduce Na* and Cl-. Muscle fibres of Carcinus regulate Na* and K+, Na+ being secreted, and K* retained by the fibre. Much variation exists in the way in which Ca++, Mg+* and SO,~ are treated (123a, 139). In lower forms the intracellular osmotic concentration 1s largely due to inorganic electrolytes, but in certain higher marine invertebrates a tendency exists for a large proportion of these ions to be replaced, particularly in muscle cells, by organic substances. Considerable quantities of amino- acids occur in molluscan and crustacean muscle, where they are believed to be important in regulation of intracellular osmotic pressure (20, 34). Ionic Concentrations in Body Fluids Turning now to internal fluids (plasma, haemolymph, coelomic fluid) we find certain disparities in the ionic concentrations of different species. LABLEE 2.9 CONCENTRATIONS OF IONS IN BoDY FLUIDS OF MARINE ANIMALS (in mM per |. or mM per kg watert) Animal Fluid Na® Ko 1Ca.* Met =| “Ch So; as Source Coelenterata Aurelia auritat Tissue fluid 454-4 10-5 9-73 | 50-8 | 554-2 15-18 | 536 1 Aequorea coerulescens* Tissue fluid 456 24-3 9-9 | 50-5 | 548 23-6 | 519 2 Cyanea capillata+ Tissue fluid 442 16-2 9-9 | 50 556 11-2 | 519 2. Polychaeta Aphrodite aculeata Coelomic fluid 467-3 12:59 | 10-24 | 52-93 | 549-9 | 28:2 | 548 1 Arenicola marina Coelomic fluid 470-6 | 10-31 | 10-22 | 53-73 | 546-7 | 26-04 | 548 1 Amphitrite johnstoni Blood 405-9 | 13:02 | 9:54] 54-9 | 477 30-8 | 483 3 Glycera dibranchiata Coelomic fluid — 9-61 | 10-0 | 61-34 | 483 27:5 | 483 3 Sipunculoidea Golfingia sp. Coelomic fluid 387-1 | 38-9 | 10-7 — | 440-3 —=- | 522 4 Echiuroidea Echiurus echiurus Coelomic fluid 440-16 | 12-62 | 9-17 | 42-48 | 480 30-72 | 483 3 Crustacea Ligia oceanica Blood 586 14 36:2 21 596 4:5 | 581 17 Palaemon serratus Blood 394 7:7 12-6 | 12-6 | 430 2°6 | 581 19 Homarus americanus Serum 465-13 | 8-56} 10-66} 4-73 | 498 5-0, 2525 3 Palinurus vulgaris* Plasma 544-42 | 10-33 | 13-45 | 16-61 | 556-7 | 21-30 | 560 1 Nephrops norvegicus+ Plasma e115 1725 TST \- 13-85: )\__8-88)| 518-97 | 218-53 5G5s0 1 Cancer borealis Serum 459-84 | 10-2 11-5 | 21-89 | 479 18-78 | 492 3 Carcinus maenas* Blood 514-84 | 11-94 | 12-9 18-91 | 541-22 | 15-93 | 542 5 Lithodes maiat Plasma 476°6 | 12-48 | 12-35 | 52°18 | 536-7 | 25.7 | 530 1 Xiphosura Limulus polyphemus Serum | 386-4 | 12:76 | 9-01 | 40-9 | 468-4 | 14-79 — 7 Mollusca Mytilus edulist Blood 501-8 | 12:7 | 12:6 | 55-8 | 586-1 | 30-7 | 592-2 18 Mercenaria mercenaria Blood 308-9 6-82 | 11:3. | 29-1 374 18-1 319 3 Pecten maximus Blood 487-3 | 13-47 | 13-8 | 44-2 | 554:5 | 30-1 568 11 Aplysia punctata Blood 587 11-6 | 13-87] 51-8 | 645-9 = — 13 Archidoris pseudoargus Blood S175, 05:0) | 12:6 | 5637) + \/528:8 — | 525 13 Sepia officinalist Blood 465 21-9" *) 11-6 | 57-7 591 6:3 | 575 14 Loligo forbesit Blood 419 20-6 11-3 51-6 \.522 7-3 | 506 14 Eledone cirrhosat Blood 438 13:0 | 11:0 | 54-6 | 513 20:7 | 510 14 Echinodermata Asterias vulgaris Coelomic fluid 460-1 8-82 | 8-89 | 30-7 | 505 25-4 | 492 3 Marthasterias glacialist Perivisceral fluid | 459-6 10-87 | 10-18 | 51-15 | 540-9 | 27-59 | 537 1 Echinus esculentust Perivisceral fluid | 444-4 9-58 | 9-97 | 50:29 | 524-29 | 27-07 | 525 6 St fe Copel aaa Coelomic fluid | 461-0 | 9-59| 8-82] 31-0 | 510 | 25-3. | 483 3 Lytechinus variegatus Perivisceral fluid | 546 14-68 | 13-92 | 35-36 | 485 17-99 | 510 12 Cucumaria frondosa Coelomic fluid 419-8 9-69 | 9°35 | 49-67 | 487 29-8 | 483 3 Tunicata Ascidia nigra Blood 403-2 | 17-6 | 10-35 | 38-5 — — 527°6 12 Phallusia mammillatay Blood 467-3 9:98 | 9-54 | 52-93 | 567-5 | 14-83 | 548 15 Vertebrata Myxine glutinosa Serum 401-9 9:09 | 5-29 | 22-48 | 448 6:03 | 483 3 Myxine glutinosa+ Serum 558 9-6 e12-Sy 1) 38-8 1 S76 13:3), |-592 15 Squalus acanthias Serum 257°3 7:01 | 4:01] 6:0 | 276:93} — — 7 Mustelus canis Plasma 270 5-6 5-8 3-0} 230 1-79 —— 8 Raja laevis Plasma 255 5:2 3°8 3-5 |, 235 0-5 — 8 R. erinacea Plasma 254 8 6 2-5 7 255 — — 20 Muraena helena Plasma 211-8 1:95 7:73 | 4:85 | 188-4 11-:35-| 659 15 Gadus callarias Serum 180-9 | 10-1()| 4:67] 2-42 | 175-5 — — U Syngnathus acus Blood 206°8 16-5 3:50 | 9-68 | 190-7 — — y Lophius piscatorius Blood 242-1 6-60 2-61 4-56 | 182-2 — — 9 Lophius piscatorius Plasma 180 5-1 11 10 172 = — 16 Sea water, chlorinity 19°/ 9, density at 20°C 1:0243 470-15 | 9-96 | 10:24 | 53-57 | 548-30 | 28-24 | — 10 (1) Haemolysis of erythrocytes may have raised level of K+. Sources: 1, Robertson (112); 2, Koizumi and Hosoi (73); 3, Cole (including analyses of Smith) (22); 4, Steinbach (133); 5, Webb (142); 6, Robertson (111); 7, Macallum (85); 8, Smith (125); 9, Edwards and Condorelli (25); 10, Sverdrup, Johnson and Fleming (134); 11, Hayes and Pelluet (45); 12, Valente and Bruno (140); 13, Bethe and Berger (9); 14, Robertson (113); 15, Robertson (114); 16, Brull and Nizet (18);17, Parry (101); 18, Potts (106); 19, Parry (102); 20, Hartman, et al.( 44). WATER, SALTS AND MINERALS 61 Nevertheless the ionic compositions of body fluids throughout the animal kingdom possess a general pattern of resemblance which is too great to have originated fortuitously. In lower animals the tissue fluids closely resemble sea water (coelenterates, polychaetes). Higher forms tend to accumulate K* relative to Na*, and to reduce levels of Mg** and SO,>. This trend is most pronounced in decapod crustaceans and vertebrates. Concentrations of ions in body fluids of representative species are shown in Table 2.9. Absolute values depend to some extent on the concentration and ionic composition of the external medium. The relative ionic composi- tion of the body fluids of a series of marine invertebrates and fishes is presented in Table 2.10 on a chloride basis of 100. This ion is in equilibrium with sea water in most animals, and forms a suitable basis for comparison. Of twenty invertebrates examined by Robertson (112), all had chloride TABLE 2.10 RELATIVE IONIC COMPOSITION OF BODY FLUIDS Group Animal Nat K+ Catt Megtt+ Ci so,- Sea water 55°5 2:01 2°12 6-69 100 14-0 Coelenterata Aurelia aurita 53 2-05 1:96 | 6:3 100 6-3 Polychaeta Arenicola marina 56 2:09 2°12 6:7 100 12-9 Aphrodite aculeata Ee] 252 2°12 6-6 100 13-9 Sipunculoidea Golfingia vulgaris | 58 2:24 2°25 4-7 100 12-9 Mollusca Pecten maximus 55 2°61 2:18 6°5 100 13°5 Mya arenaria 56 2-15 2-26 6:6 100 14-1 Ensis ensis 55 3-14 2°30 6°6 100 12:2 Pleurobranchus membranaceus 56 2:36 2°38 6:7 100 14-3 Neptunea antiqua | 35 2-30 2:25 6-8 100 13-1 Buccinum undatum 54 2°88 2°31 | 7:0 100 12-6 Eledone cirrhosa | 54 3:07 2:46 | 7:0 100 10-5 Sepia officinalis 51 4:10 2:22 6-7 100 2:9 Loligo forbesi | 52 4-35 2:44 6:8 100 3-9 Crustacea Squilla mantis 64 2°69 2°59 2-2 100 I1-2 Homarus vulgaris 62 1-75 3-20 1-0 100 4-4 Nephrops norvegicus 65 1-61 3-02 1-2 100 9:7 Palinurus vulgaris | 63 2:05 213 2-0 100 10-4 Carcinus maenas 62 2°43 2:69 2°4 100 8-0 Cancer pagurus 64 2°44 3-00 3-2 100 13-8 Lithodes maia ests) 2:56 2-60 6-7 100 13-0 Echinodermata Marthasterias glacialis 55 2:22 2°13 6:5 100 13-8 Echinus esculentus 55 2-09 2°15 6:6 100 14-0 Holothuria tubulosa | 56 2:06 ZS 6:9 100 13-9 Tunicata Phallusia mammillata 53-3 1-95 1:91 | 6:38 100 T1 Salpa maxima | .54-4 2-22 1-99 6:20 100 8-8 Vertebrata | Myxine glutinosa 62-9 1-84 1-23 2°31 100 3-1 Lampetra fluviatilis 80-9 3-69 2-32 1-51 100 ACD Mustelus canis iy 16 2:68 2:79 0-82 100 2°11 Raja laevis 70 2-44 1-82 0-29 100 0-46 Gadus callarias 67 6°35* 2:62 0-95 100 — Lophius piscatorius 86 4-0 1-62 1-72 100 — Muraena helena 72-9 1-14 2-32 0-88 100 8-2 * Haemolysis of erythrocytes probably responsible for release of intracellular K~. (Data from Robertson (111, 112, 113, 114) and earlier sources) 62 values within 4% of sea water, and most within | °%. In marine fishes, which regulate osmotically and keep their blood concentrations below 200 mm, the internal chloride level is directly determined by the homoi- osmotic powers of the species. THE BIOLOGY OF MARINE ANIMALS Physiological Media Suitable physiological media for marine animals are described in the appendix (p. 675). Some of these are based on analyses of body fluids, others have been determined empirically. Hydrogen Ion Concentration. Sea water is alkaline, about pH 8-1. The body fluids of marine animals are generally more acid than sea water, usually above neutrality, but slightly below in some species. Some values for several species are shown in Table 2.11. A small variation in hydrogen TABLE 2.11 HYDROGEN ION CONCENTRATIONS (pH) OF BODY FLUIDS Animal Fluid pH Polychaeta Arenicola marina Coelomic fluid 7:2-7:3 Amphitrite johnstoni Coelomic fluid 6-80 Glycera dibranchiata Coelomic fluid 7:40 Echiuroidea Echiurus echiurus Coelomic fluid 7:60 Sipunculoidea Sipunculus nudus Body fluid 7:25-7:79 Mollusca Strombus gigas Blood 75 Aplysia fasciata Blood 7:23-7:46 Mercenaria mercenaria Mantle fluid 7-90 Crassostrea angulata Blood 7-2 Sepia officinalis Blood 7:24-7:90 Octopus vulgaris Blood 7:8 Crustacea Homarus americanus Serum 7:55-7-61 Palinurus vulgaris Blood TI Cancer borealis Serum 7°81 Carcinus maenas Blood 7:53 +0:37 Callinectes hastatus Serum 7:24-7:49 Xiphosura Limulus polyphemus Serum 6:98-7:47 Echinodermata Cucumaria frondosa Coelomic fluid 3-7-8 Asterias vulgaris Coelomic fluid 7:2-7:54 Solaster endica Coelomic fluid 6:90 Strongylocentrotus drobachiensis Coelomic fluid 7:20-7:84 Lytechinus variegatus Coelomic fluid 7-7-7°8 Echinarachnius parma Coelomic fluid 6-90 Tunicata Salpa maxima Blood 7-5 Chelyosoma siboja Pericardial fluid Ube Ciona intestinalis Blood 6:47-6:56 Vertebrata Myxine glutinosa Serum is) Scyliorhinus stellaris Blood 7-32 Torpedo ocellata Blood 7-64 Conger conger Blood 7-67 Scomber scombrus Oxygenated blood at 2:17 mm CO, 7:94 Prionotus carolinus Oxygenated blood at 1:21 mm CO, 12 Opsanus tau Oxygenated blood at 1:37 mm CO, 7:64 (Various sources) WATER, SALTS AND MINERALS 63 ion concentration is tolerated by tissues or organs. The lobster heart, which has a pH optimum of 7-4, continues to beat normally over the range pH 7:0-8-0, but values beyond these limits affect tonus, amplitude and frequency. Cardiac and smooth muscles of other animals show similar sensitivity. Herring eggs develop normally in sea water ranging from pH 6:7—8-7, but development is retarded in sea water with lower pH values (58). Ionic Regulation The ionic differences which exist between body fluids and the surround- ing sea water could result from passive physical agencies or depend upon active regulation by the animals’ tissues. The body fluids of more primitive groups, namely coelenterates, polychaetes and echinoderms, usually con- tain very little protein, but in more active crustaceans and cephalopods blood protein attains high levels, up to 100 g/l. in Lo/igo and Eledone (see Tables 2.12 and 2.13). The presence of protein affects ionic diffusion and concentrations in several ways. Proteins form undissociated complexes with calcium and retain calcium at high levels. Other cations may be bound by negatively charged protein molecules which are prevented from diffus- ing across bounding membranes because of their large size. According to the Donnan equilibrium, the product of diffusible cations and anions inside must equal the product of diffusible cations and anions outside, e.g. Na; x Cl; = Na, x Cl,. Since some of the cation is held by protein the actual situation is— Na;Cl,; Na,Pr tierciore, Na, > Na,, and Cl, < Cl,. inside outside Na,Cl, For the Donnan equilibrium to be operating under these circumstances without endosmosis implies impermeability to certain ions, or an internal hydrostatic pressure equal to or greater than the colloidal osmotic pressure. The magnitude of the ionic differences resulting from the undiffusibility of proteins and protein/calcium complexes across gills and other bounding membranes can be estimated by dialysing the body fluids against sea water and comparing the analyses of undialysed and dialysed samples. Data obtained by Robertson and Webb for a series of marine animals are shown in the accompanying tables (2.12 and 2.13). These investigations show that the ionic differences between body fluids and sea water become greatly reduced following dialysis, and reveal the small part played by protein-binding in determining the concentration levels of ions in blood and other internal media. Most of the disparity between internal and external media is due to controlled regulation of ionic levels by the animal. The extent to which different animals regulate separate ions can be appreci- ated from an examination of Table 2.10 (relative ionic composition of body fluids). The ability to regulate ionically is a universal characteristic of marine animals, differing only in magnitude in various groups. In more primitive (uOs}IoqgoYy Wor eyed) I9}eM 3/SU _ 1-986 = 9V9°C 68°81 VICI 00v-0 O8t-0 Sv-0l JOJEM BOS [BULION VS6 8e £9S:C 87-81 Tle-1 Ovv-0 c8t:0 9¢-01 SISATBIP 1OIJe vS6 8e OLV:C ¢0-61 697: I S6v:0 887-0 96:01 SISATPIP d10J9q BLUSeI DIDUL Sapoy ITT c:196 (DS-VE LEO"C SS-81 687: 1 v9v-9 96£:0 OL-OL. 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HTaV.L WATER, SALTS AND MINERALS 65 groups, embracing coelenterates, echinoderms, polychaetes, lamelli- branchs, gastropods and ascidians, regulation is slight. There is some regu- lation of potassium and, to a lesser extent, of calcium ions; ionic sulphate is reduced slightly, except in Aurelia and Phallusia, where it is halved. Sodium, magnesium and chloride ions are in equilibrium with sea water. In contrast to these forms, the two active invertebrate groups, cephalopods and decapod crustaceans, show well-marked regulation of all ions in the haemolymph. Cephalopod bloods show a range (expressed as percentage of concentration in dialysed plasma): Na+, 93-8; K+, 152-219; Ca++, 91-107; Mg**, 98-103; Cl-, 101-5; SO,-, 22-81. Potassium is held at high levels, magnesium is accumulated slightly and sulphate is greatly reduced. The range in decapod crustaceans is: Na+, 94-113; K*, 77-156; Catt, 84-137; Mg**, 14-99; Cl-, 87-104; SO,-, 32-135. Potassium is usually accumulated (anomurans and brachyurans) and calcium is main- tained at higher levels than sea water in most species examined. The most striking feature is the great reduction in magnesium, ranging from equi- librium with sea water in Maia and Lithodes, to 14°% of sea-water values in Homarus. Sulphate is likewise reduced. A peculiarity of decapod blood is seen in the accumulation of sodium, in correlation with a reduction of magnesium. Hermit crabs (Eupagurus) are unusual among decapods in accumulating sulphate, and in correlation with this fact have reduced levels of chloride to preserve the balance of anions. The isopod Ligia oceanica likewise shows well-marked ionic regulation (Table 2.9). Sodium, potassium, calcium and chloride are all more concentrated than in sea water, whereas magnesium and sulphate are much reduced. Data relating to ionic regulation in lower vertebrates are presented in Table 2.10. The serum of Myxine is isosmotic with sea water (449 mm when external Cl- is 483 mm). Salt levels of lampreys and all higher forms are only about one-half sea-water values (280 mM in selachians and 200 mm or less in teleosts). In lower vertebrates sodium, and often potassium, are accumulated in the plasma relative to chloride, to a small extent in Myxine, and at relatively high levels in marine selachians and fishes. Magnesium is greatly reduced relative to chloride in correlation with the increase of sodium and sulphate is held at very low levels. Ionic regulation is more highly developed in vertebrates than in the most advanced invertebrate phyla (114). Mechanisms of Ionic Regulation. There are two aspects of ionic regula- tion which have excited much interest, namely the historical basis for the ionic similarity of body fluids, and the mechanisms by which particular concentrations are maintained. The similarities in ionic composition of the body fluids of animals are more striking than the differences. Thus, all fluids show relatively high levels of Na* and Cl-, and low levels of K*, Mg** and SO,=. Moreover, the relative concentrations are roughly of the same order as those found in sea water. This general similarity between body fluids and sea water has been regarded as a relic of the probable marine origin of animals. Macallum (85) has pointed out that the fluids of M.A.—3 (UOs}IOGOY WO.jy ve) 968 0:SOI CLL L101 L:col -LOT [cst V-L6 Sc6 259 L:08 0-101 re €-V6 v-SLI 9:L6 C68 601 [cc I-SOl 1-86 S:06 c:S0C S°C6 998 L-6vl €:6C S-€Ol L101 9-101 V-617 c:S6 996 €:ST 0:06 9:66 8-cOl €-vOl 6:Itl1 t-L6 896 L-V7 8:L6 6-001 I-10] L101 6tIl 9-001 186 vv $98 c:66 L:86 9-LOI cost L:86 C86 BT 0-101 6:66 0:66 9901 L901 8-001 £86 9:C $:96 0-001 €-L6 §-cOl 6:6C1 L:66 vS6 8t 96 tol L6 9! 87l vol 9¢6 Ss 19 vol ve 801 Sit OI 196 Lt 66 86 Ag SII SII 601 976 69 sel 96 6v Lel OtT Sor 196 tt 69 66 jE) vcl LL ell 96 67 ct 101 vl lel $8 Orl = aa 101 OOT OOT 101 col OOT Toye ul9}01d —"OS ID ++5IN ++P89 +A +PN [wi /suu SONA SISATRIP JO SosejUddIOd Se SUOI}eI]UIOUOD SANIVA SISATVIC dO SHOVLNAOUAd SV SGINTY AGOG AO NOILISOdWO) tlc ATAVL psoysdd auopayy syoursyfo vidas Isaqsof O81]OT winjopun wnulong pnbyuv vaunjdan Sisua SISUT DIADUIAD DAY SNUIXDUL UdJIAg pIDU SapoysIT SDUADUL SNUIIADD snanspd daduvy Snpapyudsag snansvdny SHI1BIAAOU SdodydanN SLUIDSINA SNADUOPT Snjuajnrsa snulyory jewuy WATER, SALTS AND MINERALS 67 primitive marine animals closely resemble oceanic water in ionic proper- ties. The blood of higher forms, particularly vertebrates, differs somewhat from sea water in the relative proportions of various ions. Macallum advanced the hypothesis that the chemical composition of the ancient seas differed profoundly from that of the oceans today. Marine animals were considered to have been originally in ionic equilibrium with sea water. As the oceans changed in composition, the fluids of more primitive animals altered correspondingly, whilst higher animals, which developed closed circulatory systems, tended to retain throughout their subsequent evolu- tion the ionic composition possessed by sea water at the time their circu- latory systems were closed off. The peculiar ionic conditions in the plasma of vertebrates were thus related to the composition of sea water at the time these animals first appeared in the ancient seas. This interesting hypothesis has served to underline the essential simi- larity of body fluids and has directed attention to their probable origin. Evidence relating to the chemical composition of the oceans in past ages indicates that they have remained relatively stable throughout most of geological time, and any variations which have occurred have been within rather narrow limits. The problem no longer bears the original stamp that was impressed upon it, since the evidence just reviewed shows that all marine animals can regulate the chemical composition of their cellular and body fluids. Ionic regulation is an evolutionary acquisition and its mode of expression has been modified with time. Peculiarities in the vertebrates must be traced back to their freshwater origin (85, 118). Differences in ionic mobilities may be involved to some extent in establishing and maintaining ionic levels in animals, but the primary fac- tors operative must be ascribed to active processes of absorption and excretion of particular ions. These processes result in the creation of ionic steady states, in conjunction with, or independent of, the maintenance of osmotic equilibrium. Phyletic Review of Primitive Groups largely in Equilibrium with Sea Water We have noted that ionic regulation is slight in inactive members of more primitive groups. The ionic composition of the mesogloea in Aurelia differs slightly from that of sea water, and is controlled by the bounding ectodermal and endodermal epithelia. Sulphate is actively eliminated together with associated cations. Active absorption of potassium appears to take place. The chloride increment is apparently a passive consequence of the reduction of sulphate, and thus acts to counterbalance the cations Ciao t2): Peculiar conditions relating to floating devices have been noted in siphonophores. In various genera, e.g. Aga/ma and Diphyes, there are special bracts (hydrophyllia) or bells (nectocalyces) which are lighter than sea water. Suggested mechanisms to achieve this condition are hypo- tonicity (reduced internal salt concentration) or accumulation of substances with lower specific gravity than NaCl (77). 68 THE BIOLOGY OF MARINE ANIMALS Various hydroids and anemones which inhabit brackish water appear to possess low salt concentrations, and to be in osmotic equilibrium with their environment. Internal concentrations of certain ions also follow closely those of the external medium. Sea anemones have calcium con- centrations nearly equivalent to sea water. When placed in dilute sea water, animals absorb water and lose calcium; in sea water plus isotonic CaCl,, calcium is absorbed. The body wall of anemones shows two-way perme- ability to water and calcium, but exchange of the latter is rather slow (52). The concentrations of ions in coelomic fluids of echinoderms are very similar to those insea water. Potassium appears to be regulated in all species, and magnesium in some instances. The quantities of protein are very small, less than 1 mg/g of water, too low to affect ionic concentrations. Concen- trations of potassium in the water-vascular system are much higher than in the perivisceral fluid (Marthasterias, Echinus). Suggested mechanisms are, active absorption of potassium via the gills or outward diffusion of potassium through the same structures, thus maintaining a concentration gradient of potassium across the vascular system and coelom to the exterior (9; 112, 113). The holothurian Caudina chilensis resembles coelenterates in the facility with which ions and water are exchanged with the environment. Swelling and shrinking take place in hypotonic and hypertonic media. When animals are placed in artificial sea water in which the concentrations of individual ions have been altered, the body fluids alter in conformity with the environment and reach equilibrium within 5 days. Relative rates for ionic movement across the body wall are K+ > Nat > Ca++ > Mgtt, and Cl- > SO,= (69, 70). The coelomic fluids of polychaetes, sipunculoids and echiuroids like- wise contain very little protein, below 1 mg/ml of water (Aphrodite, Arenicola and Golfingia). There is slight regulation of ions: potassium is accumulated in most species, and sulphate is reduced in Arenicola and some others. The mechanism of ionic regulation in these animals is un- known. Selective absorption of ions by the body wall, and secretion of a urine low in potassium and rich in sulphate may be involved. It has been suggested that nephridia may be concerned in ionic regulation in these forms. Arenicola marina adjusts in dilute sea water and soon reaches osmotic equilibrium. Potassium and calcium are increased relative to the medium, sulphate reduced, while other ions attain equilibrium with the diluted medium (Table 2.14). Heightened values of potassium and calcium may be due to accentuated ionic regulation in dilute media, or to differ- ences in diffusion rates. An analogous situation is presented by Golfingia muscle immersed in solutions of artificial sea water in which potassium or calcium is altered. Intracellular potassium is kept at a higher level, and calcium is held below that of the external medium over wide ranges of concentrations (133). Decapod Crustacea. Marine decapod crustaceans generally possess high levels of sodium, potassium and calcium ions, and reduced levels of WATER, SALTS AND MINERALS 69 magnesium and sulphate ions. Lobsters are peculiar in having low potassium values. Protein occurs in high concentrations, up to 8% of the plasma, and forms indiffusible complexes with calcium. Ionic regulation involves: active absorption by the gills of sodium, potassium and calcium TABLE 2.14 IONIC REGULATION OF COELOMIC FLUID BY Arenicola marina IN FULL STRENGTH AND DILUTED SEA WATER (75-50% Concentrations of coelomic fluids as Medium ~ percentages of sea-water values | mg/ml Nat Kt Catt | Mgt* | Cl | SO, | Protein | Water In normal sea | 100-1 103-5 99-8 100-3 99-7 92:2 0-2 983 water In diluted sea | 100 118 113 100 100 § 90 — = water | | (15-23 hours) (From Robertson (112)) against a concentration gradient; inward diffusion of magnesium and sul- phate along the concentration gradient; differential excretion by the anten- nary glands, tending to lower blood magnesium and sulphate, and con- serve sodium and potassium. The gills and integument of the lobster are relatively impermeable to magnesium and sulphate ions, which enter largely through the gut. Permeability to ions has been measured in various ways. When Carcinus is placed in dilute sea water, changes take place in the relative proportions of ions in the blood. Sodium and chloride decrease more than potassium and calcium, and blood protein rises. Percentages of sodium, potassium, calcium and chloride increase relative to the medium, while magnesium and sulphate decrease (Table 2.15). The greater divergency between ionic TABLE 2.15 COMPOSITION OF THE HAEMOLYMPH OF Carcinus maenas IN NORMAL AND DILUTE (67 °%) SEA WATER Percentages of concentration in external medium Medium Naz Kt Care Mgrtt Cie SOr In normal sea water 110-9 120-9 126°72 35°4 99-79 771 In dilute sea water 134-5 142-7 133-6 31-3 12552 46:4 (From Webb (142)) ratios of the blood and external medium which results is due to increased salt absorption and excretion. The overall effect is a reduction of osmotic gradient, largely owing to an absolute decrease of sodium and chloride and increased ionic regulation. Permeability has also been tested by altering the concentration of ions individually in the medium. 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For example, Carcinus in calcium-free sea water suffers a reduction of internal calcium to one-fifth in 8 days. The gills are far more permeable than the general integument. Those of Carcinus actively absorb sodium, potassium, chloride and probably calcium against a concentration gradient. The gills of Eriocheir take up the same ions from very dilute media. Measurements on isolated gills of Eriocheir show that NaCl is absorbed at a rate of 2:5 mg/g tissue/hour. Urine A Excretory organ t Mg*t CL S04= Hy0 Haemolymph By z aoe Nat Kt Mg ++ CasceCl> ial Nas Ke Water Cat Cle Fic. 2.18. DIAGRAM ILLUSTRATING THE SALT AND WATER EXCHANGE OF Carcinus Broken arrows represent movement of solutes and water brought about by diffusion and ultrafiltration; continuous lines, movements (absorption and secretion) occasioned by active regulation on the part of the animal. (After Webb (142).) Absorption of NaCl is halted by cholinesterase inhibitors, and it is likely that cholinesterase is involved in the mechanism of ion-transport. The accompanying diagram (Fig. 2.18) illustrates salt and water exchange in Carcinus. Examination of crustacean urine reveals differential excretion. Sodium, potassium and calcium are conserved, while magnesium and sulphate are eliminated (Table 2.16). Together, the patterns of absorption and excretion broadly account for the levels of individual ions in the haemolymph (18a, 22, 68, 77, 102, 109, 142). Mollusca. In lamellibranchs and gastropods potassium and calcium are regulated, and in cephalopods regulation extends to all ions. Plasma pro- (P2 THE BIOLOGY OF MARINE ANIMALS tein is very high in certain prosobranchs and in cephalopods, largely owing to the occurrence of haemocyanin (Table 2.13). Mechanisms of ionic regulation in marine forms are imperfectly understood. Examination of fluid from the renal sac of cephalopods shows that resorption of potassium, calcium and magnesium ions, and secretion of sulphate ion take place in the formation of urine (Table 2.16). As a result, levels of the former ions are raised, and levels of the latter lowered in the blood. The low level of sodium in the renal fluid of Sepia, compared with plasma, is ionically balanced by secretion of ammonia (146 m-equiv./kg water). As far as absorption from the medium is concerned, it would appear that potassium, magnesium, etc., are taken up against a concentration gradient, while sodium and sulphate enter along a diffusion gradient. Gills and general integument are probably involved. The body wall of Ap/ysia (tectibranch) is freely permeable to the ions of sea water (106, 112, 113). Tunicates. The body fluids of tunicates are in osmotic equilibrium with sea water and ionic regulation is feebly developed (Table 2.10). All ions except sodium are kept at different values from those of sea water: potas- sium is significantly raised in Sa/pa, and the divalent ions—calcium, mag- nesium and sulphate—are reduced in both Thaliacea and Ascidacea. The pattern of ionic regulation is not dissimilar from that of Aurelia (cf. Table 2.10). Excretory tubules being absent in tunicates, the whole burden of ionic regulation is borne by the external surfaces (114). Fishes. Marine elasmobranchs tend to be almost isosmotic with sea water owing to high internal concentrations of urea. From the fluids taken into the alimentary canal, magnesium and sulphate ions are absorbed to only a slight extent, compared with potassium, calcium and chloride ions. Some magnesium and sulphate are lost in the urine and extrarenal excre- tion of chloride is believed to take place. Blood protein levels are rather high in elasmobranchs and teleosts (1-8 %), and the proteins interact with inorganic ions. The mechanism of ionic regulation is somewhat similar in marine teleosts except that the blood is hypo-osmotic. Monovyalent ions, sodium, potassium and chloride, are absorbed from the sea water which is drunk, while calcium and especially magnesium and sulphate are concentrated in the intestinal fluid. Excess magnesium and sulphate are excreted in the urine, while sodium, potassium and chloride are excreted extrarenally via the gills (Table 2.16). Branchial excretion of chloride has been measured by Keys in the heart-gill preparation of the eel Anguilla (77). MINOR ELEMENTS IN TISSUES AND SKELETONS In addition to the principal elements which we have just reviewed, we find various minor elements in many species. These show a random distribution, and their functional significance is not always known. We exclude here the common constituents of the three main groups of organic compounds— carbohydrates, fats and proteins. Apart from chlorine, the three halogens, bromine, iodine and fluorine, WATER, SALTS AND MINERALS 18 are sometimes present in small amounts. Iodine is a normal constituent of thyroxine in vertebrates, and in Amphioxus it is localized in the endostyle and in mucous secretions of that gland. Further evidence is thus provided of the homology of the protochordate endostyle and the vertebrate thyroid gland. Both iodine and bromine occur in gorgonians and sponges, in haloaromatic amino-acids which are normal constituents of skeletal scleroprotein. Iodine is accumulated to some extent by lamellibranchs and other marine animals. Organic secretions of many species besides corals show high levels of iodine, possibly in combination with organic sub- stances, namely tubes of Diopatra, Chaetopterus, Bispira (polychaetes), byssus of Mytilus, test of Pyura (tunicate), etc. Fluorine occurs in traces in the shells of some lamellibranchs, in remarkably large quantities in mantle and other tissues of the nudibranch Archidoris (2% of cations), and in the body wall of the brittle star Ophiocomina (2b, 15, 40, 53, 81, 115, 116, 136, 136a, 141). Some of the heavy metals are accumulated in appreciable amounts by various marine invertebrates. Certain metals, particularly iron, are essential elements in the prosthetic groups of many enzymes. Iron is found in cytochrome, a widely distributed intracellular haemochromogen, and in certain respiratory pigments (haemoglobin, chlorocruorin, haemerythrin, see Chapter 4). The radular teeth of chitons (Chitonidae) and limpets (Patellidae) contain large amounts of iron as Fe,O, (54% of ash in Patella vulgata). The iron content of sea water is low (0-002-0-02 mg/kg), and Patella relies mainly on algal food for supplies of iron. The iron content of certain gastropods, Lineus (nemertine) and Nephthys (polychaete) is high (56). Copper is probably universally distributed among animals. It occurs in the respiratory pigment of crustaceans, xiphosurans and molluscs (haemo- cyanin, Chapter 4), and in some respiratory enzymes. Marine invertebrates lacking haemocyanin generally have copper concentrations of 0-2-8 mg % dry weight, but oysters show unusually large concentrations of copper, up to 300 mg %. Manganese is another element which appears to be a normal constituent of all animals, and occurs in traces. Amounts in Pecten and Ostrea vary from 1-18 mg % dry weight. Gills and ripe ovaries contain the largest amounts and there is an increase of manganese during the repro- ductive period (36, 88). Certain other trace elements have an interesting biological distribution. Vanadium is accumulated by some ascidians, Stichopus mobii (holothurian) and Pleurobranchus (nudibranch). In ascidians, at least, the vanadium is organically bound and is concentrated in blood corpuscles (Chapter 4). Other trace elements reported for various marine animals are aluminium, zinc, nickel, cobalt and titanium. Minor elements may be taken up directly from solution. However, certain ions are adsorbed by hydrated oxides of iron and manganese, and if the latter are collected by filter-feeders the adsorbed elements become available to the animals (8, 38, 39, 74). Calcium is extensively utilized in skeletons of animals, and in calcareous M.A.—3* 74 THE BIOLOGY OF MARINE ANIMALS tubes. Because of their importance these structures are described in more detail in a separate chapter (15). We note here the occurrence of CaCO, in coral skeletons and molluscan shells, and of CaCO; and Ca,(PO,). in the exoskeleton of decapod crustaceans. Magnesium is an important constituent of the skeleton of certain animals—Foraminifera, Alcyonaria, Echinodermata and Crustacea. Magnesium concentrations in some gastro- pods are very high (1:58 % wet weight in Archidoris). Strontium occurs in the skeleton of some radiolarians (Acantharia). Silicon is important in the skeleton of diatoms, most radiolarians and siliceous sponges. The radular teeth of limpets (Patella) contain much silica (33% of ash), as well as iron (56, 141). EGGS, EMBRYOS AND LARVAE Eggs of marine invertebrates are usually in osmotic equilibrium with the surrounding sea water, but differ considerably from the latter in ionic composition (Table 2.8). Echinoderm eggs have been extensively studied, and over a limited range of dilutions they behave in conformity with the gas laws. The plasma membrane is largely impermeable to salts and the egg behaves like an osmometer when placed in dilute sea water. Deviations from expected values for volume changes are explained as due to the presence of osmotically inactive materials. The osmotically inactive frac- tion amounts to 7:3% in unfertilized eggs of Arbacia, and increases to 27:4% after fertilization. Cyclical changes take place in the egg prior to and subsequent to fertiliza- tion, one of which is permeability to water. Sensitivity to dilute sea water and rate of swelling in hypotonic media increase greatly after fertilization. The permeability of the egg membrane (Arbacia, Paracentrotus) alters greatly at this time: potassium, calcium and magnesium ions are released shortly after fertilization and are subsequently resorbed (95, 122). A proportion of the total osmotic pressure of eggs is due to organic molecules, particularly in higher forms. Relative amounts of organic constituents are low in echinoderm eggs, whereas in eggs of Maia, Sepia and Torpedo they are responsible for one-quarter to one-half of the total osmotic concentration. Among mineral constituents potassium is high, as in many cells, sodium occurs in relatively low concentrations, calcium and magnesium are present in colloidal combinations and chloride is mostly dissociated, the relative amounts varying in different species. To maintain these ionic differences from the environmental medium requires a highly impermeable barrier about the egg. In reality, the egg is selectively perme- able, and possesses the power of secreting and absorbing particular ions according to metabolic requirements. During the course of development salt is absorbed, and the ash content and density steadily increase (84, 94). Ionic regulation in developmental stages of marine animals has been reviewed by Krogh (77), and Needham (94) has collated much relevant information. An interesting study of an estuarine polychaete Marphysa gravelyi reveals that the demersal eggs of this species are protected by a WATER, SALTS AND MINERALS hs jelly coat. Eggs denuded of jelly swell or shrink in anisotonic media, and disintegrate in sea water of salinity less than 14%, whereas eggs protected by jelly will develop even in distilled water. Larvae show volume regulation in hypotonic sea water (down to 22%.) presumably due to salt loss (Fig. 2.19). Sex cells and larvae of the California mussel Mytilus californianus are susceptible to dilutions below 29-6%,, and survival declines below this concentration. This species is characteristic of coasts with high salinity, and the narrow salinity tolerance of the larvae is one factor restricting its distribution (75, 149). There is space to examine only certain aspects of water and salt relations in fish eggs and larvae. Oviparous selachians produce what Needham has termed a cleidoic type of egg which is independent of the medium for its 25 20 15 10 Percentage Increase in Volume 20 40 60 80 100 120 140 160 Time (minutes) Fic. 2.19. VOLUME REGULATION IN LARVAE OF A POLYCHAETE, Marphysa gravelyi Smoothed curves showing percentage increases of volume in waters of lowered salinity. (After Krishnamoorthi (75).) supply of water and salts. The egg membrane is largely impermeable to urea, and the accumulation of this substance as the result of protein metabolism gives rise to final concentrations approaching those of the adult. The eggs of marine teleosts are at first permeable to water and salts, but during the course of development osmoregulatory ability develops and the chloride content of the embryo is reduced. 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T., ““The distribution of the mussel (Mytilus californianus) in relation to the salinity of its environment,” Ecology, 22, 379 (1941). CHAPTER 3 BODY FLUIDS AND CIRCULATION But the circulation which I discovered teaches clearly that there is a necessary outward and backward flow of the blood, and this at different times and places, and through other and yet other channels and pas- sages; that this flow is determined also, and for the sake of a certain end, and is accomplished in virtue of parts contrived for the purpose with consummate forecast and most admirable art. WILLIAM Harvey: Letter to P. M. Slegel, 1651 GENERAL FUNCTIONS OF BODY FLUIDS IN the preceding chapter the composition of body fluids has been described, and it has been shown that many animals are capable of maintaining an internal medium of composition different from the environment. In primi- tive animals the dissolved constituents of the body fluids consist pre- dominantly of mineral substances, but in higher forms the blood and body fluids are found to contain large amounts of organic substances, sometimes in colloidal form, including respiratory pigments (Tables 2.12, 2.13, 4.9, 4.10). Experimentally it has been demonstrated that isolated tissues of many simpler marine invertebrates are capable of functioning in sea water, e.g. polychaetes, holothurians and sipunculoids. In these animals the body fluids differ only slightly from sea water in mineral composition (Table 2.9). Organs from more complex animals, such as decapod crustaceans and vertebrates, require balanced media approximating their natural blood or haemolymph in composition in order to maintain optimal activity (Appen- dix, p. 675). This is a reflexion of the increasing degree of homoeostasis attained by these forms. As animals have become more complex they have achieved a greater degree of control over the internal milieu, the composi- tion of which is maintained within rather narrow limits, favourable to efficient functioning of the body cells. The Protozoa are of such small size that functional exchange of material between the protoplasm and environment is carried out efficiently by diffusion across the plasma membrane. The simplest metazoans—sponges and coelenterates—lack body cavities, and the spaces between the tissue layers are filled with a solid gelatinous matrix. Internal and external mem- branes are exposed to sea water and even when the jelly layer becomes very thick, as in the mesogloea of large pelagic medusae, it is noteworthy that the cellular content is very sparse. Moreover, metabolic activity of these lowly animals proceeds at a low rate, which can be satisfied by processes of diffusion. 84 BODY FLUIDS AND CIRCULATION 85 In higher groups tissue and body fluids of various kinds are invariably present. These fluids are concerned with transporting oxygen, foodstuffs, metabolic wastes, hormones, phagocytes, erythrocytes and other haemal cells. In soft-bodied invertebrates the body fluids are also concerned with maintenance of body turgor. Diffusion is such a slow process that in all but the smallest animals some mechanism must exist for circulating body fluids so that exchanges of substances may be facilitated. We shall now consider the various kinds of body fluids, and the circulatory systems which control their movements. Owing to the great diversity existing in the organization of body spaces and circulatory systems in different animals, any classification must be arbitrary and repetitive, but the following will indicate the various categories which can be recognized. CLASSIFICATION OF BODY FLUIDS The External Milieu. This is utilized in some groups of marine animals as a fluid medium subserving transport. Sponges are literally a network of channels through which sea water is propelled by the unco-ordinated activity of numerous flagella. The sea water carries oxygen and food to the organism and waste products away. The hydraulics of this system are described in Chapter 5 dealing with feeding and nutrition. In coelenterates the gastrovascular cavity or coelenteron, with its ramifying passages, is filled with sea water which forms an internal vehicle for transporting dis- solved substances and foodstuffs to various parts of the body. In Cyanea, for example, currents pass peripherally along the roof and return along the floor of the gastrovascular channels (63). The water vascular system in many echinoderms communicates with the exterior at the madreporite, and acts as a hydraulic mechanism. In the cavities and channels of these various animals the fluids are moved to and fro by ciliary activity or muscular contractions. Tissue Fluids. These are important in all triploblastic animals as the intimate mi/ieu bathing the cells of the body and filling the spaces between organs. It is through this medium that the cells receive or unload gaseous, mineral and organic substances, and it is vis-a-vis the interstitial fluids that ionic and other exchanges occur which are essential to the functioning of nervous and contractile tissues (124). In simple metazoans, such as platyhelminths which lack a body cavity, the fluids are confined to inter- stitial spaces. The tissue spaces of those animals with open circulatory systems, such as bivalve molluscs, are continuous with the haemocoele and contain haemolymph. But in various higher forms with closed circula- tory systems, especially the vertebrates, a distinct interstitial fluid perme- ates the intercellular matrix, and communicates with the blood stream by diffusion through the walls of the finest blood vessels, and by the lymphatic circulation. There are few estimates of the volume of intercellular fluid in lower animals. On the basis of thiocyanate determinations (injection of a known amount of thiocyanate and subsequent estimation of its concentration in 86 THE BIOLOGY OF MARINE ANIMALS body fluids), the total extracellular space in mammals is found to lie around 20-30% of body weight. About one-third of this volume is occupied by blood; the remainder is lymph and interstitial fluids. In the mussel Mytilus edulis, intercellular fluid or haemolymph is estimated to be about 12%; the extracellular space in crabs Carcinus and Eriocheir forms 33-37% of body weight. Haemolymph volume in these animals is equivalent to total extracellular space in vertebrates. In cephalopods, which also possess a closed vascular system, total extracellular fluid as measured by sucrose injection amounts to about 33% (78, 114). In the open circulatory system of the mussel, the haemolymph follows alterations of the external medium fairly closely. Mytilus californianus maintains a constant ratio of about 1-6—-1-0 between Cl- concentrations of external medium and intercellular fluid over a restricted environmental range (Cl- concentrations of 0-9-2°8 %). Within these limits, Cl- exchange occurs only between haemolymph and external medium. Beyond this range, however, Cl- exchange takes place between the cellular contents and the haemolymph, and the cells are unable to maintain a steady internal concentration in the face of marked changes in external chlorinity. Where a well-developed closed circulatory system is present, as in vertebrates, the tissue fluids form a reservoir of water and salts which can be called upon to buffer alterations in the composition of circulating fluids (40, 78). Fluids in Primary Body Cavities. Many invertebrates retain the primary body cavity or blastocoele in the adult as spaces of various extent filled with intercellular fluid. In the preceding section reference has been made to the relationship between interstitial and circulatory fluids, and we have noted that in various primitive groups all the body fluids of the organism can be regarded as a continuous medium occupying primary body spaces, sinuses and intercellular meshes. In platyhelminths the parenchyma con- tains large intercellular spaces filled with tissue fluid. In nemertines the diffuse body cavity is reduced to a system of one or more definite vascular channels. Longitudinal vessels are connected together by contractile trans- verse vessels and these, aided by body movements, circulate the contained fluids. This is the beginning of a true circulatory system concerned, among other things, with transport of oxygen to more deeply lying tissues (27). In those animals with open circulatory systems (Mollusca, Crustacea, Xiphosura) the primitive body cavity is retained in large haemocoelic spaces through which the blood or haemolymph slowly passes on its way back from the tissues to the heart. Coelomic Fluids. Extensive coelomic spaces filled with fluid are found in polychaetes, sipunculoids, echiuroids, ectoproct polyzoans, phoronids, chaetognaths and echinoderms. In leeches the coelomic spaces are exten- sively invaded by mesenchymatous tissue and are reduced to a system of longitudinal channels. In molluscs the coelomic spaces are restricted to small cavities in the kidneys, gonads and pericardium. A similar condition obtains in crustaceans. Coelomic cavities are, of course, well developed in BODY FLUIDS AND CIRCULATION 87 the lower chordates, and coelomic fluid occurs in the spaces between the body wall and the internal organs. Bleod in Closed Vascular Systems. Closed vascular systems are found in the following marine groups: nemertines, polychaetes, leeches, sipuncu- loids, phoronids, cephalopods, holothurians and chordates. Usually such systems contain a relatively small volume of blood which is pumped con- tinuously around the organism. The hearts or other devices which provide the motive force for propelling the body fluids display great variation in structure and efficiency, and some are subject to various degrees of nervous control: “7 Lymphatic systems resembling those occurring in vertebrates are not encountered among invertebrates. Gnathobdellid leeches, however, are peculiar in having blood vessels communicating with coelomic sinuses and with intracellular capillaries of botryoidal tissue in a manner analogous with the lymphatic vessels of vertebrates. In the following pages attention will be restricted to fluid systems of marine invertebrates and the lower chordates. For a comparative account of the physiology of circulatory systems see Scheer (116), Prosser (108) and @arter (21); VOLUME OF CIRCULATING FLUIDS In higher animals with closed circulatory systems the blood forms only a small and relatively fixed fraction of the body weight. Although much attention has been devoted to blood volumes of mammals, especially man, very little information is available for invertebrates, and it would be valu- able to have more comparative data and information dealing with body fluids in lower forms. Observations dealing with blood volumes of various animals have been summarized by Reichert and Brown (113) and Prosser (108), and some selected values are given in Table 3.1. These have been obtained by several methods. One consists in determining the haemoglobin content of a sample of blood, and then bleeding the animal and washing out the vessels to obtain the total haemoglobin in the body; with these data the blood volume is computed. Others involve addition of a known quantity of some sub- stance which remains confined to the blood stream, and determining the degree of dilution which takes place after it becomes uniformly distributed in the circulation. For this purpose dyes are usually employed (Evans’ blue, vital red). Total extracellular fluid has been estimated by using substances such as thiocyanate and sucrose, which are not taken up by tissue cells. In fishes, with closed circulatory systems, blood volume is small (1-5-4 % in teleosts; 4-18% in selachians, with majority of values below 7%). These values contrast with the large blood volumes found in invertebrates with open circulatory systems (17—37 % in crustaceans; 8-13 % in lamelli- branchs). Utilization of a small volume of circulatory fluid is more efficient than a large one, when it is pumped around the body faster and made use of more often. In these terms the blood circulation of a fish is more TABLE 3.1.—BLOooD VOLUMES OF LOWER ANIMALS Blood volume Animal ml1/100 g body Method Source we ight (or other) Teleostei Tautoga onitis 57 Bleeding and Hb estimation 1 Sebastodes sp. 2:84 Vital red 2, 14 Ophiodon elongatus 3-1 Evans’ blue 2 1-6-47 ditto 14 1-97 Vital red 14 Cottidae, sp. pF | ditto 2 1-9-2-7+ ditto 14 Solea solea 1-76, 2°355t Bleeding and estimation of 9 erythrocyte volume Belone belone 4:08, 4-44 ditto 9 © less roe 4-80, 5:91 ditto 9 Selachii Squalus acanthias 3-717 Bleeding and Hb estimation 1 6:667-0-26 Evans’ blue 10 S. acanthias male a2 Vital red 2 3:9-5-9F ditto 14 S. acanthias female 4-4 Vital red 2 3:9-5-1t ditto 14 1-7-17-9F Evans’ blue 14 female with young 11-2 Vital red 2 (embryos small) 4-5—5-S5t ditto 14 (embryos large) 10-7-137 ditto 14 4-77 Vital red and Evans’ blue 14 Raja rhina Ses) Evans’ blue 2 1-5-2t Vital red 14 3-6-77 Evans’ blue 14 R. binoculata 4-4 ditto 2 2:2-7:3F ditto 14 0-8—2-7+ Vital red 14 R. binoculata and 4-:3-4:5 Bleeding and Hb estimation [3 R. erinacea R. clavata 2:18-2-49+ Bleeding and estimation of 9 erythrocyte volume Chimaeroidea Hydrolagus colliei 6-77 Vital red 14 JTS RRS re i Evans’ blue 14 Cyclostomata Petromyzon marinus 4:97 Bleeding and Hb estimation 3 Crustacea Carcinus maenas Sy/ Thiocyanate 4 Eriocheir sinensis 33 Thiocyanate 2 Homarus americanus 16-6 Evans’ blue 15 Mollusca Mytilus edulis 11-13 Thiosulphate 5 Anodonta and 8 Evans’ blue 6 other FW mussels 8-1 Thiocyanate 6 Echiuroidea Urechis caupo 37-387 Bleeding 11 Priapuloidea Priapulus caudatus : Halicryptus spinulosus b 30-50* Bleeding a Polychaeta Arenicola marina 13-44 Bleeding and Hb estimation 7 31-50 ditto 8 + g blood per 100 g body weight. * Volume per cent. Sources: 1, Derrickson and Amberson (33); 2, Martin (84) and quoted by Prosser (108); 3, Welcker, quoted by Reichert and Brown (113); 4, Nagel (93); 5, Krogh (78); 6, Prosser and Weinstein (110); 7, Barcroft and Barcroft (9); 8, Borden (15); 9, Korjuiev and Nikolskaya (74); 10, Burger and Bradley (19); 11, Hall (53); 12, Fange and Akesson (39); 13, Hyde (62); 14, Martin (85); 15, Burger and Smythe (20). BODY FLUIDS AND CIRCULATION 89 efficient than that of the crab (open system). But the haemolymph of the latter is really equivalent to blood plus interstitial fluids of animals with closed circulatory systems, and a comparison on a basis of this kind is difficult. The values for Arenicola (minimal estimates) show that large blood volumes are not confined to animals with open circulatory systems. In addition to volumetric considerations, overall efficiency will also depend on hydrodynamic factors, especially the configuration of the peripheral bed and the mechanics of the pumping system. The amount of coelomic fluid in some of the soft-bodied lower invertebrates appears to be very high. Thus, in echiuroids and priapuloids it forms 30-50% of body weight or volume. BLOOD VESSELS In those animals with open circulatory systems the blood or haemolymph comes into direct contact with the tissue cells. When a closed vascular system is present the blood constituents must diffuse across the walls of sinuses, small capillaries, etc., to reach the tissue fluids and cells. In the tissues the blood unloads gases, nutrients, etc., and picks up metabolites originating in the tissue cells. The composition of the blood is restored by exchanges which take place in the circulatory bed of skin, gills, excretory organs, etc. In vertebrates the walls of the larger vessels are composed of layers of muscular and connective tissue (elastic and collagen fibres), with an in- ternal lining of endothelium. Changes in diameter of these vessels serve to accommodate alterations in blood volume, either locally or widespread, and changes in vascular tonus participate in regulation of blood pressure. The capillary wall is a semipermeable membrane, freely permeable to water, salts and small organic molecules. Temporary gaps between the endothelial cells allow the escape of colloidal particles (protein, particulate matter) and occasional blood cells. The residual osmotic pressure, exerted by plasma protein, counterbalances in large part capillary hydrostatic pressure. Colloidal osmotic pressures in the plasma of marine teleosts lie between 4-28 cm H,O. Values are much lower in elasmobranchs, ranging from 17-46 mm H,O (70, 128). Pressure-drop along the capillary results in some degree of circulation in the tissue spaces, fluid escaping from the arterioles and capillaries proximally returning again at the venous end. Among invertebrates muscular blood vessels occur in many groups, but little is known of the role they play in regulation of circulatory conditions. The histology of vessels has been most intensively investigated in the Annelida. Generally these consist of endothelial, skeletal and peritoneal layers. The endothelium is discontinuous and is made up of flat branched cells. The skeletal coat consists of collagenous material. Muscle fibres are frequently present in the external peritoneal layer (55, 56, 96). In holothurians the larger haemal vessels (intestinal vessels) have rela- tively thick walls composed of internal endothelium, loose connective tissue and muscle, and an external peritoneum. Vessels of the rete mirabile 90 THE BIOLOGY OF MARINE ANIMALS consist of two epithelial layers; capillaries and lacunae contain only a single layer of endothelium. Nemertines, the most primitive animals to possess a closed vascular system, have two kinds of vessels. The larger contractile vessels are provided with a wall of four layers, namely lining endothelium (flat or bulging cells), gelatinous connective tissue, circular and longitudinal muscles, and an outer non-nucleated covering. Smaller non-contractile vessels lack the muscle layer and consist of inner and outer epithelia, with intervening membrane (27, 64, 109). Among other invertebrates with closed circulatory systems the cephalo- pods are worthy of special attention. There is a rich peripheral capillary network; arteries and veins are provided with striped muscle, which is hypertrophied in arterial and branchial hearts (121). The vascular systems of lower chordates and invertebrates invite further investigations from many aspects. For each of the major groups in which the systems are closed it is desirable to obtain a sound histological picture of the structure of the vascular walls, and to relate this to permeability, inherent contractility, maintenance of tonus and nervous regulation. Partition of fluids and alterations of volume with changing functional states are obvious physiological variables. In the case of open systems there is much to be learnt about volume changes in different sinuses, participation in hydraulic mechanisms and pressure changes under differ- ent functional conditions. In all cases these factors have to be related to the output of the heart and the contribution of contractile vessels as main or subsidiary pumping agencies. Pressure in Circulatory Systems Blood is propelled through vascular channels by the pumping action of special hearts and contractile vessels, aided in many animals by somatic movements. The pressure head which is built up by the pumping system is gradually dissipated in the vascular channels through frictional losses. The level of blood pressure is determined by the volume of circulating fluid, the force exerted by the heart or other contractile structure and by the peripheral resistance. Closed Circulatory Systems Fishes. Haemodynamics and regulation of blood pressure have received relatively little attention in lower animals, and data are insufficient for a satisfactory comparative treatment. Pressure regulation in fishes provides an interesting contrast with regulation in mammals. Some selected values for blood pressures in arterial vessels of fishes are given in Table 3.2 (26, 34, 58, 62, 81, 82, 108). In fishes the blood which leaves the heart passes through the branchial vessels before reaching the dorsal aorta. Interposition of the branchial capillaries produces a pressure drop before the blood actually reaches the systemic circulation, and pressure continues to drop in peripheral vessels. Pressures are lower and circulation more sluggish in fishes than in homoeo- BODY FLUIDS AND CIRCULATION 91 TABLE 3:2 BLOOD PRESSURES OF FISH Blood pressure Species Vessel mm He Scyliorhinus canicula Ventral aorta 29:4-36°8 36°8/33-1 Branchial arteries 48/38-5 45/38 Skin artery 7-10 Intestinal artery 8-1-8-9 S. stellaris Ventral aorta 12-25 Squalus acanthias Ventral aorta 28-2/14-9 Dorsal aorta 11-28 mean 15-4 S. acanthias Ventral aorta 39/28 Dorsal aorta 30/23 Carcharias taurus Ventral aorta 22-39 mean 32 Dorsal aorta 13-30 mean 23-3 Raja “punctata”’ Ventral aorta 16°1/7-4 Intestinal artery 5-1 R. ocellata Ventral aorta 16-9/12-5 17°7/15-5 Raja spp. Ventral aorta 13, 20 Torpedo torpedo Ventral aorta 16°1/7-4 Intestinal artery =| Anguilla anguilla Ventral aorta 30-40 mean 35-5 Pneumogastric artery 10-22 mean 16 Branchial arteries 65-70 Oncorhynchus tschawytscha Ventral aorta 45-120 mean 74-6 Dorsal aorta 44-58 mean 53-3 Dorosoma cepedianum Bulbus arteriosus 47-7/38-2 Lophius piscatorius Ventral aorta | 36°8 therms of comparable size. Clark observes that the heart ratio (heart weight over body weight) of a fish is much lower than that of mammals. Since the oxygen requirements of fish are only a fraction of those of warm- blooded animals, a much smaller circulatory volume per minute is ade- quate for fish metabolism. Cardiac activity in fish, as in other vertebrates, is subject to autonomic control, resulting in changes in blood pressure. With increase of heart rate there is a rise of blood pressure and a decline of pulse pressure, as the arteries become increasingly distended. The circulation rate depends on the volume output of the heart per beat (stroke volume), and the frequency of heart beat, and as the latter increases there is an increase in the velocity of blood flow. Measurements made on the eel (Anguilla anguilla) show that the blood leaving the heart takes from 12 to 60 sec to reach the various posterior veins. 92 THE BIOLOGY OF MARINE ANIMALS Certain reflexes, initiated peripherally, affect cardiac activity and blood pressure. If respiratory flow over the gills of a shark or skate is stopped, cardiac inhibition ensues and blood pressure falls; mechanical handling and a variety of other external stimuli produce the same effect. Electrical stimulation of the central ends of cut vagus, hypobranchial and lateral line nerves results in respiratory and cardiac inhibition. When branchial arteries are perfused, raising the arterial pressure evokes sensory dis- charge in the branchial nerves (glossopharyngeal, vagus), and a similar rhythmic sensory discharge takes place in the normal animal at each heart beat when pressure in the branchial arteries rises. Raising the pres- sure in the first pair of afferent branchial arteries also produces cardio- inhibition. It seems likely that there is a normal branchial depressor reflex in fish: this reflex is stimulated by presso-receptors in the afferent branchial arteries, and produces cardio-inhibition and fall of blood pressure. Its function appears to be protection of the delicate branchial capillaries lying near the heart (92). Afferent visceral impulses from many regions are fed into autonomic medullary centres, from which efferent impulses proceed peripherally in autonomic pathways. Efferent cardioregulatory routes available are cardiac branches of the vagus (inhibitory fibres) and pre-ganglionic sympathetic fibres to the suprarenal organs. There is no sympathetic supply to the heart in fish. Vagus activity stops the heart and results in fall of blood pressure. Experimentally, adrenaline and noradrenaline usually produce acceleration of the heart and increased strength of beat, as well as causing prolonged rise of blood pressure. It is possible that adrenaline and noradrenaline, secreted by chromaffine tissue of the suprarenal bodies, have pressor effects in the normal animal. Efferent pathways to blood vessels in fish require further study. Peri- pheral somatic vessels in the trunk of elasmobranchs are innervated by sympathetic fibres only; visceral (gut) vessels are within sympathetic and parasympathetic fields: branchial and cephalic vessels receive parasympa- thetic fibres. The following pharmacological observations are pertinent to the morphological pattern. Injection of acetylcholine, although causing cardiac inhibition, raises blood pressure owing to a vasoconstrictor action on peripheral arteries. Adrenaline dilates branchial and constricts systemic vessels, resulting in overall rise of blood pressure (Raja). This suggests that sympathetic control of somatic vessels is mediated by adrenergic fibres; parasympathetic fibres to the branchial vessels may be cholinergic. In teleosts the autonomic supply of heart and vessels is more complex: noteworthy features are a demonstrated sympathetic innervation of branchial and cephalic vessels, and sympathetic plexuses on splanchnic vessels (Fig. 3-1). Vagus activity causes inhibition of the heart. Stimulation of sympathetic trunks in the eel (Anguilla) causes vaso-constriction of peripheral vessels. Adrenaline has a vaso-dilator effect on branchial vessels and a constrictor effect on systemic vessels as in selachians (7, 19, 94556930708; 6182. 95-97). ° BODY FLUIDS AND CIRCULATION 93 Invertebrates. In cephalopods, which have closed circulatory systems, blood pressures are high, greater than those of many cold-blooded verte- brates. In Octopus pressures in the cephalic artery range from 48-60 mm Hg, and in one large animal reached 88 mm Hg. Differences between systolic and diastolic pressures usually lie around 10 mm Hg, but may attain 25 mm. In the gill veins blood pressures fall to 5-4-6-1 mm Hg; Fic. 3.1. DIAGRAM OF SYMPATHETIC PATHWAYS IN A TELEOST FISH a, artery; c, chromatophore; ma, mesenteric artery; mrc, mixed ramus communicans; pr.gn, pre-ganglionic neurone; 0, oviduct; sg, sympathetic ganglion; sb, suprarenal body; da, dorsal aorta; e, gut; pgn, post-ganglionic neurone. pulse pressure here is small, around 0-8 mm Hg. Cephalopods are the largest and most active of invertebrates, with correspondingly high metabolic levels (121). In contrast, pressures in the vessels of sluggish polychaetes are very low. In Neanthes pressure in the dorsal artery is 1-1—2-2 mm Hg when the animal is at rest, and increases to 17-6 mm Hg during activity (Zucker- kandl in Prosser) (108). 94 THE BIOLOGY OF MARINE ANIMALS Open Circulatory Systems Animals with open circulations have low and highly variable blood pressures. In small crustaceans—e.g. cirripedes, many copepods and ostracods—a heart is often wanting and blood is circulated solely by movements of the body wall and alimentary canal. Increase in size and activity is attended by development of a heart pump, but even in large crustaceans the pressure head developed by the heart is low, and circula- tion of the blood owes much to movements of the appendages and the body wall. Recorded blood pressure in the sternal sinus of the shore crab Carcinus maenas, a relatively small but active animal, is 13 cm H,O. In large specimens of Maia, a relatively sluggish crab, systolic pressure in the heart is 55 mm H,O, and the difference between systolic and diastolic pressures is 8 mm H,O. Pressure drop from the arteries to the thoracic sinus is about 25 mm H,O (absolute pressures in the latter 24-34 mm H,0). In the lobster (Homarus), a large active animal, the intraventricular blood pressure is 17-7-1-36 cm H,O, and the aortic pressure immediately posterior to the ventricle is 17-7 cm during systole and 12 cm at diastole. Pressures in the haemocoele range from 27-82 mm H,O; pericardial pres- sures are the lowest of the circulatory system, 0-16 mm H,O. Vascular pressures vary with activity of the heart, with body tonus and body move- ments. In decapod crustaceans blood pressures in arteries and the haemo- coele rise greatly during activity (20). An open circulatory system is also characteristic of gastropods and lamellibranchs. Arterial pressures in freshwater mussels (Anodonta) are lower than in decapod crustaceans, about 35 mm H,O at systole, falling to 10 mm H,O during diastole. Muscular contraction, causing ejection of water from the exhalant siphon, doubles arterial pressure. Pressures in the haemocoele of the sea hare Aplysia, a gastropod, are much above those of Anodonta, and approximate those of decapod crustaceans. Pressures in the body cavity of a resting animal were 2:5—4 cm H,O, and rose to 6 cm during activity. The heart is a weak and rather inefficient pump in crustaceans and molluscs with open circulatory systems. Some of the work involved in moving the blood about the body is performed by the somatic musculature through changes in body tonus and activity; during periods of higher activity, when oxygen requirements are greater, blood pressure and circu- latory movements are automatically increased (26, 102, 108, 121). HYDROSTATIC PRESSURES IN BODY CAVITIES Fluids in the body cavities of many soft-bodied animals, as we have already noted, have several functional roles including circulation of essential substances, removal of wastes, provision of a constant internal milieu and participation in hydraulic mechanisms essential for movement and locomotion. In lamellibranchs in which the foot is employed for locomotion and BODY FLUIDS AND CIRCULATION 95 burrowing, the haemolymph supplies the necessary turgor for movements to be executed. When the foot is extended the pedal muscles relax and blood flows through the pedal artery into the foot. Retraction of the foot is brought about by contraction of the pedal muscles, and blood is shifted largely into spaces in the mantle. Burrowing movements are carried out with great rapidity by the razor-shell Ensis. In downward burrowing the foot is extended into the sand, blood flows into it and the tip swells out into a bulbous disc (Fig. 3.2). This acts as an anchor while the pedal muscles contract and draw the animal down. This is followed by return of the blood to the _ GaeE hE body, while the foot extends once more, re- peating the manoeuvre. In upward progression the pedal muscles remain relaxed while the tip of the foot becomes distended with blood. ‘ With the tip anchored, blood is forced into ee = the upper region of the foot, which elongates, thus pushing the animal upwards. The con- ( ( secutive muscular contractions involved in burrowing can still take place in an ex- Sanguinated animal, but in the absence of turgor these are weak and ineffective (42). Mechanisms used in siphonal extrusion vary in different species of lamellibranchs. The siphons of Mya are extended by water forced into their lumina from the mantle cavity. The mantle and siphonal cavities form a fluid-tight system, and force is applied by contractions of the adductor muscles. Elongation proceeds stepwise; water is taken into the system be- & tween successive elongations, so that the in- gastric plexus -> lateral gastric vessel —> lateral heart -- ventral vessel —> sub-intestinal and affer- ent vessels to body wall, nephridia and gills > intestinal plexus and vessels —> gastric and dorsal vessels. Contractile vessels are the dorsal vessel, the lateral, oesophageal and some nephridial vessels. The two lateral hearts pump blood into the ventral vessel. Contraction begins in the auricle (a thin-walled expansion of the gastric vessel) and proceeds ventrally in the ventricle towards the ventral vessel. There is no relation between contractions of dorsal vessel and hearts, nor any correspondence in the beating of the two hearts. Usually the heart beats faster and more irregu- larly than the dorsal vessel; the auricular region sometimes contracts several times for each ventricular beat (111). The blood vessels of many polychaetes contract rhythmically. The vascular system of Nereis contains a contractile dorsal and non-contractile ventral vessel, from each of which segmentally arranged branches extend to capillary plexuses in the body wall, gut and parapodia. Contraction in the dorsal vessel takes the form of a peristaltic wave which begins in the posterior region and travels anteriorly at about 7 mm/sec (17°C). Many of 100 THE BIOLOGY OF MARINE ANIMALS the smaller vessels also are contractile, including blindly ending capillaries serving internal organs. In sabellids nearly all vessels, including trunks and blind capillaries, are contractile; the latter fill up with successive lots of blood passing by in the trunk vessels. In these animals peristalsis in the main longitudinal vessels occasionally reverses direction, indicating absence of fixed functional polarization. The blood vessels of worms con- tract at rather slow frequencies, around 5-20 per min (Table 3.3) (38, 41, 66, 96). In Amphioxus the heart is a simple tube comparable to sinus venosus plus conus arteriosus, and many other vessels are also contractile. A peristaltic contractile wave begins in the hepatic vein and proceeds to sinus venosus (heart), endostyle artery (ventral aorta), bulbils and afferent branchial arteries. Velocity of propagation is slow, around 0-3 mm/sec. Contractile activity is rather irregular and after several heart beats there may be irregular pauses. In general, the glomus (a buccal plexus) and subintestinal vein beat with twice the frequency of the heart (Table 3.3). Antiperistaltic waves have also been observed (120, 137, 138). The heart of brachiopods is a contractile muscular vessel lying above the gut. Rhythmically pulsatile vessels occur in the mantle of the oyster (Ostrea), where they drive blood into the circumpallial artery. Certain of the haemal (lacunar) vessels on the intestine of holothurians are spon- taneously contractile. The beat is rather irregular and slow (2-10 per min) in different species (Table 3.3) (61, 109). Chambered Hearts Molluscs. These occur in vertebrates and molluscs. In the latter group they range in complexity from the rudimentary heart of scaphopods, connected by ostia with venous sinuses, to the highly developed organs of cephalopods. Typically the heart consists of two auricles (one in certain gastropods, four in tetrabranch cephalopods), opening into a ventricle. Auricles are receiving chambers with slight musculature; the ventricle is more muscular and strongly contractile. Cardiac muscles are sometimes striated (Murex, Octopus, etc.). Guarding the aperture between auricles and ventricles is an A.V. or semilunar valve. In lamellibranchs valves are also present at the origin of the aortae, and prevent blood from being driven backwards into the heart when the foot or siphons suddenly con- tract. Blood is carried away from the heart by anterior and posterior aortae, and is returned by veins, differently organized in the various groups. In lamellibranchs and gastropods with open circulations the blood passes into lacunae and venous sinuses. The branchial circulation takes its origin largely from the renal sinus. Not all lamellibranchs have a com- plete branchial circulation; in Mytilus, for example, some of the venous blood returns to the heart without passing through the gills. In cephalo- pods the venous return from the peripheral capillary network involves vena cava, abdominal and pallial veins, which lead into a pair of branchial hearts. These are muscular dilatations serving to pump blood through the BODY FLUIDS AND CIRCULATION 10] ctenidia, from which efferent vessels return blood to the auricles. Many of the veins are also contractile, showing peristaltic movements, and venous valves occur which regulate the direction of flow. Rate of heart beat is generally slow in lamellibranchs and gastropods, around 10-20 beats per min, and higher in cephalopods, up to 80 beats per TABLE 3.3 REPRESENTATIVE FIGURES FOR HEART FREQUENCIES . Temperature Frequency Animal Vessel (°C) (beats per min) Neanthes virens Dorsal vessel 20 20 Perinereis cultrifera Dorsal vessel 14:5 7-9 Arenicola marina Lateral heart — 13-22 Branchial vessel 19 Bie, ; Lateral vessels 19 ka Sabella pavonina 1 Ventral vessel 19 10-9 | Peri-intestinal sinus 19 24 Talorchestia longicornis Heart 15 175-200 Homarus americanus Heart 16-20 50-136 mode 100 Palaemon serratus Heart 17 181 Spirontocaris securifrons Heart 18 139-199 Cancer irroratus Heart 16-20 150 Limulus polyphemus Heart — 12-28 Ostrea edulis Heart — 25-30 Pecten sp. Heart 22 18-22 Mya arenaria Heart 20 | be Cryptochiton sp. Heart — 5-7 Acmaea limatula Heart 14 35-48 Phyllirrhoe sp. Heart 20 46:5 Tiedemannia neapolitana Heart 14-15 25 Pterotrachea coronata Heart 14-15 50 Loligo vulgaris Systemic heart — 70-80 Octopus vulgaris Systemic heart — 33-40 Stichopus californicus Intestinal vessel 18-22 4-5:5 Caudina chilensis Intestinal vessel — 1-2-2 Molgula manhattensis Heart — 43 Phallusia sp. Heart 12 10 Salpa fusiformis Heart 11 57-60 Endostyle artery 20 0-88 Amphioxus Glomus 20 1-70 Vena sub-intestinalis 20 1-64 Scyliorhinus stellaris Heart 16 44 Carcharias sp. Heart 16 18, 30 Anguilla anguilla Heart 13-16 46-68 Gadus callarias Heart 16 26-40 Pleuronectes platessa Heart — 54-76 Dorosoma cepedianum Heart 19 20-50 | min (Table 3.3). Cephalopods are active animals with high respiratory rates, and have need of an efficient circulatory system. Blood pressures are high, due in part to rapid heart beat, and in part to the closed circulation and strength of the cardiac pump; the circulatory volume is certainly small (unknown, but only some fraction of total extracellular fluid). In 102 THE BIOLOGY OF MARINE ANIMALS consequence of these various factors the velocity of circulation is rapid, but the actual circuit-time has never been measured. Turning now to the properties of molluscan heart muscle, we find cer- tain differences from vertebrate cardiac muscle. The molluscan heart is excitable at all stages of the cardiac cycle, but threshold is high during systole, and a strong stimulus is required to elicit a contraction. A condi- tion approaching absolute refractoriness exists during early systole, after which excitability gradually returns (Fig. 3.6). It follows from these charac- teristics that the molluscan heart can be tetanized by repetitive stimulation. ITI SSE SOS. . a ad a 6 Fic. 3.6. EFFECT OF ELECTRICAL STIMULI ON VENTRICULAR CONTRACTION OF Aplysia Stimulus ineffective during systole (a), but effective during diastole (5), and diastolic pause (d). (From Carlson, 1906.) The frequency and force of contractions are dependent on the internal pressure (cf. Starling’s law of the vertebrate heart), and some molluscan hearts fail to beat unless sufficiently distended. The heart of Octopus ceases to beat when the internal pressure falls below 2 cm H,O, and the frequency increases with rise in pressure (26, 77, 121). Fishes. The fish heart is derived during ontogeny from a tubular struc- ture and consists of three successive chambers, namely sinus venosus, auricle and ventricle. These are guarded by valves: S.V. (sinus venosus) valves where the ducts of Cuvier enter the sinus, S.A. (sino-auricular) valves between sinus and auricle, and A.V. (auriculo-ventricular) valves guarding the auriculo-ventricular junction. The ventricle leads into a muscular and contractile truncus arteriosus in elasmobranchs, and a fibrous bulbus arteriosus in teleosts. These latter structures are provided with semilunar valves. The heart discharges into a ventral aorta from which afferent branchial arteries carry the blood to the gills before it enters the systemic circulation. The sequence of cardiac contraction is from sinus to ventricle. The sinus and auricle have thin muscular walls, especially the former, and are essen- tially receiving chambers; the ventricle has strong muscular walls and is the effective pump. The valves are arranged so as to prevent reflux of blood, and close when the pressure on the outgoing side exceeds that on the incoming side. Conditions affecting venous return in fishes are poorly understood; venous pressures measured near the heart lie around zero, and it is likely that swimming movements and contractions of the body wall are important in moving blood back to the heart. The sinus fills during diastole, and when contraction ensues the S.A. valves open and the auricle fills. As the auricle fills its pressure rises and during systole the S.A. valves close, the A.V. valves are forced open and blood fills the ventricle. BODY FLUIDS AND CIRCULATION 103 Systolic contraction of the ventricle in turn is accompanied by closure of the A.V. valves and discharge of blood into the ventral aorta, the semilunar valves in turn closing when the ventricle passes into diastole and intra- ventricular pressure falls below that in the ventral aorta. Vertebrate heart muscle is both striped and syncytial, so that the whole organ forms a single excitable unit without internal cellular boundaries. Consequently, when an electrical stimulus is applied to the intact heart, or to a piece of heart tissue, the excitatory wave which is evoked spreads throughout the tissue. The contraction of the vertebrate heart, either dur- ing rhythmic activity or following electrical stimulation, is all or nothing in character, i.e. it is maximal for the condition of the heart at that time and is not affected by stimulus-strength. When tested by electrical stimula- tion during various phases of a normal contractile cycle the heart proves to be inexcitable during most of systole (absolute refractory period), after SIDA AASSL PLA A JUL 17sec Fic. 3.7. REFRACTORY PERIOD AND RECOVERY OF EXCITABILITY IN THE HEART OF Torpedo ocellata Effect of a strong induction shock on the ventricle at different intervals after the occurrence of an auricular contraction. Exact moment of stimulation is shown by the black spots on the tracings. (From Mines, 1913.) which excitability gradually returns (Fig. 3.7). At the end of systole and throughout most of diastole an interpolated stimulus evokes a contraction of submaximal height (relative refractory period); recovery becomes com- plete by the end of diastole. Any extra contraction interposed between two normal contractions is followed by a compensatory pause longer than a normal diastolic pause. The long absolute and relative refractory periods preclude tetanization of heart muscle. When quiescent strips of vertebrate heart muscle are stimulated by repeated shocks at suitable intervals (longer than the refractory period), they respond by single contractions of increas- ing magnitude, an effect known as staircase. According to Starling’s law of the heart the strength of contraction is dependent on its degree of distension. This is an effect common to all muscle by which the force of contraction increases with stretching, up to some maximal value. By this means the heart adapts itself to a given load. The mechanics of cardiac output in several freshwater teleosts have been investigated by Hart (58). The blood pressure rises with increase in rate of 104 THE BIOLOGY OF MARINE ANIMALS heart beat up to frequencies of 45-50 beats per min, beyond which there would appear to be little opportunity for further increase of pressure. As the heart rate increases, pulse pressure declines, while systolic pressure rises. The output per heart beat is known as the stroke volume. The stroke output of teleosts varies with the weight of the fish. For the eel Anguilla bostoniensis it ranges from 0-12 g for a 200-g fish, to 0-30 g for a 600-g fish. The cardiac output (stroke volume) of the dogfish Squalus acanthias is 0-4-1-5 c.c. for a normal animal weighing 1,600 g, with a maximal value of 3 c.c. At a cardiac frequency of 36 per min this gives an average minute volume of about 20 c.c per kg fish. Heart frequencies of some representa- tive species of selachians and teleosts are given in Table 3.3. These show about the same range as in cephalopods, invertebrates of comparable size and blood pressures (19, 119). Tubular Hearts Hearts of arthropods, when present, usually take the form of contractile tubes or are derived therefrom. In Crustacea the heart lies in a large peri- cardial sinus, with which it communicates by several ostia guarded by valves. Isopods and amphipods have long tubular hearts (Fig. 3.8). In decapods they are polygonal-shaped chambers, lying freely in the peri- cardium and suspended at several corners by strands (Fig. 3.17). Blood is discharged from the heart through several arteries (five in front and two behind in Homarus), which are also supplied with valves. The arteries carry blood to all parts of the body; after bathing the tissues the venous blood passes by a lacunar system into a large ventral sinus, whence it is carried by afferent branchial vessels to the gills. In the latter there is a complicated through circulation by which venous blood is brought close to the surface of the gill filaments in distinct afferent and efferent capillaries. From the gills the blood passes by efferent branchio-cardiac veins to the pericardium, and thence to the heart. The heart of Limulus is a long segmental tube perforated by eight pairs of ostia, which mark it off into eight segments (Fig. 3.9). In the anterior half are five pairs of arteries plus one antero- median artery. A receiving chamber (“‘auricle’’) covers the posterior region of the heart. Arthropod heart muscle is cross striated, and consists of circularly or spirally arranged fibres. There is some doubt whether the heart muscle is truly syncytial. Arthropod heart muscle resembles the molluscan heart in its contrac- tile properties. There is a condition of reduced excitability early in systole, after which excitability gradually returns (Fig. 3.10). This condition is relative, however, and by increasing the strength of stimulus a contraction can be produced at any stage of the cardiac cycle. At slow frequencies of stimulation (1 per sec), staircase, or progressive increment of consecutive responses, can be demonstrated in crab heart (Cancer). An investigation of haemodynamics in the lobster (Homarus americanus) shows that the cardiac stroke volume is about 0-1—0-3 c.c. An animal hav- h : AY SIN \\ ! \ \ an \ aN a \ Ali \ . : VA f \ | \ / \ \i N') \ \ \ f! (a) Fic. 3.8. HEART OF Ligia oceanica; (a) VIEW OF THE HEART FROM THE VENTRAL SIDE, SHOWING ARTERIES AND ALARY MUSCLES; (b) INNER SURFACE OF THE HEART, SHOWING NERVES AND GANGLIA gc, nerve cells in nerve trunks; nc, nerves connecting local system with c.n.s.; os, ostia (From Alexandrowicz (1).) Cardiac ganglion Latera/ nerve 10 ) Carlson.) Fic. 3.9. HEART OF KING CRAB Limulus, SHOWING ARRANGEMENT OF NERVES 7-8, cardiac nerve from brain; 9-11, cardiac nerve from abdominal ganglia. (From M.A.—4* 106 THE BIOLOGY OF MARINE ANIMALS ing a blood volume of 75 c.c. and a cardiac rate of 100 beats per min can turn over its entire blood volume in 3-8 min. The heart is the main pump- ing mechanism in decapod crustaceans, but body movements assist to some extent in propelling blood through the gills and back to the heart (20). In solitary ascidians such as Ciona the heart is a simple “\-shaped tube. The walls are composed of curious muscle fibres, differentiated into an Fic. 3.10. MYOGRAM OF THE HEART OF Palinurus, STIMU ED WITH INDUCTION SHOCKS AT VARIOUS STAGES OF THE CARDI CYCLE Stimulus ineffective at the beginning of systole (a), but effective during diastole (b, c, d). (From Carlson, 1906.) inner striated, and an outer sarcoplasmic portion. There is no endothelium. Periodically, the heart reverses the direction of its beat, at the same time reversing the course of circulation about the body. Alternation of direction of heart beat is characteristic of all tunicates—salps, pyrosomae and ascidians. Blood pressure is very low, around 2 mm Hg in Ascidia (36, 90). Ampullar Hearts Accessory devices for propelling blood through peripheral channels sometimes take the form of contractile ampullae. In cephalopods the branchial hearts are booster devices which drive systemic venous blood through the gills towards the systemic heart. The walls consist of spongy tissue lined with faintly striated endothelial cells; exit and entrance are guarded by sets of valves. The two branchial hearts contract simultane- ously and rhythmically (121, 127). In lancelets there are small contractile bulbils at the bases of the gill bars. Lymph vessels of fishes sometimes bear contractile lymph hearts which drive lymph into the veins. In the tail of Angui//la, for example, there is a lymph heart which opens into the caudal vein. Lymph hearts are composed of striped anastomosing muscle fibres, and are provided with valves to prevent reflex of blood or lymph. Evidence exists that activity of the lymph heart is controlled by the c.n.s. (central nervous system): it stops beating when the spinal cord is destroyed, and its frequency is altered by stimulation of the cord. HEART RATES. Some representative data on heart rates of different animals are presented in Table 3.3. In general the frequencies of con- tractile vessels in such animals as polychaetes and holothurians are rather low, often less than 10 per min. The hearts of sluggish animals beat at slower rates than those of more active forms (e.g. lamellibranchs versus cephalopods). Heart rates are related to general metabolism and are an BODY FLUIDS AND CIRCULATION 107 important factor, varying in significance in different groups, in maintain- ing an efficient circulation suited to the animal’s requirements. Both cardiac rate and amplitude are usually subject to regulatory mechanisms, described in more detail in sections to follow. Heart rates vary with temperature and Q,, values range between 2-3 in many species. Like certain other activities, there is evidence that heart rates tend to show temperature adaptation (acclimatization) in various species. This is established for the limpet Acmaea limatula, and is implicit in the adaptation of rate function to latitude shown by many forms. For example, the rate of pulsation of the dorsal blood vessel of Perinereis cultrifera has been compared in specimens at Plymouth (England) and Tamaris (Mediterranean). The rate in northern specimens was the same at 14°C as in Mediterranean specimens at 20°C (respective summer environ- mental temperatures). In a similar manner Crustacea from northern lati- tudes show faster heart rates than more southern forms, when comparison is made with the same or closely allied species at a given intermediate temperature. Differences in rate processes obtain even in a single locality. Thus, limpets (Acmaea limatula) from the low inter-tidal region have faster heart rates than high littoral animals at a given temperature of measurement. Adaptation of rate processes is one aspect of compensation for variable environmental temperature (18, 52, 112, 117, 138). INITIATION AND REGULATION OF CARDIAC ActTiviTy. Hearts are charac- terized by rhythmic and continuous contractility, the initiation and main- tenance of which are primary intrinsic functions. The classical object of cardiac research is the vertebrate heart, in which the beat is demonstrably muscular in origin (myogenic). There are certain invertebrate groups in which cardiac contractions are dependent on the nervous system (neuro- genic hearts). Myogenic and neurogenic hearts are distinguished on several grounds. Myogenic hearts are usually inhibited by acetylcholine in low concentrations (< 10-8), while neurogenic hearts are usually accelerated. Search should reveal ganglion cells in the latter type of heart, or in the immediate vicinity. The electrocardiogram of the myogenic heart consists of regular slow potentials, while that of the neurogenic heart shows fast oscillations. Other suggestive features are ease of tetanization, simultaneity or sequence of contraction throughout the organ, and degree of autonomous activity versus central nervous control. A differential effect of ether on the activity of the two types of hearts is explained by the greater sensitivity which the c.n.s. shows to this anaesthetic compared with heart muscle (26, 94, 108). Myogenic Hearts The hearts of vertebrates and molluscs are myogenic, and the beat originates in the cardiac musculature. Ontogenetically, contractions appear in the vertebrate heart before it is innervated, and automatic contractions sometimes continue in fragments of adult heart tissue which lack ganglion cells (6). 108 THE BIOLOGY OF MARINE ANIMALS Fish Heart. In the fish heart contractions are initiated in the sinus venosus and spread to other chambers through the myocardium. Other regions capable of producing automatic rhythmic contractions under experimental conditions are: the veins of Cuvier, the auricle, the auriculo- ventricular canal and the ventricle. Local warming or cooling of the pace- makers alters cardiac rate and provides a means of delimiting their boun- daries and observing their influence on other regions. Local warming of the sinus region, for example, increases cardiac rate. When a Stannius ligature, blocking conduction from sinus to ventricle, is applied to the heart of selachians and eels, the anterior chambers beat at a slower rate than the sinus. This effect is not always apparent in the teleost heart. A wave of excitation can proceed in either direction through the myo- cardium. The unidirectional course of excitation, from sinus to auricle to ventricle, results from differences in the rhythmicity of the several regions, aH HEEL Hensaaiii of sea" ——— Fic. 3.11. ELECTROCARDIOGRAM OF THE HEART OF Platichthys flesus PRT waves shown. Calibration, 100 wV. (From Oets (98).) the sinus showing a more rapid recovery of excitability and thus acting as pacemaker for the whole heart (119). The electrocardiogram (ECG) from the fish heart resembles that of other vertebrates, and gives information about the spread of excitation and contractions of the several chambers. Typically there are a series of slow waves consisting of upward inflexions (negative P, R waves) and downward deflexions (positive Q and S waves) (Fig. 3.11). The electrocardiogram represents a wave front of excitation (depolarization) spreading over the heart. The P wave corresponds to conduction in the auricle, the PQ interval is delay at the auriculo-ventricular junction and the QRS complex represents conduction in the ventricle. A terminal T wave, which may appear negative or positive, follows after an interval and is linked with repolarization of the ventricular surface. Contraction of the S.V. is cor- related with a small negative V wave, immediately preceding the P wave. This appears as a simple diphasic wave in the isolated sinus, and stands revealed after extirpation of the ventricle. In selachians a B wave, repre- senting activity of the truncus, is sometimes registered (71, 72, 98). The fish heart receives inhibitory fibres in cardiac branches of the vagus nerve, and these terminate around the sino-auricular opening on post- ganglionic neurones which influence the pacemaker. Stimulation of the vagus slows the heart or brings it to a standstill (Fig. 3.12). The vagus is believed to exert its effect on the cardiac musculature, especially the pace- maker of the sinus, by release of a chemical transmitter, acetylcholine. BODY FLUIDS AND CIRCULATION 109 Acetylcholine and pilocarpine have a vagomimetic action (Fig. 3.13) and are blocked by atropine (p. 440). It has been observed that the inhibitory potentialities of these drugs are not realized in fish embryos (Fundulus) Noeeagebs Fic. 3.12. EFFECT OF STIMULATION OF THE VAGUS NERVE ON THE HEART OF Scyliorhinus canicula Time scale, above, 1/sec; stimulation shown on second line from top. Lower two curves, heart contractions; auricle above, ventricle below. Some sinus activity is manifest during vagus-arrest of the auricle. (From von Skramlik (119).) 1, 30sec, Fic. 3.13. EFFECT OF DRUGS ON THE FISH HEART (Squalus acanthias) 1: adrenaline (1 x 10~*); 2: noradrenaline (5 x 10~”); 3: acetylcholine (5 x 10~’) (heart sensitized with physostigmine 4 « 10-°). (From Ostlund (97).) until the heart is innervated. There is no sympathetic acceleratory supply, either in selachians or teleosts. Evidence dealing with the effect of adrena- line has been rather conflicting. Adrenaline and noradrenaline usually accelerate the elasmobranch heart and increase the force of beat (Squalus, 110 THE BIOLOGY OF MARINE ANIMALS Raja) (Fig. 3.13). There is occasionally an initial inhibitory effect, which is blocked by atropine. The effect of these amines on the teleost heart is slight (6, 19, 60, 97). Molluscan hearts are myogenic, and all regions can show autonomous contractions. During contraction there is a well-marked A-—V interval, amounting to about 0-5 sec. Electrocardiograms have been recorded from several mulluscan hearts, and show the slow waves characteristic of myo- genic types. In Octopus there is an initial fast deflexion succeeded by a prolonged wave of negativity, and the same sort of pattern is shown by the gill heart of Loligo. The electrocardiogram of Ap/ysia consists of slow waves with superimposed irregular deflexions; those of bivalves (Ostrea, Anodonta) normally display a diphasic component near the beginning of = (a) (db) Fic. 3.14. ELECTROCARDIOGRAMS OF MOLLUSCAN HEARTS (a). Aplysia. From above downwards, myogram, electrocardiogram, time in seconds (from Hoffmann, 1911.) (b). Crassostrea virginica. Electrical record, small waves; mechanical tracing, large waves. Upward deflexion of myogram indicates contraction. Time scale below, 1 sec. Temp. 22°C (from Taylor and Walzl (126).) contraction and one or several slow waves associated with contraction (Fig. 3.14). Apparently the fast component in the ECG represents the spread of excitation; and the slow waves, potential changes taking place during contraction. Although showing automaticity, the hearts of molluscs are subject to nervous regulation. Cardiac nerves arise from the visceral ganglion in lamellibranchs; the presence of nerve cells in the heart is disputed. In gastropods cardiac nerves arise from the visceral or accessory visceral ganglion, and terminate in auricle and ventricle. The existence of nerve cells is in doubt; if present they probably represent secondary regulatory neurones. The nervous supply of the cephalopod heart has been traced in detail. In these animals the cardiac branches arise from the visceral nerves and proceed to the heart via ganglia. From the latter, branches pass to auricles, ventricle and branchial hearts. Inhibitory and acceleratory cardiac fibres have been identified in certain molluscs. Inhibitory nerves are particularly in evidence among lamelli- branchs but are also recorded for certain gastropods (nudibranchs and pulmonates). Stimulation of the visceral nerves and ganglia causes well- marked inhibition of the lamellibranch heart (Fig. 3.15). Acceleratory BODY FLUIDS AND CIRCULATION 11] nerves have also been demonstrated in various amphineurans, gastropods and lamellibranchs: these arise from the pleuro-visceral cords in chitons, and the visceral ganglia of prosobranchs and tectibranchs. Stimulation of these nerves causes acceleration of the heart, or initiates contractions in a quiescent organ. In cephalopods both branchial and systemic hearts receive inhibitory fibres. Stimulation of the visceral nerves causes slowing or inhibition of the heart. The distribution of the two visceral nerves is such that each nerve causes inhibition of ventricle and ipsilateral auricle. Reflex afferent Fic. 3.15. INHIBITION OF THE HEART OF Mercenaria mercenaria BY STIMULATION OF THE VISCERAL GANGLION (a). Brief bursts of shocks; figures indicate position of secondary of induction coil (in centimetres). (b). Effect of repeated bursts of shocks. Time scale in (a) 3 sec. (From Prosser (104).) pathways linking the two auricles are believed to traverse the visceral nerves. Evidence is also available that the cephalopod heart is provided with acceleratory fibres. In some preparations stimulation of the visceral nerves produces acceleration of rhythm in the systemic and branchial hearts, and augmentation of beat in the systemic ventricle (E/edone, Octopus). Under repetitive stimulation the amplitude of beat rises with increase in number of stimuli, the augmentation amounting to 40-300 % with thirty stimuli (Octopus). It is believed that both inhibitory and accel- eratory fibres coexist in the cardiac nerves, the latter showing higher threshold to electrical stimulation, and that these two categories of fibres exert their specific effects by different chemical transmitters (26, 43, 44, 45, 46, 77, 105, 121, 126). 112 THE BIOLOGY OF MARINE ANIMALS Much recent work is concerned with the participation of chemical mediators in the neural regulation of cardiac activity. Acetylcholine inhi- bits the hearts of certain gastropods, lamellibranchs and cephalopods, sometimes in very low concentration (10-” in Mercenaria (= Venus) (Fig. 3.16)). A similar inhibitory effect is shown by pilocarpine in the lamelli- branch heart (Anomia), and by nicotine (Mercenaria, Ostrea, Sepia). Within the body acetylcholine is quickly hydrolysed and rendered ineffective by acetylcholine esterase, the activity of which is blocked by certain tertiary ammonium compounds, e.g. eserine (physostigmine) and prostigmine. Eserine sensitizes the heart of Mercenaria to acetylcholine, resulting in enhanced inhibition when the latter drug is applied. This effect, however, is not apparent in the cephalopod heart. If we accept the hypothesis that acetylcholine may be a normal chemical transmitter produced by inhibitory fibres in the molluscan heart, other Fic. 3.16. EFFECT OF ACETYLCHOLINE ON THE HEART OF Mercenaria mercenaria Concentrations: 1: 2: Oo S 02 Consumption (4000 eggs/day) c.c. do 2 4 6 8 10 12 Days Fic. 4.9. OxYGEN CONSUMPTION DURING EMBRYOGENESIS OF THE KILLIFISH Fundulus heteroclitus. (From Amberson and Armstrong, 1933.) responses to temperature changes than larger ones (beach flea Ta/orchestia, sand crab Emerita, killifish Fundulus). The interrelations between external oxygen tension and oxygen consumption are also affected by external temperature, since high temperatures raise the metabolic rate (26, 39, 41, os. 142, 1505162,..171). TEMPERATURE ACCLIMATIZATION. Animals are capable of adjusting themselves to altered environmental conditions, a process known as acclimatization. Differences in the physiological responses of individuals of the same species from different latitudes may be due to acclimatization rather than to differences in hereditary constitution. Studies on tempera- ture acclimatization have been carried out on diverse functions: survival at extreme temperatures, rates of many activities, e.g. heart beat, ciliary — — ae == ({) DIDUsIS DJauIZ0y Podos] PZ — — — vispsup pue snavuag duiitys ze) os — XDpAOU DIV QeID II[PPIy ; E — — IPAOIIA DULADSAS QeAD Pur'y — — — — ASUALADUOG UO[NWAVDET 1OYVOAD — — — snpodp snuvynT zsddeus jeordor |, Q.] snoaiyospuUuy snuvjoy podsdoy — JOUUUUNS onIp IOJUIM DUDYJYdOJVSIU DIJSIYIAOJD [, CILJ PUBS —_ JOUILUNS op — IO}UIM pplodjv] vjlsaUyA Geis pues JOuUUunNs op as JOUIM = snssadspp snaqojosojnvy, soUUND 9} e10d Wd |, uowojua DaYyJopIsa-F= podos] SNJDI1AO] SNYJuvIDADUUYYH podiydury SYDAOJIY Snjoaquopnasg podiydury SDA] pure sajavaoulyy) qeid topids ‘dds sninspg Qeio WUIOH SyvAojad vIOG YSyAxor|_ DpIDS SnpdsOIAOg POD Ie{Og smMAsoIdponb snpoydavoxod py Uldjndg NOLV ee ANQAAAA aANAAAM N a eS ot oe OTD N GCGi= | | | | | por o eT N TOOLAAG Onset po el eS CA ao ANAKHAANAN om NARA MN AnMANnAAA ae) BEES é AAN A Once A | | al iy ee i ee a | =O —Qeotominm tao MOM Ase we Se RN SO eae ae EK NOT SE OM OoSE OOE JST O07 VST 0.01 OS J$:0-0 (9.) ornyerodus J, jewiuy (Zyl) 7) 42 Jopurjoysg Wolf eed) STVWINY ANIAVJ, YOA NOLLAWASNOZD *O dO "'O Vey AIEAVL \ RESPIRATION 159 motility and body metabolism. An instance of the latter is provided from experiments on the goby Gillichthys mirabilis, which shows reduced rates of metabolism when exposed for several weeks to high environmental temperatures (I5a, 164). Several species of animals are now known to exhibit seasonal acclima- tization of metabolism, dependent on temperature changes. Fundulus parvipinnis taken from cold water in winter possess a higher rate than 2° S SS g body wt,) 2 So a I ° s) a 2 S OH — ~) ° 12) rs ° S wo S Oxygen uptake (ml/hour CS % Ventilation rate (l./hour) Percentage 0, extracted Bey 5 7 9 11 13 Aa) 17 ig <2] Temperature (°C) Fic. 4.10. EFFECT OF TEMPERATURE ON THE RESPIRATION OF THE LOBSTER Homarus vulgaris Curves refer to oxygen uptake by the gills, rate of gill ventilation and percentage of oxygen extracted by a lobster weighing 345 g in sea water having an oxygen concentra- tion of 5-3 ml/l. (From Thomas (150).) specimens measured during the summer (tested at 12°C and 20°C). More- over, when kept at a constant temperature, these fish still show a seasonal rhythm with higher winter respiration. In the sand crab Emerita talpoida oxygen consumption in winter is greater than in summer at temperatures below 20°C; augmentation of winter metabolism is about fourfold at 3°C. Similarly, excised tissues of the bivalve Mercenaria mercenaria tend to show higher winter rates of oxygen consumption. These animals become adjusted to seasonal changes in temperature with the result that the metabolic rate 160 THE BIOLOGY OF MARINE ANIMALS is preserved at a relatively high level in winter. Other species become dormant, e.g. beach flea Ta/orchestia megalophthalma, and show no adap- tive rise in winter metabolism (41, 42, 79, 142, 162, 163). Many instances of temperature compensation or acclimatization involve a shift of the metabolism/temperature curve along the abscissa. Thus in metabolic adaptation to cold, the entire O, consumption curve is displaced towards low temperatures. Theoretically, a low temperature-coefficient Pumping Rate (ml/g/ hour) 0 2 4 6 8 10 12 14 16 18 20 Temperature (°C) Fic. 4.11. PUMPING RATES OF THE MussEL Mytilus californianus, AT DIFFERENT TEMPERATURES Measurements made on three samples of animals from different localities (latitude shown after each curve). Animals weighed 50 g. (From Bullock (21).) (Q,,) would be advantageous in offsetting the effects of temperature changes, but evidence for such is still equivocal (Table 4.4) (21, 142). Adaptations to temperature differences associated with geographic distribution are now well established for a series of poikilotherms. Respira- tory rates and heart rates of some but not all cold-water polychaetes and crustacea are often higher than those of comparable warm-water forms when measured at the same intermediate temperature; mussels (Mytilus californianus) have greater pumping rates in higher than in lower latitudes, other factors being equal (Fig. 4.11) (21, 34a). RESPIRATION 161 Among lamellibranchs, Arctic or boreal cold-water species have a higher metabolism than warm-water Mediterranean species, when measured at the same temperature. Thus Cardium ciliatum from East Greenland has an oxygen consumption twice that of C. edule from the Mediterranean, determined at 5°C. In specimens of one and the same species, Mytilus edulis, from different latitudes the same metabolic rate was recorded in the Mediterranean at 15°C as in Danish waters at 5°C. Curves of oxygen con- sumption versus graded temperatures are displaced towards the left in some but not all cold-water as compared with warm-water species. Thus graphs of O, uptake for prawns Pandalus montagui show that Kristineberg animals have a higher metabolic rate than those from Plymouth, when measured at the same intermediate temperature (10°C). These relations are brought out in detailed studies on a wide series of arctic and tropical fish and crustaceans investigated in Alaska and Panama. The environmental sea temperatures of the arctic species ranged from — 2°C to 9-4°C (mean 6°8°C). Sea temperatures in the Canal Zone showed a range of 25-6°C to 30°C. In all the arctic aquatic forms (amphipods, isopods, decapod crustaceans, teleosts), the O, consumption curves were displaced to the left, toward cold temperatures, when compared with tropical species (Fig. 4.12). The arctic animals, at the normal temperature of their habitat, O°C, have metabolic rates from three to ten times less than tropical species at a habitat temperature of 30°C. When the corresponding metabolic curves of tropical species are extrapolated to O°C, the metabolic rates of these animals would be lowered from thirty to forty times (Fig. 4.13). Consequently in the arctic species examined there is a very appreci- able amount of metabolic adaptation to low temperatures (142). Metabolism of isolated tissues sometimes reflects the same temperature influence measurable in the intact animal. In a study of Mercenaria mercenaria, excised gills from cold-water animals showed a higher Q,, than gills from warm-water animals (at 20°C and 25°C). Of the same nature are results obtained from a comparison of the metabolism of brain and liver tissue of the polar cod Boreogadus saida and golden orfe [dus melano- tus. The former is an arctic fish living at environmental temperatures of around O°C, the latter a freshwater temperate fish having an environmental temperature of 25°C. Polar cod tissues showed a higher respiratory rate over a temperature range of 0°C—25°C, and a relatively much greater rate below 10°C (79, 125a). INTERRELATIONS OF OXYGEN CONSUMPTION AND OXYGEN TENSION. The relations between oxygen consumption and oxygen tension are complex. A convenient generalization divides animals into those which maintain a steady respiratory state over a wide range of external oxygen tensions and those in which the amount of oxygen consumed is directly dependent on the oxygen tension of the environment. Among those factors which deter- mine relative independence are: size of the anirnal, existence of an efficient circulatory system and magnitude of diffusion distances; degree of loco- motory activity and effect of temperature variations; ability to regulate M.A.—6 162 THE BIOLOGY OF MARINE ANIMALS external respiration; existence of respiratory pigments and their physico- chemical characteristics. Metabolism Directly Dependent on O, Tension. In many animals O, consumption varies directly with O, tension, e.g. Actinia, Nereis, Calanus, Homarus, Callinectes, Limulus, Asterias. In Actinia equina, for example, 10007 Tom Cod 1,000 = Sculpin Oxygen Consumption (c.c./kg/ hour) 20 Temperature (°C) 000 Sergeant Major ju ~ 100 (c.c./kg/ hour) Oxygen Consumption 0) 10 20 30 40 0 10 20 30 40 lemperacus-e (2G) Fic. 4.12. RESPIRATION IN TROPICAL AND ARCTIC FISH Ordinates in c.c. O,/kg/hour. Abscissae, temperature (°C). Tropical fish: sergeant major Abudefduf saxatilus; snapper Lutianus apodus. Arctic fish: tom (polar) cod Boreogadus saida; sculpin Myoxocephalus quadricornis hexacornis. (From Scholander et al. (142).) oxygen consumption increases threefold when the external tension is raised from 55-220 mm Hg. When the O, concentration falls below 2 c.c./l. Actinia moves to the surface, and if this is prevented the animal secretes some mucus, closes up and enters upon a period of latent life. In large coelenterates diffusion is a limiting factor in the supply of O, to the tissues. In very small animals, larvae and eggs, O, uptake remains steady RESPIRATION 163 over wide variations in O, tension, showing the adequacy of diffusion for supplying O, requirements. The oxygen consumption of some animals continues to rise at tensions above normal atmospheric pressure, e.g. Actinia, Limulus, suggesting that the tissues are unsaturated with O, under normal conditions. Relative Independence of External O, Tensions. In contrast are those animals in which the respiratory rate remains fairly steady over a wide range of gaseous tensions. Such ability is relative, and critical tensions 10 j TW! TT tt 1 "3 CRUSTACEANS Marine hermit cr&b ae TROPICAL Spider © Blue crab, crab (©) ARCTIC cc | Si iS | Land crab =¥ = Fiddler crab crab x= Shrimp» = a He ~~ / Cc ; 4 9 100 Hermit 9\ Isopod ~ 4 crab 7 Q a 9 7 E a : ve : an ms it, Be c Isopod Gloricatus 8 0 d Approximate /evei of. 10 Pa : re standard non-adapted C 30° hee a oxygen consumption 0 lepi aia at 0°C. aD Zi x< se i) is Pseudalibrotus inm31 0) y 7 i Fair shrim 3 wr b | e i 0-] Img 10 100 1g 10 100 1kg Wet weight Fic. 4.13. OXYGEN CONSUMPTION OF TROPICAL AND ARCTIC CRUSTACEANS AT THEIR NORMAL HABITAT TEMPERATURES The tropical forms extrapolated down to O0°C on the basis of Krogh’s standard temperature curve would fall along the dotted line, corresponding to a rate 30-40 times lower than at 30°C. Arctic forms show appreciable adaptation in that their metabolic rates are only 4-10 times lower than tropical forms. (Double log plot, regression lines correspond to a weight/rate exponent of 0-85.) (From Scholander ef al. (142).) below which independence ceases differ greatly from one animal to another. Some examples of critical O, tensions for various animals at a temperature of 20°C are as follows (mm Hg)— Arbacia eggs 20 Fundulus heteroclitus 16 Loligo pealei 45 Tetraodon maculatus 100 Palaemonetes vulgaris 80 Stenotomus chrysops 30 Even fairly closely related animals show great differences in this respect, Callinectes, for example, being unable to regulate whereas Palaeomonetes 164 THE BIOLOGY OF MARINE ANIMALS regulates down to about 50% saturation (80 mm Hg). Sea anemones do not regulate, but jellyfishes, containing a small proportion of dry matter (Pelagia, Geryonia), show relative independence of external O, tensions. Diffusion in the latter animals is adequate for the small amount of respiring tissue present. Very active animals, because of high levels of oxygen utilization, are more dependent upon high oxygen tensions (e.g. mackerel compared with angler fish). At low temperatures the activity of poikilo- therms is reduced, and they are able to tolerate lower O, levels, 1.e. critical tensions are lowered with fall in temperature. Aquatic invertebrates differ greatly in their tolerance of abnormal levels of O, over long periods. Thus, Sabella pavonina, a tubicolous poly- chaete, will tolerate 100, 21 and 10% dissolved O,, but succumbs in 4% dissolved O, after 4 days. Young Arenicola marina, however, tolerates 21, 10 and 4% O,, but 100% dissolved O, is slowly toxic. Sabella lives in well-aerated water, whereas Arenico/a is a burrowing species that en- counters periodic anoxia (62). Relative independence of external oxygen tensions may result from low levels of metabolism, small size, short diffusion distance and so forth. But many animals possess some degree of control over their respiratory mechanisms, and in these forms low O, tensions may stimulate respiratory centres and evoke increased ventilation movements. Frequently, animals with well-developed circulatory systems possess respiratory pigments which, by virtue of their combining power with oxygen and their saturation characteristics, continue to provide the tissues with adequate oxygen in oxygen-deficient environments and confer relative independence of external oxygen concentrations (11, 15, 30, 32, 109, 114, 150). RESPIRATORY RHYTHMS AND REGULATION OF VENTILATION The ventilation patterns of aquatic animals show specific peculiarities and vary in accordance with environmental conditions, including alterations in temperature and levels of oxygen and carbon dioxide. These responses can usually be interpreted as functional adaptations to respiratory stress. Since there is much variation in ventilation patterns of different groups the sub- ject can be reviewed conveniently on phyletic lines. Ventilation volumes for various species are presented in Table 4.5, and should be consulted in conjunction with the following account. Polychaetes. Tubicolous and burrowing polychaetes show certain characteristic responses to respiratory stress. By an elegant method of recording G. P. Wells has been able to study irrigation by these animals under substantially natural conditions. The lugworm Arenico/a regularly pumps water headwards, occasionally tailwards, through its burrow. Irrigation is rhythmical and occurs in bursts with definite pauses between Successive outbursts of pumping (Fig. 4.7). This rhythmicity is set by an internal pacemaker located in the ventral nerve cord, and the timing of the irrigation cycle is unaffected by reduction of the oxygen tension of the sea RESPIRATION 165 water or by accumulation of possible excretory products. Under condi- tions of oxygen lack, however, the rigour of the outbursts is much reduced. When aerated water is restored after several hours of oxygen deficiency, the worm responds by greatly increased irrigation. Arenico/a utilizes 30-50% of the oxygen in the affluent water, and the utilization percentage is not increased when the animal is exposed to low oxygen tension (5% of normal value), or restored to fully aerated water after prolonged exposure to an oxygen-deficient environment. Nereis diversicolor and Chaetopterus variopedatus resemble Arenicola in showing regular cyclical patterns of irrigation activity (Fig. 5.4). In Spiro- graphis spallanzanii pumping movements are nearly continuous under normal conditions; in Sabella pavonina pumping is sometimes interrupted by long quiescent periods. Oxygen deficiency has different effects on these species: Nereis and Sabella respond by decrease or cessation of irrigation; Chaetopterus by an increase. The determining agent—O, lack, CO, increase or accumulation of harmful metabolites—has not yet been determined. The echiuroid worm Urechis caupo also shows periodical pumping activity, bursts alternating with quiet periods, and about a third of the O, taken into the hind-gut is utilized. In Nereis virens, utilization of the dissolved oxygen is variable, ranging from 20-75% in water saturated with air. After a period of oxygen lack, ventilation, but not utilization, is increased (2842.30.75, 100,158, 159, 161). Molluscs. Ventilation in some aquatic molluscs is greatly affected by environmental conditions. In Mya arenaria the ventilation current is increased after a period of oxygen deficiency (low tide). Following a pro- longed period of anaerobiosis (21 hours), normal levels are not restored until 34 hours later (Fig. 4.14). The utilization of oxygen is normally low, between 3-10°%, but after a period of low tide when the animal has con- tracted an oxygen debt it rises appreciably (to about 25%). In lamelli- branchs the flow of water over the gills shows little variation when the valves are open. When submerged, Mytilus edulis keeps its valves open most of the time and pumps almost constantly. In other bivalves the valves are periodically closed and ventilation ceases. The oyster (Ostrea virginica), for example, closes its shell, on the average, 7 hours out of 24 (29, 32, 55, 87, 88). The differential effects of O, decrease and CO, increase have been dis- tinguished in several species. Ventilation in the oyster (Ostrea virginica) 1s affected by acidity. When HCl is added to sea water so as to lower the pH to 7:0-6:75, the oyster responds by increased pumping activity. In con- formity with their more complex organization and physiology, cephalopods show a high degree of respiratory control. Oxygen lack in Octopus causes increased ventilation up to ten times normal. Increased CO, at tensions below 6 mm also produces an increase, but higher tensions inhibit respira- tion. Oxygen utilization varies between 50-80% (76). Crustacea and Xiphosura. Respiratory control shows much variation in different groups of crustaceans. The respiratory (and feeding) movements 166 THE BIOLOGY OF MARINE ANIMALS of acorn barnacles are not accelerated by decreased O, or increased CO, tension in the water. Both decrease in O, and increase in CO, quicken the rhythm of respiratory movements in Gammarus locusta but the effects are transitory. In amphipods and isopods generally, respiratory movements are performed by beating of pleopods, and the rate is accelerated by low O, and high CO, tensions in most subaquatic species examined (Cirolana, Cymodoce, Idotea, Melita). Ventilation in stomatopods is accomplished both by beating of the pleopods and movements of the thoracic branchiae, and in Squil/la these 1 Bec Utilization O27 ——> Cie. (Op Pierwt, Velocity Respiratory Current 0 / 2 3 4 5 6 Time (hours) after Low-tide Period Fic. 4.14. CURVES SHOWING THE COURSE OF OXYGEN UTILIZATION AND VELOCITY OF RESPIRATORY CURRENT AFTER A LOW-TIDE PERIOD OF 20 HOURS IN THE GAPER Mya arenaria Current velocity = 1/streaming time. (From van Dam (29).) movements are quickened by lack of O, and heightened CO,. In decapods oxygen lack is sometimes effective as a stimulus increasing respiratory movements (Pandalus, Eriocheir, et al.), and increased CO, is also accelera- tory. Exceptions are the shore crab Carcinus maenas and the lobster Homarus vulgaris. Oxygen withdrawal (utilization) in general is rather high, between 25-88 % (average value about 50%) (Fig. 4.10) (76, 77, 90, 100 120.5150, 15285154): The abdominal appendages in Limulus perform ventilation movements at a frequency of 25-50 beats per min. Respiratory movements are con- trolled by branchial ganglia in the ventral nerve cord, and are influenced by a variety of internal and external stimuli. Anoxia leads to a steady RESPIRATION 167 decline in frequency and amplitude of gill movements, resulting in com- plete arrest after 30 min, with the gills closed. Excess CO, causes disorgan- ization of the normal respiratory pattern, and in about 15 min leads to complete stoppage with the gills gaping. The rapidity with which these respiratory changes are effected has prompted the suggestion that extero- ceptors are involved (155). Ascidians. In ascidians ciliary activity produces a more or less continuous current over the gills, and this is augmented by rhythmical branchial and atrial pumping or squirting movements (Fig. 5.17). Oxygen is taken up by the vascularized test as well as by the branchial and atrial epithelia. Pump- ing rates in Pha/lusia are estimated at about 225 c.c./hour, of which 60 c.c. are due to ciliary through-current, the remainder resulting from contrac- tions of the body wall. Oxygen utilization in solitary sea-squirts is low, around 4-7 % (17-20°C) (76, 81). Fishes. Breathing in fishes is regulated by a relatively autonomous respiratory centre located in the medulla. In the skate this centre is capable of continued activity when isolated from anterior and posterior levels of the c.n.s., and possesses segmental functional regions corresponding to the several gill arches which it controls. Respiratory movements are markedly affected by peripheral stimuli, revealing well-developed reflex control. Thus, decreasing the flow of water over the gills depresses respiratory and cardiac rates and may lead to gasping movements (Raja, Scyliorhinus). The circulatory centre is only weakly affected by changes in the blood circula- tion; and when afferent pathways are anaesthetized by cocaine, respiratory movements still continue at a reduced rate. It is believed that a double mechanism for respiratory control exists in selachians, namely weakly- developed automaticity of bulbar centres, on which are superposed strong reflex reactions evoked principally by peripheral stimuli. Experiments dealing with respiration have been carried out on teleosts, in which various combinations of the IXth and Xth cranial nerves have been cut immedi- ately medial to the gills. The results indicate that reflex activity, originating in branchial receptors served by the IXth cranial nerves, is essential for continued respiratory movements (126). Ventilation in fishes is very efficient and a large proportion of the oxygen in the inspiratory current is withdrawn. Under normal conditions utiliza- tion values for different species range from 46% (Scyliorhinus, Spheroides) to as much as 80% (Anguilla, Salmo, Uranoscopus). Fishes react to adverse respiratory conditions (low O, and high CO, tensions) in several ways. Oxygen lack and heightened CO, result in more active swimming and struggling in some fishes (Scyliorhinus, Anguilla, Opsanus), responses which may have value in promoting escape from an adverse environment. Low O, and high CO, tensions increase respiratory activity and ventilation. The ventilation volume is altered by changes in depth, or frequency of breathing, or both. In the eel ventilation may be increased fivefold when the O, content of the water falls below 4 c.c./l. In selachians (Mustelus, Squalus) rhythmic respiratory movements show Q,, values of from 1-2—2-4. 168 THE BIOLOGY OF MARINE ANIMALS Ventilation affects utilization since the faster the animal ventilates, the shorter will be the period of contact of the water with the gill filaments. With the same oxygen content a greater ventilation volume results in decreased utilization. In the eel utilization falls when the O, content of the inspired water drops below 2-1-5 c.c./I., but down to a level of about 1 c.c./l. the animal still obtains sufficient oxygen for its needs. Increased respiratory activity causes a rise in metabolism, amounting to 40% in the eel, and 70% in the trout; this is due to increased work by the respiratory muscles. In Spheroides, utilization remains approximately the same over a fivefold decrease in tension (at 46% for O, levels of from 4-7-1 c.c./I.), and is not altered over a temperature range of from 12—22°C (11, 30, 76, 100). Conclusions In slow-moving and sedentary animals oxygen consumption and utilization are low. Oxygen withdrawal from the respiratory current is of the order of 20° or less in sponges, lamellibranchs and tunicates. These are all sedentary filter feeders. The magnitude of ventilation currents in such animals is determined primarily by nutritive requirements, and there is usually a very large margin for respiration. Absolute values show very great variation, depending on a complex of environmental and intrinsic factors. Greater regularity is obtained by expressing ventilation volumes in terms of dry weight or amino-N (Tables 4.5 and 4.6). Oxygen utiliza- tion in worms with muscular pumping mechanisms stands at a higher level than in filter feeders, ranging from 30-75% in different polychaetes and echiuroids. Utilization depends upon the oxygen content of the inspiratory current, the rate and amplitude of pumping, and the metabolic condition of the animal. Active cephalopods and fish have high levels of oxygen consumption and utilization. Oxygen lack is generally a more effective respiratory stimu- lus than rise in CO, tension. The CO, tension of sea water is very low (free CO, around 0:23 mm Hg) and relatively constant, and changes in this factor are probably outside the physiological experience of most marine animals. Responses to temporary oxygen deficiency take a variety of forms: increased ventilation (fish, cephalopods, crustacea, Chaetop- terus), decrease in activity and lowering of metabolism (sponges, various polychaetes, Limulus), utilization of oxygen stores (Arenico/a), incurrence of oxygen debt (lamellibranchs) and escape responses (fish). In all aquatic poikilotherms (invertebrates, fish) the ventilation rate is directly propor- tional to the circumambient temperature over normal tolerable ranges Cle 25..28a.,29) 67,16, O15 80, 095 Lan): RESPIRATION IN DIVING VERTEBRATES Three main factors are involved in respiration among air-breathing diving vertebrates, namely accessory aquatic respiration (marine reptiles), adaptation to prolonged submergence in absence of oxygen renewal TABLE 4.5 VENTILATION AND PUMPING VOLUMES on Ee . Temperature Vent./vol. zane (C) ml/hour Sponges Grantia compressa 17-—20° 594-1,148 Sycon ciliatum 17-—20° 360 Spinosella sp. -- 3,200 Polychaetes and echiuroids Arenicola marina 20-4° 177 Nereis (Neanthes) virens 17-—18° 92:7 Sabella pavonina 1D; 30 ditto 18-20° 73 Urechis caupo not feeding -— 660 ditto feeding — 1,740 Molluscs Mytilus edulis 17—20° 100—29,500 mean 1,850 ditto 17> 1,900—2,600 mean 2,000 ditto 11-8-14-7° 200—4,600 mean 1,800 Cardium edule 17-3—19-5° 200-2,500 mean 500 M. californianus 20-23° 500-18,100 mean 2,600 ditto 20° 4,000—5,500 Crassostrea virginica 24-26:9° 500—3,900 mean 2,700 ditto —— 1,180 ditto 20° 1,027—212,000 Pecten irradians 21-9-25:-8° 3,260-14,720 Tunicates Molgula sp. 17—20° 207-513 M. manhattensis 600—1,200 Ciona intestinalis 17-20° 552-750 ditto — 1,800—3,400 ditto — 2,000 ca. Phallusia mammillata 14° 225 Ca, Fishes Salmo shasta 10-12° 700—9,000 Anguilla anguilla 17-18° 330—3,600 ea ge i ea 8 = a ee TABLE 4.6 PUMPING RATES OF SPONGES, LAMELLIBRANCHS AND ASCIDIANS (From Jergensen (87, 88)) Average content Animal amino-N (mg) Grantia compressa < ES 8-5 Sycon ciliatum 2 Halichondria panicea — Mytilus edulis Z 25 Molgula sp. 1:8 Ciona intestinalis 6 M.A.—6* Pumping rate ml/hour/mg 170 THE BIOLOGY OF MARINE ANIMALS (homoiotherms) and resistance to pressure and pressure effects (deep- diving whales). Physiological information is very scanty for all marine reptiles. The possibility of bucco-pharyngeal and cloacal respiration requires considera- tion. Quantitative studies on typical marine reptiles will have to be carried out before their respiratory needs can be evaluated. Sea snakes and turtles are said to survive many hours under water (Table 4.7). Diving marine birds usually stay under for only brief periods, from 1-2 min, but can stand submergence up to 6—12 min (guillemot, penguin, etc.). Seals remain submerged up to 15 min. Compared with these times the duration of dives in some whales is striking: about half an hour in fin and humpbacked whales, and 1—2 hours in Greenland, bottle-nosed and sperm whales (Table 4.7). TABLE 4.7 DURATION OF SUBMERGENCE OF MARINE REPTILES, BIRDS AND MAMMALS Animal Submergence time Reptiles Loggerhead turtle Caretta caretta Up to 25 min ae snakes Hydrophiidae Possibly up to 8 hours Birds Razorbill Alca torda 52 sec Black guillemot Uria grylle 1-12 min Little auk Alle Alle Up to 68 sec 6-12 min forced submergence Puffin Fratercula arctica 30 sec or more 4 min forced submergence Adélie penguin Pygoscelis adeliae 30-45 sec normal; can survive 6 min forced submergence Velvet-scoter Melanitta fusca Upto Si sec Mammals Sea otter Enhydra lutris 4-5 min ditto 10 min ditto 15-30 min Sea elephant Mirounga angustirostris 6 min, 48 sec Common seal Phoca vitulina 15 min Grey seal Halichoerus grypus 15 min in net Sperm whale Physeter catodon 20-75 min Bottle-nosed whale Hyperoodon rostratus 2 hours (1 hour wounded) Common rorqual Balaenoptera physalus 14-9 min normal; up to 20 min Blue whale B. musculus 4-15 min normal; harpooned 30-49 min Greenland right whale Balaena mysticetus % to 14 hours when harpooned. Normal 5-20 min Black right whale B. sieboldii 50 min Humpback whale Megaptera nodosa 15-20 min Sources: Murphy (117); Irving (82); Scholander (140); Witherby, et a/. (168); Norman and Fraser (119); Gunther (73); Layne (102) ef al. Respiration in diving birds and mammals does not differ fundamentally from that in typical air breathers. Adaptations which allow divers to remain submerged for long periods are diminished sensitivity to CO,, shunting of blood to essential organs, and ability to contract a large oxygen debt. RESPIRATION eit In true diving animals contact of the respiratory openings with sea water reflexly inhibits breathing. During submergence O, is used up and CO, accumulates in the blood. Compared with terrestrial species divers are relatively insensitive to CO,. Diving birds (puffins and guillemots) show unusual tolerance to CO,, up to a concentration of 15%. The low sensitivity to CO, results from higher threshold of the respiratory centre in birds and diving mammals. Divers carry a supply of oxygen, but this 1s insufficient for aerobic respiration during a long dive. The oxygen store of the bladder-nose seal Cystophora has been estimated to be sufficient for about 5 min at rest, but the animal is of course highly active when diving and its energy consumption will be much greater than at rest. A seal can remain submerged for 15 min and is enabled to do so by contracting an oxygen debt in addition to depleting its oxygen stores. The gaseous capac- ity of the lungs of divers is only slightly if at all larger than that of terrest- rial mammals of comparable size. Only a small proportion (about a third) of the oxygen store is carried in the lungs. The remainder is carried largely in the blood and to a lesser extent in tissue fluids and in cells. Some divers (seals, ducks) have somewhat larger blood volumes and higher concentra- tions of blood haemoglobin than terrestrial forms. In addition many divers, especially whales and seals, have large stores of muscle haemoglobin, which surrender their oxygen to the muscle cells during submergence. It appears, then, that oxygen capacity and stores of divers may be somewhat higher than in terrestrial mammals, but still insufficient to account for the fact that they can remain submerged so much longer than the latter. On surfacing after a dive lasting 15 min (suspended respiration) the seal dis- plays deep and continuous respiration, lapsing to normal in 15 min. There is a large increase in gaseous exchange, and around 80% of the oxygen debt is paid off in the first 20 min. During a dive lasting 15 min the seal carries only enough O, for at most a third of its requirements. How is this allocated and utilized? In all divers submergence causes reflex slowing of the heart. In the seal the heart rate falls from a resting value of 80/min to 10/min when the animal has submerged, and the oxygen consumption falls during the first minute of the dive to about one-fifth of resting value (Fig. 4.15). At the same time the peripheral circulation (in the muscle mass and probably the viscera) is largely suspended, and the reduced circulation and oxygen supply are reserved largely for the brain. In whales there seems to be a method for shunting blood through retia mirabilia to the brain. While the peripheral circulation is occluded during a dive, the muscle haemoglobin of the seal provides a local store of oxygen for the muscles, enabling them to carry on aerobically for 5-10 min without recourse to lactic acid formation. Once these stores are depleted, lactic acid accumulates in the muscles (anaerobic respiration). On surfacing, lactic acid appears with a surge in the blood once the peripheral circulation is opened, and is gradually removed over the course of the next half-hour (Fig. 4.15) (82, 83, 84, 85, 140, 143). Li THE BIOLOGY OF MARINE ANIMALS Whales can dive deeply, and there is evidence that some species regularly go down to great depths. Harpooned right and fin whales are reported as descending to 500—1,200 metres, and there is a record for the sperm whale of 1,600 metres. How do they tolerate these pressures and escape caisson disease? The lungs of whales are not unusually large in proportion to their size and may not be fully inflated during a dive. The whale takes down only a limited supply of air (including N,), which is not replenished as in human diving apparatuses. Moreover when the whale makes a deep dive 3min periods Arterial Blood Respiration Fic. 4.15. CHANGES IN THE ARTERIAL BLOOD OF THE GREY SEAL Halichoerus grypus DURING A DIVE UNDER EXPERIMENTAL CONDITIONS. (From Scholander (140).) its lungs are compressed and reduced in volume, air is displaced into bronchial and tracheal dead space, and the alveolar surface becomes re- duced and thickened, all of which are factors reducing or slowing diffusion of nitrogen into the blood. In addition much of the peripheral circulation is closed off. It is probable then that the limited amount of N, available is insufficient to supersaturate the blood seriously, and the limited blood volume exposed to nitrogen at high pressure is diluted with blood from the periphery when the animal surfaces (100, 140). RESPIRATORY PIGMENTS The oxygen capacity of water is low (sea water 0:54 vol% at 20°C), and is inadequate as an oxygen carrier except for animals having low meta- bolism. Many animals which possess circulatory systems have blood pigments which serve to increase greatly the oxygen-carrying capacity of the blood or haemolymph. Blood pigments are compounds which combine loosely with oxygen. They belong to several different chemical categories but they have this in common that they contain some metal, usually iron or copper, in combination with protein. In the following section we shall examine their role in oxygen transport among marine invertebrates and RESPIRATION 173 lower chordates. Reviews dealing with the comparative physiology of respiratory pigments have been prepared by Redfield (129, 130), Florkin (53), Prosser (127) and Eliassen (43). Occurrence and Chemical Characteristics of Respiratory Pigments in Marine Animals The distribution of respiratory pigments in marine animals is shown in Table 4.8. The principal categories are haemoglobin, chlorocruorin, haemocyanin and haemerythrin. Several additional pigments, the func- tional roles of which remain obscure, are also known. Haemoglobin is a reddish pigment containing an iron-porphyrin complex, widely but sporadically distributed through the animal kingdom. It is found in the blood of all vertebrates, with the exception of certain pelagic transparent fish larvae (Leptocephalus) and certain antarctic teleosts of the family Chaenichthyidae. It is the dominant respiratory pigment in anne- lids (polychaetes, oligochaetes and echiuroids). Elsewhere it is distributed in isolated instances among hemichordates, holothurians, phoronids, arthropods, lamellibranchs and nemertines (137). In animals with closed circulatory systems haemoglobin occurs in corpuscles (erythrocytes) or dissolved in the plasma. Where haemoglobin is found in the coelomic fluid or haemolymph it is always enclosed in cor- puscles. In certain terebellids (Terebella and Travisia) both conditions coexist: the blood contains haemoglobin in solution, and the coelomic fluid is provided with erythrocytes. Among invertebrates with closed circulatory systems there is a general tendency for haemoglobin to be dissolved in the plasma. Haemoglobin has also been recognized in other tissues. As muscle haemoglobin (myoglobin) it occurs in heart muscle of vertebrates; striated muscle of fish (Hippocampus) and homoiotherms; pharyngeal and radular muscles of gastropods (Busycon); locomotory muscles of Arenicola, Potamilla and Urechis. It is also found in the nervous system of worms (Urechis, Aphrodite, nemertines) and lamellibranchs (Tivela). Haemoglobins are complexes made up of a protein globin in combina- tion with a prosthetic haem group. The latter is a metalloporphyrin con- taining ferrous iron combined with protoporphyrin (p. 477). The haem component of haemoglobin is identical in all species but the protein (globin) fraction shows specific differences. The oxygen-transporting function of haemoglobin is due to haem, which combines loosely with oxygen in the proportion | molecule of O, to | atom of Fe (oxygenation). More than one haem unit is combined with globin, the exact number vary- ing with different species. The iron content of haemoglobins will differ accordingly. When present inside corpuscles, haemoglobin has its own chemical environment which may be of functional significance. Haemo- globins dissolved in plasma have high molecular weights. The large mol- ecular size will tend to retain the haemoglobin in the vessels but increases the viscosity of the blood. The haemoglobin and erythrocyte contents of 174 THE BIOLOGY OF MARINE ANIMALS various bloods are shown in Table 4.9 (4, 5, 54, 58, 59, 61, 68, 69, 94, 124, 138, 14la, 148). Chlorocruorin. This is a greenish respiratory pigment related to haemo- globin. It is confined to certain families of polychaetes—namely Chlor- haemidae, Ampharetidae, Sabellidae and Serpulidae—in which it is dissolved in the plasma, and it may coexist with haemoglobin in the same species. In marine invertebrates the porphyrin-containing blood pigments, haemoglobin and chlorocruorin, are restricted to relatively inactive sedentary species, the majority of which live in tubes, burrows or crevices. Their occurrence in such animals is often related to conditions of tem- porary or permanent oxygen deficiency. The prosthetic group of the chlorocruorins is also a haem, but contains a different porphyrin from haemoglobin. The affinity of chlorocruorin for oxygen is on the basis of 1 molecule O, per atom of Fe (27, 58, 125). Haemocyanin. This is a copper-containing pigment which appears light blue in the oxygenated condition. It is found in the blood of some gastro- pods, in cephalopods and in higher crustaceans, and is the most important respiratory pigment of invertebrates, yielding to haemoglobin in vertebrates. Some gastropods containing haemocyanin have myoglobin as well, e.g. Busycon. The haemocyanins are copper-protein compounds in which the copper is contained in a prosthetic group showing polypeptide characteristics. The oxygen-combining capacity of haemocyanin is dependent upon its copper content, 1 molecule of oxygen combining in proportion to 2 atoms of copper. The haemocyanin and copper contents of the blood of Various animals are given in Table 4.10. Haemocyanins differ in their copper content, those of molluscs generally containing more copper than arthropod haemocyanins. Copper in the haemolymph is predominantly located in haemocyanin, and hence is indicative of haemocyanin content. It has been pointed out that the high copper content of animals containing haemocyanin demonstrates remarkable ability to concentrate this element. The copper content of some haemolymphs may be as much as 3 x 10° times greater than that of sea water (1-10 wg Cu per 1.) (16, 130). Haemerythrin. Another iron-containing respiratory pigment found in invertebrates is haemerythrin, which differs from haemoglobin in the absence of a metallo-porphyrin group. It occurs in the polychaete Mage/ona, in sipunculoids and priapuloids, and in the brachiopod Lingula. Haemery- thrin is always enclosed in corpuscles, and appears pink when oxygenated. It combines with oxygen in the ratio 3 Fe/O,; the coloured prosthetic group is not a porphyrin (34, 48, 93). ADDITIONAL PIGMENTS. Several other pigments of doubtful function have been recognized among invertebrates. These include vanadium chromogens found in blood and body fluids of ascidians; reddish naphtho- quinones (echinochromes and related substances), found in tissues and coelomic cells of echinoids (Chapter 11); and pinnaglobin, a manganese- containing pigment found in the haemolymph of the lamellibranch Pinna. RESPIRATION | ap fs Vanadium Chromogens. The presence of surprisingly high levels of vanadium in ascidians has long presented an intriguing problem. Vanadium content varies from 0:04°%% dry weight in Ciona intestinalis to 0-186% in Ascidia mentula; nine-tenths of the vanadium is in the blood. Present in the circulation of certain ascidians are large numbers of cells, some of which are green in colour and contain vanadium chromogen. These cells, known as vanadocytes, make up 1:2% of total blood volume in some species of Ascidiidae. Vanadium chromogens are confined to the order Ascidiacea; they occur in the plasma of Cionidae and Diazonidae; and in vanadocytes in several other families, including the Ascidiidae and Pero- phoridae (Table 4.11). In view of the low vanadium content of sea water the ability of ascidians to concentrate such large quantities of this element appears remarkable. Experiments with Ciona and Ascidia using radioactive vanadium suggest that the element may be largely taken up from sea water through the alimentary tract, possibly in colloidal or adsorbed form on mucus sheets. Vanadium chromogen (native haemovanadin) contains vanadium and hydrosulphuric acid bound together into a complex (disulphate-vanadium acid), probably linked with protein. Vanadium forms about 10-15% of the haemovanadin obtained from haemolysis of the blood cells. Inside the corpuscles (vanadocytes), the vanadium is kept in reduced form by a remarkably high concentration of H,SO,, reaching 9% (1:83 N). The physiological characteristics of haemovanadin appear to exclude it as a respiratory pigment: recent studies suggest it may have an oxidation- reduction role in the animal (6, 8, 14, 23, 67, 156). The oxygen capacity of blood depends on the amount and nature of the respiratory pigment present. In Table 4.9 information is summarized for haemoglobin concentrations and erythrocyte contents of the blood or body fluids of various animals. These figures show certain trends. There is much variance in haemoglobin concentrations and oxygen capacities of divers (birds and mammals); the erythrocytes of some diving mammals show high oxygen capacities (seals, sea lions, porpoises, Table 4.13). Haemoglobin concentrations are lower in poikilotherms, around 5% in most fish, and less in the few invertebrates examined. Highly active pelagic fish have large oxygen capacities and haemoglobin concentrations (menhaden, scombrids), whereas sluggish bottom forms show low haemo- globin values. A series of deep-sea fish examined proved to have average haemoglobin values (Table 4.12) (22, 33, 141a). TRANSPORT OF OXYGEN Oxygen Capacity and the Dissociation Curve The effective transporting ability of blood is given by its oxygen capacity, which is a measure of the maximal amount of O,, in volumes per cent, with which the blood will combine. Oxygen capacities of the blood of most invertebrates are rather low, 3-18% in worms, I-5% in molluscs and 1-3 % in arthropods; levels are higher in fishes, from 5-15 %, and very high 176 THE BIOLOGY OF MARINE ANIMALS capacities are shown by some diving mammals, up to 30% (Table 4.13). Most bloods become saturated with oxygen below atmospheric tension. The affinity for O, is characterized by the dissocation curve, connecting degree of saturation of the blood (percentage) with tension of O,. Stand- ard values, which indicate the nature of the dissociation curve and its useful range, are given by tensions of loading and unloading. Loading tension (t;) refers to the tension at which the blood becomes 95 % saturated with O,; tension of unloading (t,,) corresponds to half-saturation. Some oxygen dissociation curves for haemoglobins of different animals are Wis 4 5 6 10 20 30. + 40 50 60 70 80 90 ~+~«+100 Oxygen tension (mm Hg) Fic. 4.16. OXYGEN DISSOCIATION CURVES FOR HAEMOGLOBIN OF SEVERAL MARINE ANIMALS Curves: 1: Arenicola marina, 20°C, pH 7:3; 2: Anguilla bostoniensis, 20°C pH 7:3; 3: Petromyzon marinus, 20°C, pH 7:4; 4: Opsanus tau, 20°C, CO, 1 mm Hg; 5: skate, 10-4°C, CO, 1 + 0-5 mm Hg; 6: Phoca vitulina, 38°C, CO, 46-47 mm Hg. (From various sources.) shown in Fig. 4.16, and values for tensions of saturation (loading) and half-saturation are listed in Table 4.13. FUNCTION OF HAEMOGLOBIN. Haemoglobins of different animals often possess quite dissimilar dissociation curves. The tension of half-saturation indicates the facility with which O, is transferred from blood to the tissues: a high value for t, increases the availability of O, to the tissues; a low value indicates that oxygen is available only when supplies in the tissues are nearly exhausted, and tensions are very low. Saturation tension is linked with unloading tension, but does not become functionally signific- ant until oxygen becomes depleted in the external medium, and falls below 100 mm Hg. The dissociation curve is typically sigmoid in shape, the slope differing greatly from species to species (Fig. 4.16). At one extreme are animals with haemoglobins which possess high ¢,, values and have a low affinity for Oz, e.g. seal, porpoise, duck; at the other are animals such as eel and lugworm, the haemoglobins of which show high affinity for O, and possess low t, and t, values, Most aquatic poikilotherms have dissociation curves lying RESPIRATION TABLE 4.8 LaF OCCURRENCE OF RESPIRATORY PIGMENTS IN MARINE ANIMALS Phyletic group Platyhelminthes Turbellaria Nemertinea Mollusca Amphineura Gastropoda Lamellibranchia Polychaeta Aphroditidae Nereidae Glyceridae Eunicidae Cirratulidae Capitellidae Arenicolidae Maldanidae Amphictenidae Opheliidae Terebellidae Sabellidae Serpulidae Echiuroidea Phoronidea Subgroup or species Haemoglobin Syndesmis echinorum Derostomum? Drepanophorus spectabilis Tetrastemma flavidum Amphiporus cruentatus Cerebratulus marginatus C. ehrenbergii Lineus gesserensis L. geniculatus Chiton Patella, Buccinum, Aplysia Busycon Ensis, Solen, Arca, Cardita, Glycymeris (Pectunculus), Phaxas, Gastrana, Tel- lina, Poromya, Astarte Mercenaria mercenaria Aphrodite Arenicola Clymene lumbricoides Thoracophelia mucronata Amphitrite rubra Pista cristata Polycirrus haematodes P.. aurantiacus Terebella lapidaria Travisia forbesii Potamilla stichophthalmos P. reniformis Spirorbis corrugatus Serpula vermicularis \. S. lobiancoi Urechis caupo Thalassema neptuni T. erythrogrammon Hamingia arctica Phoronis, Phoronopsis Location Body fluids ditto Blood corpuscles ditto ditto Cephalic ganglia ditto ditto ditto Pharyngeal muscle ditto Heart, radular muscle Blood corpuscles Heart, adductor muscle Brain and nerve cord Blood plasma Coelomic corpuscles Blood plasma ditto Coelomic corpuscles Blood plasma, muscles Blood plasma ditto ditto ditto ditto Coelomic corpuscles ditto Blood plasma and coelomic corpuscles Blood plasma and coelomic corpuscles Muscles ditto Blood plasma Haemoglobin and chloro- cruorin in blood plasma Coelomic corpuscles, muscles, nerve cells Coelomic corpuscles ditto ditto Blood corpuscles 178 THE BIOLOGY OF MARINE ANIMALS TABLE 4.8—OccurRENCE OF RESPIRATORY PIGMENTS IN MARINE ANIMALS (continued) Phyletic group Echinodermata Ophiuroidea Holothuria Arthropods Crustacea Copepoda Rhizocephala Amphipoda Insecta Chordata Hemichordata Vertebrata Polychaeta Mollusca Amphineura Gastropoda Cephalopoda Arthropoda Xiphosura Crustacea Polychaeta Sipunculoidea Priapuloidea Brachiopoda Subgroup or species Haemoglobin Ophiactis virens Cucumaria frauenfeldi C. miniata, C. elongata C. planci Thyone aurantiaca T. gemmata Leptosynapta minuta Caudina chilensis and x Molpadia roretzii ifs Lernanthropus, Clavella, Congericola, Mytilicola Septosaccus, Peltogaster, Galatheascus Urothoé Chironomus Discoglossus pictus All classes Chlorocruorin Chlorhaemidae Sabellidae \ many Serpulidae f{ species Haemocyanin Tonicella marmorea Littorina, Buccinum, Busycon, Neptunea, et al. Loligo, Sepia, Octopus, Eledone, Rossia, Sepietta Limulus Malacostraca—many species Haemerythrin Magelona papillicornis Sipunculus, Golfingia, Phymosoma Priapulus caudatus Halicryptus spinulosus Lingula unguis Location Vascular corpuscles Coelomic corpuscles ditto ditto ditto ditto ditto Corpuscles in coelom and vessels Haemal vessels Blood in external sac Muscle, blood? Blood plasma Blood corpuscles ; Blood corpuscles, heart, stri- ated muscle (many species) Blood plasma ditto ditto Haemolymph ditto ditto ditto ditto Blood corpuscles Coelomic corpuscles Coelomic corpuscles ditto Blood corpuscles RESPIRATION 179 TABLE 4.9 ERYTHROCYTE AND HAEMOGLOBIN CONTENT OF BLOODS OF MARINE ANIMALS : 7” Erythrocytes Haemoglobin pecies Flui g per 1 0/ Count Vol 7, 108 per man? c.c. blood Mammals | Seal Phoca vitulina Blood — 6-01 16-5 Sea lion Eumetopias stelleri Blood 29 — — Porpoise Blood 42-5 8-4-11-2 — Porpoise Phocaena phocaena Blood 35-6 —_ — Dolphin Tursiops tursio Blood md lee — = Blue and fin whales Balaenoptera| Blood —- == 9 Birds Surf scoter Melanitta perspicillata| Blood 45 — — Reptiles Marine turtle Blood a —- 6:94-8:98 Fish Sole Solea solea Blood | 13 0-7, 1:29 223°! Gar-fish Belone belone Blood oe Ae | a 5:6 Angler Lophius piscatorius Blood | 1335 0-867 -— Toadfish Opsanus tau Blood 19-5 0-585 — Tautog Tautoga onitis Blood — 1-5—2-36 5-4-7-5 Shark sucker Echeneis naucrates | Blood — 3°75 1-5 Puffer Spheroides maculatus Blood 17:5 2:28-4:38 7:2-7°5 Scup Stenotomus chrysops Blood 32:6 2:685 — Lingcod Ophiodon elongatus Blood | 21-68 2-:094-4-24 a Sea robin Prionotus carolinus Blood 24 2:49-2:54 7:1 Mackerel Scomber scombrus Blood | 37-1 3-0-3-9 14-2 Trout Salmo trutta Blood — 1-5 — Eel Anguilla anguilla Blood — ie | — Eel A. japonica Blood 27-41 2:22-3-13 — Skate Raja ocellata Blood 20 G2 — Skate R. laevis Blood — 0-21—-0-29 3-2-4:0 Skate R. erinacea Blood 11-24 — = mean 18 Skate R. binoculata Blood 4-24 — — Skate R. clavata Blood 14-25 0-15-0-24 1-3-4-3 Stingray Dasyatis centroura Blood — 0-29 2°3 Stingray D. pastinaca Blood 19-30-5 0:20-0:29 3-1-4-6 Dogfish Squalus acanthias Blood 6-25 — — Smooth hound Mustelus canis Blood — 0-41-0-52 3-7-4-2 Ratfish Hydrolagus colliei Blood 18:21 — — Echiuroidea Urechis caupo Coelomic 18-3-40-3 -—— 1-4—-5:1T Annelida Arenicola marina Blood — _- 3°25 Terebella nebulosa (= johnstoni) | Blood — — 3°31 Lamellibranchia Arca inflata Haemolymph 6:5 = = Arca sp. Haemolymph = = 1:06-1:64 + Calculated on basis of 1 vol % O, = 0-746 g Hb per 100 c.c. cells. (Sources: Redfield (129); Irving et al. (85); Duthie (38): Kisch (95); Martin (110); Korshuiev and Bulatova (97); Korshuiev and Nikolskaya (98); et al.). 180 THE BIOLOGY OF MARINE ANIMALS TABLE 4.10 HAEMOCYANIN CONTENT AND OXYGEN CAPACITY OF SOME INVERTEBRATE BLOODS Species Cu mg/100 c.c. eon ca Oe ty Busycon canaliculatum 9-16-16-2 3:7-6°6 2:1-3-35 B. carica 4-94 a 1-36 Sepia officinalis 237 _- od Loligo pealei 18-8-22°8 7:2-8:'8 3-8-4:5 Octopus vulgaris 1-48-23-5 5-9-9-1 3-1-4-5 Squilla mantis 6:1 -- — Homarus vulgaris 4-14-14-8 — 1-22 sem H. americanus 8-3 4:4 1-95 Palinurus vulgaris 5-34-9-5 _ 1-43-1-80 Panulirus longipes 4-3-20°8 _ — Cancer pagurus 6:0-10-3 _- 1-6-2:3 C. irroratus 4-2-6:82 -- 1-23-1-69 C. borealis - 5-16 = 1-40 Carcinus maenas 3-69-9-0 — 1-14-1-16 Callinectes sapidus 4-54 — 1-29 Ovalipes ocellatus T3833 — 1-78 Maia squinado 1-99-7-16 — 0-84-1-75 Limulus polyphemus 1-42-12-6 0-8-7-3 0-74-2-7 (Data from Redfield (130); Elvehjem (45); Beck and Sheard (7).) TABLE 4.11 VANADIUM CHROMOGENS IN TUNICATES Family Location Perophoridae Perophora viridis Vanadocytes Ecteinascidia turbinata ditto Ascidiidae Phallusia mammillata ditto P. fumigata ditto P. hygomiana ditto Ascidia mentula ditto A. conchilega | ditto A. nigra ditto A. samea ditto Diazonidae Piazona violacea | Plasma Cionidae Ciona intestinalis ditto Rhodosomatidae | Plasma? (From Webb (156); Kobayashi (96).) RESPIRATION 181 TABLE 4.12 HAEMOGLOBIN CONCENTRATION OF THE BLOOD OF MARINE FISHES (Selected values. Expressed in terms of iron per unit volume) Fish—Teleosts me die oats ee Bonito Sarda sarda 45-5 Mackerel Scomber scombrus 43-0 Menhaden Brevoortia tyrannus 41-0 Cunner Tautogolabrus adspersus 21 Butter-fish Poronotus triacanthus 27:4 Scup Stenotomus chrysops 25°3 Rosefish Sebastes marinus 24-9 Silver hake Merluccius bilinearis 19-4, 24-0 Sea robin Prionotus carolinus ye Oe | Simenchelys parasiticus 22-0 Puffer-fish Spheroides maculatus Ziss Eel Anguilla bostoniensis 20-4 A. japonica | 26-0-37:1 Macrourus berglax 16:5 Angler Lophius piscatorius 14-7 Toadfish Opsanus tau | ae Coryphenoides rupestris 11-9 Sand dab Lophopsetta maculata 11-5 Fish—Elasmobranchs Smooth dogfish Mustelus canis 15-4 Electric ray Torpedo nobiliana 8:8 to the left of those of homoiotherms. The oxygen affinities of some in- vertebrate haemoglobins are very high. EFFECT OF CO, AND TEMPERATURE ON DISSOCIATION CURVE. The oxygen affinities of haemoglobins are affected, sometimes greatly, by temperature, CO, and pH. An appreciation of the normal functional role of haemo- globin in a species is best obtained from the dissociation curve determined under conditions of temperature and CO, tensions actually existing in the animal and at its respiratory surfaces. In mammalian blood the addition of CO, reduces the affinity for oxygen, and shifts the dissociation curve to the right—the Bohr effect. At low CO, tensions, such as encountered in normal sea water, loading tension (f;) is lower than in the tissues, where CO, tensions are high. The Bohr effect therefore raises loading tension and facilitates unloading at sites where O, is required. The same results are achieved by varying the pH. The Bohr effect is quite pronounced in certain bloods (Figs. 4.17, 4.18). Haemoglobins of marine teleosts are very sensitive to changes in CO, tension and pH. Among invertebrates the O, affinities of Arenicola and Thalassema haemoglobins are increased by CO,; the blood of Urechis is not affected by variance of CO, or pH over wide physiological ranges. A rise in temperature also shifts the haemoglobin dissociation curve to the right, reducing affinity for O,. In poikilotherms (fish, marine inverte- 182 THE BIOLOGY OF MARINE ANIMALS brates), warming raises ¢,, and tf, values, promoting unloading in the tissues (7): Functioning of Haemoglobin in Marine Fishes. Marine fishes live in a fairly constant environment, where the O, tension is high (100-160 mm Hg), and the CO, tension generally less than | mm. The blood in passing through the gills is brought into equilibrium with the sea water in the branchial cavity. The haemoglobins of many teleosts have high values of t, and ¢, at the summer environmental temperatures to which the fish are normally exposed. There is some correlation between habits of the fish and haemo- globin characteristics. Active pelagic teleosts such as mackerel (Scomber), wo +t ttt Lee 90 ee ©) <0 eee | ee Oxyhaemoglobin (%) BB G oS S Jo 20 30 40 50 60 70 80 90 190 110 120 Oxygen Tension (mm Hq) Fic. 4.17. THE EFFECT OF CARBON DIOXIDE ON OXYGEN DISSOCIATION CURVES OF MACKEREL BLOoD (Scomber scombrus) AT 20°C Curve 1 for 1 mm CO,; curve 2 for 10 mm CO,; curve 3 for 25 mm CO, tension. (From Root, 1931.) which have bloods of relatively high oxygen capacity, possess haemo- globins with low oxygen affinities, whereas sluggish species such as the toadfish (Opsanus), with bloods of low oxygen capacities, have haemo- globins with high affinities. Other fish are intermediate in these respects, e.g. scup (Stenotomus). The Bohr effect is often quite pronounced, an increase of CO, raising loading tension and reducing oxygen affinity (Figs. 4.17, 4.18). There is, however, considerable interspecific variation in this respect, haemoglobins of sea-robin (Prionotus) and mackerel (Scomber), for example, being much more sensitive to CO, than tautog (Tautoga) and toadfish (Opsanus). Reduction of O, affinity with rise in acidity and CO, tension would be advantageous in promoting unloading RESPIRATION 183 in the tissues and, owing to the relative constancy of sea water, can but rarely hinder loading in nature. Elasmobranch haemoglobin is less affected by CO, than that of teleosts, but there is still a distinct acid effect within physiological ranges of CO, tension, namely 1-2 mm Hg. Haemoglobins of sharks possess higher oxygen affinities than in rays. In the skate Raja 66% of the oxygen is utilized or removed from the arter- ial blood in passing through the tissues (74, 103, 105, 132, 135, 136, 153). 2:0 04 0 6-0 6°5 7:0 5 8-0 Fic. 4.18. BoHR EFFECT ON HAEMOGLOBINS OF VARIOUS MARINE ANIMALS Ordinates, log p;) (O: pressure in mm Hg at which Hb is half saturated). Curves: 1: mackerel blood (Scomber scombrus), 25°C; 2: lamprey Petromyzon marinus, 20°C; 3: porpoise Phocaena phocaena, 38°C; 4: echiuroid Urechis caupo, 19°C; 5: lugworm Arenicola marina, 20°C; 6: skate Raja ocellata, 10-4°C (from various sources). Haemoglobin in Invertebrates. Although restricted in occurrence, the haemoglobins of marine invertebrates are of much theoretical interest and are deserving of detailed study. They are probably as variously adapted as those of vertebrates to their diverse functional roles, and further re- search on their physiology will be aided by correlated studies on oxygen consumption and ventilation of the species in question. Haemoglobin possesses strong affinity for carbon monoxide, which blocks it as an oxygen carrier. In Nereis diversicolor, treatment with CO 184 THE BIOLOGY OF MARINE ANIMALS reduces O, consumption by 50% at high O, tensions (6—7 c.c./l.), and stops O, consumption at a level of 3-4 c.c./l. Nereis, therefore, is dependent on Hb to a high degree for oxygen transport under normal conditions (92). Urechis caupo 1s a littoral echiuroid which inhabits a U-shaped burrow through which sea water is circulated. Oxygen capacities lie between 3-7 vols %, comparable to those of marine fishes, and oxygen affinity of the haemoglobin is fairly high (t, = 12 mm Hg). Urechis haemoglobin is little affected by changes in pH over the physiological range (pH 6-6-7°5). The critical tension, which in this species is 70 mm Hg (17°C), is probably determined by the dissociation characteristics of the blood (t; = 80-90 mm Hg). The haemoglobin of Urechis is almost completely saturated when well-aerated sea water is available, and only becomes operative in oxygen transport under conditions of relative anoxia. When there is a shortage of oxygen, it has been calculated that the oxygen requirements of the animal can be met for about 14 min by oxygen dissolved in the coelomic fluid and water of the hindgut (water-lung), whereas the oxygen- ated blood pigment would permit normal metabolism for another 55 min. This would be of value to the animal during rest periods between pumping, periods which may last as long as an hour. In addition, the burrows of Urechis are exposed during tidal ebb and the oxygen pressure in the sea water in the burrow falls to 14 mm Hg (0-06 vols %). At this pressure the haemoglobin of Urechis is 60% saturated, and functions effectively as an oxygen carrier. In Urechis, then, blood Hb subserves O, transport at low levels (to 14 mm Hg), and probably also provides an oxygen store during temporary periods of oxygen lack. Somewhat similar considerations apply to Arenicola marina. The Hb of Arenicola is completely saturated at 7 mm Hg and shows high affinity for oxygen (t,, about 2 mm Hg); there is likewise a pronounced Bohr effect (Fig. 4.18). Oxygen stores in the blood are not exhausted, therefore, until the positive pressure of O, in the tissues falls below t, level. During low tide the water in the animal’s burrow contains dissolved O, at a tension of 13 mm Hg, at which level the Hb is saturated (Fig. 4.19). The blood is depleted of O, on passing through the tissues, and on returning to the gills the pressure gradient of O, between the external medium and the venous blood permits the latter to be recharged with Og. The vascular Hb of Arenico/a thus functions as an O,-transporter during periods of tidal exposure when the O, supply in its burrow is depleted. A storage function is probably negligible (2, 91). Nephthys is a burrowing polychaete with both coelomic and vascular haemoglobins. The oxygen affinity of Nephthys Hb is low compared with that of Arenicola, and the tension of half-saturation is below the oxygen tension of interstitial water in its burrow during tidal ebb (Fig. 4.19). The worm has an O,-combining potential which would suffice its metabolic needs for only some 10 min, and the Hb is thus inadequate to serve as an oxygen store during exposure. It would seem that the pigment of Nephthys RESPIRATION 185 serves as a high-tension oxygen transport system only when the sand is covered by the sea, and the oxygen tension of water in its burrow is high (91). In Caudina and Anadara (Arca), with corpuscular haemoglobin, O, becomes available only at low tensions (8-10 mm). In two closely related holothurians, one, Cucumaria elongata, possesses Hb, while another, C. saxicola, lacks it. The former is a mud-dweller, the latter lives among rocks, and the presence or absence of Hb appears to be correlated with availability of oxygen. The possession of haemoglobin is exceptional in arthropods. Among entomostracans it is sometimes limited to parasitic species (parasitic 100 Arenicola Hb pH 75, 19°C Nephthys Vascular Hb 4° 15°C Cc 8 80 pH? q Nephthys Eero Hb 3 pH 7-4, 15°C 00 4 eo 43 40 Cc o c Sy) a 20 Oxygen Tension Oxygen Tension of Water of Interstitial in Arenicola Burrows | Water ! 1 0 o 10 1S 20 pie) Oxygen Tension (mm Hg) Fic. 4.19. COMPARISON OF OXYGEN DISSOCIATION CURVES OF POLYCHAETE HAEMOGLOBINS, NAMELY VASCULAR AND COELOMIC HAEMOGLOBIN OF Nephthys hombergi AND VASCULAR HAEMOGLOBIN OF Arenicola marina The vertical dotted lines above the abscissa show the levels of oxygen tension in the interstitial water from the sand in which Nephthys lives, and in the residual water of exposed Arenicola burrows (from Jones (91).) copepods, cirripedes). In the brine shrimp Artemia salina, haemoglobin functions in oxygen transport when the animals are living in concentrated brines (such media have low oxygen content, about one-third that of sea water when saturated with air). Treatment with CO, leading to formation of carboxyhaemoglobin, significantly reduces oxygen consumption (external medium, salinity 195%,, O, 2 c.c./l.). Artemia is an example of a species that gains or loses haemoglobin in response to low or high oxygen concentrations of the surrounding medium. The haemoglobins of Chirono- mus (tidal-pool and freshwater insects) have remarkably high oxygen affinities (t,<1 mm Hg), and function in oxygen-deficient environments (57, 66, 104). 186 THE BIOLOGY OF MARINE ANIMALS TABLE 4.13 RESPIRATORY CHARACTERISTICS OF BLOODS OF MARINE ANIMALS CONTAINING HAEMOGLOBIN O, Oo Res capacity, ee tymm| fmm | , coe Temp. se ae cells Hg mm Hg CO) ood Mammals Sea lion Eumetopias stelleri 19-8 67-0 40 -— 44 38 Seal Phoca vitulina 29-3 61-3 26 40 40 38 Porpoise Phocaena phocaena_ |19-7—22-2|55:4-62:2) 31 ca. 105 46 38 Dolphin Tursiops truncatus 19-1 = — = == = Dolphin T. tursio — 61-5 — — — — Blue and fin whales Balaenoptera 14-1 — — = — = Birds Murre Uria aalge 26:0 — — sa == = Puffin Fratercula arctica 24-0 — — = — = Surf scoter Melanitta perspicillata 22 47 — a -— —_ Reptiles—Marine turtles Chelonia mydas —_ — 19 65 DH 744255 Caretta caretta — =. 28:5 — pH. 7-43), 2335 Fish Eel Anguilla japonica 13:5 35 2 25 0 i A. anguilla == — 3 10 0-3 il Trout Salvelinus fontinalis 11-0-13-9 -— 17 43-5 1-2 15 Salmon Salmo salar 12:3 31-6 25 2) 1 15 Cod Gadus callarias ~ 6:5-7°8 39 15 S 0) Percentage Saturation Ww S 20 10 20 30 40 50 60 70 80 90 100 Oxygen Tension (mm Hg) Fic. 4.22. OxYGEN DISSOCIATION CURVES OF HAEMOCYANINS OF SEVERAL ARTHROPODS AND MOLLUSCS Limulus polyphemus, pH 7:35, 25°C; Busycon canaliculatum, pH 8:35, 25°C; Homarus americanus, pH 7:72, 25°C; Loligo vulgaris, 5:3 mm COs, 20°C (from various sources ). waters, and their haemocyanins function to the full in oxygen transport under such conditions (130, 169). The oxygen capacities of other haemocyanin-containing bloods are rather low, ranging from | to 3 vols % in gastropods, decapod crustaceans and Limulus. The dissociation curves are similar in shape to those of haemoglobins and are very diverse. On warming there is a marked increase in loading and unloading tensions, but the effects of acidity and CO, 190 THE BIOLOGY OF MARINE ANIMALS are variable. In the decapods Homarus, Cancer and Maia, haemocyanin shows minimal affinity for O, below pH 7, whereas in Busycon (conch) and Limulus (king crab) minimal affinity lies above neutrality. The normal reactions of all these bloods lie slightly above pH 7. The blood of Limulus appears reduced under normal conditions when drawn and contains about the same amount of O, as ordinary sea water. Oxygen capacities of Limulus and Busycon blood are 2-6 times that of sea water, indicating maximal amounts of O, which can be taken up under optimal conditions. In Busycon about 80% of the O, is utilized in passage through the tissues, and most of this O, is carried by haemocyanin. The increased oxygen affinity which attends rise in CO, may be of value in promoting loading in oxygen-deficient environments, but it is otherwise i cy a aa Ea RO Sees I ray ee ee (ceca ae) ee : aad s 60 % 40 3 eZ 2 | Cite |e & qa op 10 20 30 40 50 60 70 80 90 100 110 120 Oxygen Tension (mm Hg) FIG. 4.23. OXYGEN DISSOCIATION CURVES FOR HAEMOCYANIN IN BLOOD OF THE CUTTLEFISH Sepia officinalis AT 14°C Curves: 1; pH 7:97, CO,, 0-6 mm; 2: pH 7-85, CO,,-1-:95 mm; 3: pH 7-60; CO,, 2:8 mm; 4: pH 7:35, CO, 9°75 mm; 5: pH, 7:24, CO., 16-1 mm Hg. (From Wolvekamp et al. (169).) rather difficult to understand. Lobster haemocyanin, like that of Loligo, shows fairly high affinity for O, in the normal physiological range (t,, Panulirus 6:5 mm at 15°C). About 0-5 c.c. of oxygen is delivered to the tissues by 100 c.c. of blood, and haemocyanin accounts for 80-90% of the oxygen exchange (Panulirus). The Bohr effect appears to be of slight signific- ance in the blood of decapod crustacea (45, 100, 130, 131). TRANSPORT OF CARBON DIOXIDE Normally sea water contains about 4:8 vols°% CO, and fluid of similar composition could serve adequately as a nearer medium in many sluggish invertebrates. Sea water and body fluids in general contain a surplus of strong cations over strong anions, which combine with CO, to make up the alkali reserve. For such fluids carbon-dioxide dissociation curves have been obtained (Fig. 4.24). In vertebrate bloods buffering is provided by bicarbonates, phosphates, plasma proteins and haemoglobin. Among invertebrates the phosphate content of the blood is usually low, and high buffering capacity, when RESPIRATION 191 present, is due to blood protein. In the coelomic fluids of Ap/ysia and Echinus, for example, there is little protein and buffering is slight (cf. Table 2.12, p. 64). Protein content is low in Urechis plasma, but haemo- globiniferous corpuscles are present and buffering is due largely to the latter. The blood of Urechis absorbs CO, by chemical combination up to about 20 mm Hg, above which the dissociation curve becomes parallel to that for H,O and transport capacity is exhausted. In bloods containing haemocyanin most of the CO, is combined with this pigment, and such bloods show high buffering capacity. Haemolymphs of Palinurus, Limulus and Sepia, for example, contain from 10-20 vols of CO,. Usually marine 70 n So Carbon dioxide content (vols Yo) 0 10 20 30 40 50 60 70 80 90 100 Carbon dioxide tension(mm Hg) Fic. 4.24. CARBON DIOXIDE DISSOCIATION CURVES FOR SEA WATER AND DIFFERENT BLOODS Curves: 1: sea water; 2: Aplysia fasciata; 3: Octopus macropus; 4: Palinurus vulgaris; 5: Scomber scombrus; 6: Phoca vitulina. (From yarious sources.) gill-bearing animals are exposed to very low CO, tensions (CO, 0:23 mm Hg) and utilize the steep portion of their dissociation curves (49, 130, 134). The reaction CO, + H,O = H,CO, is relatively slow and is often catalysed by an enzyme, carbonic anhydrase, found in tissues and cells. Carbonic anhydrase occurs in vertebrate erythrocytes and in many invertebrate tissues. Variable, often high, concentrations are found in gills of fishes, especially in the pseudobranch; in the gills of Loligo, Limulus, Homarus and Libinia (absent, however, in Palinurus); in gills of some polychaetes; and in mantle tissue of lamellibranchs and gastropods (50, 63, 107, 146). In animals with calcareous shells and skeletons, e.g. bivalves, crustacea and starfish, the skeleton is an important source of buffer substance. 192 THE BIOLOGY OF MARINE ANIMALS During periods of anaerobiosis in lamellibranchs, when the valves are shut, the blood and mantle fluids show only a slight increase in acidity, while the CO, and calcium content increase greatly, along with CO,- binding capacity (Mercenaria, Mya, Ostrea). In Mercenaria mercenaria, for example, the CO, content increases from 6-150 vols % in the mantle fluid of animals kept in air for 15 days (Fig. 4.25). 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Bacteria break down waste and dissolved organic matter into forms which can be utilized as food by marine animals, and bacteria are themselves consumed by many animals (119). The background conditions of nutrition in the sea have been subjected to intensive study during the present century. These investigations have been concerned with the spatial distribution of phyto- and zooplankton, and with variations in abundance, both long-term and seasonal, of planktonic organisms. No less important has been the study of physical conditions—namely temperature, light and nutrient salts—which affect the growth of floating plants and, indirectly, the animals dependent upon them. A knowledge of these interrelated conditions is a necessary pre- requisite for a fuller understanding of factors regulating the abundance of marine organisms, the annual turnover of organic matter and the limits of economic exploitation of marine resources. For an extended treatment of this subject, the reader is referred to Harvey (43, 44) and Sverdrup, Johnson and Fleming (97). Flagellates, including both green and colourless species, contain various forms that bridge the gap between phototrophic and heterotrophic organ- isms. Certain flagellates are strictly autotrophic and can subsist on in- organic sources of nitrogen and carbon; others require one or more organic substances which they take from solution. Apart from these flagellates all animals are dependent upon a source of preformed carbohydrates, fats, proteins and certain other essential substances, i.e. they are holozoic (86, 90). The zooplankton is largely responsible for harvesting the phyto- plankton crop in the sea, and in turn provides food for other organisms. Herbivores predominate in the zooplankton and are generally of small size. Chief among them are copepods, especially nauplii and juvenile stages. In the southern oceans the large krill, Ewphausia superba, sometimes 202 NUTRITION AND FEEDING MECHANISMS 203 occurs in immense numbers, to the exclusion of all other planktonic animals, and smaller swarms of Nyctiphanes are sometimes encountered in the North Atlantic. Along with the permanent plankton occurs a small and variable proportion of larvae of bottom-dwelling forms, especially in shallow waters. Many of the medium-sized planktonic animals are omni- vores; the remainder, forming a small proportion of the community, are carnivores. These are generally of fairly large size, e.g. medusae, cteno- phores, arrow worms, Tomopteris, etc. Adding their depredations to the latter are pelagic fish and various bottom-feeding organisms, particularly in shallow water. At the top of the food chain are carnivorous fish, cephalopods, etc., which feed on the larger benthic and pelagic animals (39). The population density of zooplankton organisms in a unit volume of sea water provides a measure of their availability as food for other organisms. The average quantity of zooplankton existing below a large unit area of sea is maintained by the phytoplankton in the water below that unit area. The average quantity of organic matter below a unit area of sea surface provides a measure of the amount of food available for pelagic animals in different regions and at different times. Sampling the bottom biomass also reveals the quantity of food available for demersal animals. FEEDING MECHANISMS Marine animals display most diverse adaptations and specializations for obtaining food; in general, their feeding mechanisms are related to their nutritive habits. Autotrophic, saprozoic and some parasitic animals lack special feeding mechanisms and absorb dissolved substances directly from the medium. In holozoic animals it is possible to recognize definite types of feeding mechanisms determined by common features of the environment and the kinds of food available. Yonge’s classification (110) for invertebrates selects three main categories of feeding mechanisms— 1. Mechanisms for dealing with small particles. 2. Mechanisms for dealing with larger particles or masses. 3. Mechanisms for taking in fluid or soft tissues. Each of these categories includes a variety of different devices for dealing with particular kinds of foodstuffs. Many animals make use of a variety of feeding mechanisms, conjointly, or separately as occasion demands (117). METHODS OF UTILIZING SMALL PARTICLES Minute particles, including fine detritus, bacteria and plankton, form the sole nutriment of a great many invertebrates and protochordates. Mechan- isms for collecting fine particulate matter are classified as pseudopodial, ciliary, tentacular, mucoid and setous. The filtering apparatus is usually made more efficient by the secretion of mucus or other viscid matter. Selection of particles is usually quantitative, and there is seldom any mastication of the food. Jorgensen (52) has reviewed certain aspects of filter feeding among invertebrates. 204 THE BIOLOGY OF MARINE ANIMALS Pseudopodial Mechanisms Feeding by the use of pseudopodial processes is characteristic of rhizopods. In amoebae food is captured by pseudopodia which extend around the food particle and engulf it, together with a certain amount of water. The pseudopodia of foraminifers (Polythalamia) are long-branching and anastomosing filaments. The protoplasm in each filament shows active streaming movements, one stream being directed centrifugally, the other centripetally, and the actual length of a pseudopod is determined by the relative magnitude of the two currents. Food particles encountering the pseudopods adhere to their sticky surfaces, and are either carried directly towards the body with the return stream, or out to the tip and thence back to the body where they are ingested. Feeding is similar in radiolarians, which also entrap food particles on sticky pseudopodia and carry them into the body by protoplasmic streaming movements (Fig. 14.18, p. 612). Feeding in these groups is largely a passive affair, and depends on chance encounters with food particles. Radiolarians are predominantly floating forms which feed upon fine planktonic materials. The Foraminifera include both pelagic and benthic forms which feed upon fine plankton. Among colourless flagellates, some are saprophytic, others holozoic in feeding habits. Holozoic species are frequently amoeboid in habit, and ingest their prey by means of pseudopodia. More specialized is the feeding of Noctiluca, which catches small planktonic organisms on a sticky fila- ment. The food particles are aggregated into balls or strands, and conveyed to the cytostome where they are ingested (35, 46). Ciliary Mechanisms Ciliary methods of feeding are widespread, especially in sedentary species, and are encountered in various guises in many phyla. Cilia and flagella which primitively possessed a locomotory function, have in these forms become concerned with food-getting. Their activity sets up water currents which carry food particles to the animal, and they are further- more arranged in definite tracts by which captured material is sorted and transported either towards the mouth, if suitable, or to the region where ejection takes place. Frequently, a viscid mucous sheet is secreted for trapping the particles. Many ciliary feeders appear to exercise mechanical rather than chemical selection of food particles, selecting or rejecting on the basis of particle-size. Protozoa. In ciliates (Infusoria) the cilia are frequently organized in complex patterns. Some are mouthless endoparasites (Astomata), in- habiting the gut of invertebrates; some of the holotrichs are raptorial. Many species, however, produce a feeding current by means of complex ciliary apparatuses in the cytopharynx, which is a funnel extending into the endoplasm. Food particles which gather in the bottom of the pharynx become incorporated in food vacuoles and are engulfed by the endoplasm. NUTRITION AND FEEDING MECHANISMS 205 In particle feeders the cytopharynx is approached by a spiral groove, the peristome, which leads from the anterior end to the pharyngeal opening or cytostome. The peristome, like the pharynx, is ciliated. The ciliary pattern is fairly simple in holotrichs (Paramecium, Colpoda); but in higher forms the cilia on the outer edge of the peristome are fused so as to form a row of cirri or membranelles, the adoral wreath, which produces a powerful current. A few characteristic marine forms from different orders may be mentioned. Bottle animalcules Folliculina (Heterotricha) have the peris- tomial area enlarged into paired wing-like extensions. In planktonic tintinnids (Oligotricha) the peristome bearing the adoral wreath and mouth opening occupies the anterior end of the body. In sessile peritrichs such as Vorticella the peristome forms a disc at the anterior end. The peristome bears a spiral groove, which is provided with cilia in the form of an undulating membrane and which descends to the vestibule (chamber leading into the cytopharynx in these forms). Free-living ciliates ingest bacteria, algal cells, diatoms and other protozoa, and probably a certain amount of detrital matter. Littoral sand-dwelling holotrichs appear to derive nourishment from diatoms. In peritrichs the formation and course of food vacuoles have been described in some detail. Food vacuoles, containing bacteria, are constricted off from an oesophageal sac at the bottom of the cytopharynx. This vacuole then passes rapidly on a fixed course through the cytoplasm, while its contents are undergoing digestion, and returns to an anal spot on the wall of the vestibule for discharge (48, 71). ; Porifera. Among sponges the feeding water cur- ie oe pelos : 8 acea. (From Min- rent is created by the lashing of flagella on collar chin; 1969.) cells or choanocytes which line internal cavities (Figs. 5.1, 5.2). The surface of the sponge is perforated by dermal pores like a sieve, and water drawn through these openings flows past the collar cells. The choanocytes beat in an independent and unco-ordinated manner, and as the water currents sweep past the cell bodies, suspended food particles are caught and ingested in an area near the base of the collar. The simplest type of sponge structure, the ascanoid type, is realized in Olynthus, a fleeting stage in the development of calcareous sponges. It is a hollow vase-shaped structure in which the internal paragastral cavity is lined with choanocytes. The pores in the body wall are surrounded by porocytes, capable of constriction; the osculum or terminal exhalant opening of the body cavity can also be closed by the action of myocytes. The ascanoid type is found modified to some extent in the calcareous sponges Clathrina and Leucosolenia, which are colonies made up of branch- ing ascon tubes, In the syconoid type, found in Sycon and Grantia (Cal- Fic. 5.1. COLLAR CELLS OF A SPONGE, 206 THE BIOLOGY OF MARINE ANIMALS carea), the choanocytes lie in elongated radial canals lying at right angles to the internal surface. Water currents enter incurrent canals via ostia, pass through prosopyles (pores) into the flagellated chambers and from thence into the central cavity or paragaster, which opens to the exterior Epidermis Fic. 5.2. SECTION THROUGH THE BODY WALL OF A CALCAREOUS SPONGE Leuconia aspera. (From Vosmaer, 1880.) through an osculum. Most sponges show a leuconoid type of structure in which the choanocytes are restricted to small chambers which communi- cate with the paragaster by exhalant canals. The hydraulics of this system have been investigated in Leuconia, a calcareous sponge of leuconoid type, from which the following measure- ments were obtained. Estimated Aggregate Velocity Structure number area (cm?) (cm/sec) Afferent canals 81,000 4:2 0-1 Surface of choanocytes — 200 0-001 Flagellated chambers 2,250,000 52 0-01 Efferent canals 5,200 25 0-2 Paragaster 1 O21 2 Osculum 1 0-03 8-5 (diameter 0-20 cm) The total pressure involved in this flow varied from | to 3 mm of water. It appears from these calculations that the maximal surface area and slowest rate of flow are found in the flagellated chambers, especially at the surface of the choanocytes, and this will permit maximal opportunity NUTRITION AND FEEDING MECHANISMS 207 for capturing food particles. The greatest velocity, on the other hand, is reached at the osculum, from which a jet of water is thrown upwards away from the sponge. Waste materials, released by the sponge, are carried upwards in this jet and then enter a slow circular eddy returning to the sponge. The farther the exhalant current goes, the greater will be the opportunity for waste materials to be dissipated and for water entering the sponge to be renewed by diffusion, or water currents. The size of the osculum determines the pumping rate and the velocity with which the water is expelled. These are conflicting demands which counterbalance each other, and for any particular sponge there is found to be an optimal oscular diameter, which is proportional to the square root of volume of the sponge. The leuconoid type of structure confers certain advantages, which become most apparent in sponges of large size and those living in quiet water. The increase in number of choanocytes brought about by folding of the walls of the sponge results in greater current flow, and frictional resistance is minimized by the smooth surface and absence of choanocytes in afferent and efferent channels. Pumping rates in sponges and other ciliary feeders are summarized in Table 4.5 (p. 169) (10, 48, 90). Coelenterates. The coelenteron in these animals is usually ciliated, and in certain forms cilia are also employed in transporting food to the mouth. In anemones the cilia may be widespread over the ectoderm (Protanthea) or limited to disc, tentacles and stomodaeum (Metridium). These ciliated areas transport food particles to the stomach. Likewise in many corals the general ectoderm participates in the capture of food, and ciliary tracts transport particles to the mouth. Normally the cilia on the disc beat out- wards, but in the presence of nutritive substances the direction of ciliary beat is reversed and the mucus is drawn inwards. In the scyphomedusa Aurelia we also find that cilia on the bell and ventral surface of the arms are concerned with transporting food particles to the mouth (91). Polychaetes. The tubicolous polychaetes for the most part are plankton or detritus feeders and make use of ciliary mechanisms for obtaining food. Detritus feeders such as terebellids and ampharetids are provided with long mobile tentacles which extend over the surface of the substratum. Food particles (detritus, small organisms) are caught in mucus and trans- ported along a ciliated groove in each tentacle towards the mouth (Fig. 4.2, p. 139). Small particles are moved by cilia, larger ones by muscular action (terebellids). Flaps about the mouth in Cirratulus exercise a selective function and sort out the edible particles. Other forms, such as Sternaspis and Pectinaria, collect detritus beneath the surface (18a). Filter-feeding is carried out by a variety of ciliary mechanisms in chaetopterids, sabellariids, sabellids and serpulids. In Chaetopterus water is propelled through the U-shaped tube by rhythmical beating of parapodial fans (Figs. 5.3, 5.4). Copious quantities of mucus secreted by the aliform notopodia entangle food particles suspended in the water current. Strings of mucus, bearing food material, are carried in ciliated grooves on the aliform notopodia to a median ciliated groove on the dorsal body surface, 208 THE BIOLOGY OF MARINE ANIMALS and thence to the buccal funnel. MacGinitie has also described an altern- ative mode of feeding by formation of a mucus-bag. This is produced by the aliform notopodia and extends posteriorly to a dorsal cup organ where it is rolled up. All the water which flows through the burrow must traverse this net, which filters out suspended food matter. At intervals the front margin is detached and the net is transported anteriorly to the mouth by Fic. 5.3. MUCUS-BAG FEEDING IN A TUBICOLOUS POLYCHAETE Chaetopterus variopedatus. (From MacGinitie, 1939.) Fic. 5.4. ExTRACT FROM AN IRRIGATION RECORD OF Chaetopterus variopedatus (DURATION 4 HOURS) The worm was carrying out mucus-bag feeding and pumping steadily, except for brief pauses at the peaks when the lever returns to null position. The frequency is about 1 fan stroke per sec, too fast for individual strokes to be distinguished on this record. At the pauses, which occur every 18 min, a mucus-bag is passed forwards to the mouth and swallowed. (From Wells and Dales (103).) reversal of ciliary beat in the dorsal groove. The mucus-bag forms a very efficient straining apparatus, and by feeding proteins of different particle sizes it has been calculated that the mesh openings are about 40 A in diameter (63, 65, 103). In cryptocephalous polychaetes exemplified by Sabella, feeding 1s carried out by a branchial crown consisting of a circlet of tentacles (Figs. 4.2, 15.9). These bear lateral pinnules which are ciliated and form a filtering apparatus. Water is drawn into the branchial funnel by abfrontal cilia on NUTRITION AND FEEDING MECHANISMS 209 the pinnules and is directed between the latter by latero-frontal cilia (Fig. 5.5). Suspended particles are thrown by the latero-frontal cilia into a ciliated groove running along the inner face of each pinnule, whence they are transported by frontal cilia to the base of the pinnule where there are ciliary tracts running along the filament towards the base of the crown. Here there is a ciliary mechanism in the basal folds by which particles are sorted into three classes on the basis of size: the largest particles are carried to the palps and rejected; medium-sized particles are carried to ventral sacs below the mouth, and are stored there for use in Abfrontal Fic. 5.5. FILTERING AND SORTING MECHANISMS IN THE BRANCHIAL CROWN OF Sabella pavonina (a) Section through two gill filaments, showing direction of flow of water across the filament, and direction of beat of the cilia which produce the current; (5) transverse section through a pinnule to show ciliation (x 250); (c) lateral lip and base of gill filament, showing ciliated tracts and sizes of particles passing along the basal folds; (d) section through basal folds, showing sorting of particles. (From E. A. T. Nicol (80).) tube formation (p. 656); the finest are carried to the mouth and ingested (80). Sabellids and serpulids depend on finely divided suspended detritus for food. Some data for filtering rates are listed in Table 4.5 (18d). Echinodermata. Many asteroids such as Porania and Astropecten make use of ciliary feeding currents to some extent. Ciliary mechanisms are universal in crinoids, in which the ambulacral grooves possess mucous glands and cilia and carry food particles (plankton and detritus) to the mouth. Polyzoa and Phoronidea. Polyzoans capture plankton and fine detritus by means of a lophophore or ring of ciliated tentacles surrounding the mouth (Fig. 5.6). In ectoprocts such as Flustrella the extended tentacles 210 THE BIOLOGY OF MARINE ANIMALS form a funnel with the mouth at the base. A water current is produced by the beating of lateral cilia on the tentacles, and proceeds straight down the lophophore and outwards between the ten- I tacles. Suspended food particles arriving at the bottom of the funnel are sucked in J\ by the muscular pharynx, which is also provided with inward-beating cilia. J\ In endoprocts a rather different ciliary mechanism prevails (Fig. 5.7). Water is drawn into the tentacular funnel, again by the action of long lateral cilia on the ten- tacles, but the current proceeds from the outside between the tentacles and upwards away from the animal. Suspended particles ) in the water passing between the tentacles Fic. 5.6. FEEDING CURRENTS are thrown by the lateral cilia on to the IN AN ECTOPROCT POLYZOAN inner surface whence they are carried by This is a longitudinal section short frontal cilia towards the base. Here through the tentacular crown. they reach a ciliated vestibular groove Arrows indicate direction of feed- Which leads to the mouth. The lateral cilia, ing currents produced by action of BGth Galecta t a d te eat lateral cilia. (From Atkins, 1932.) | et aie haan oaths de intermittently. In Loxosomella, for ex- ample, all the lateral cilia may suddenly become motionless, and activity is subsequently resumed in a somewhat irregular manner. It may be that the lateral cilia are subject to nervous regulation (5a). The lophophore of Phoronis is similarly employed in feeding. The cilia on the two sides of the tentacles beat in opposite directions, towards and away from the mouth. An inhalant current passes downwards between the two circles of tentacles forming the lophophore and outwards between the tentacles. Particles are carried by tentacular cilia towards the mouth and, if not ingested, they are borne outwards by distally-beating cilia to the tips of the tentacles, where they drop off. Phoronids feed on fine plankton and detritus. Brachiopods. These animals are provided with an internal ciliated lophophore, bearing fringes of tentacles, which forms a complicated filtering apparatus. In Crania, the lophophore divides the mantle cavity into a lower inhalant and an upper exhalant chamber (Fig. 5.8). An inhalant current enters on either side and is drawn upward through the superposed whorls of the lophophore, which bear a double row of filtering filaments. Currents are created by lateral cilia on the lophophore filaments, body of the lophophore and mantle. Heavy particles drop down on the lower mantle surface and are rejected by ciliary action, whereas the finer particles become entangled in mucus on the filaments. These trapped food particles are carried by frontal cilia to a buccal groove at the base of the filaments, and are thence conveyed by a strong ciliary current to the mouth. After passing through the lophophore the water currents become NUTRITION AND FEEDING MECHANISMS 21 confluent, and are directed to the exterior as a single dorsal exhalant current. In another brachiopod, Neothyris, the mechanism is somewhat different. Two lateral arms of the lophophore lead into a spiral confluent chamber, enclosed by tentacles. On either side of the gape the tentacles are spread apart so as to form inhalant openings (Fig. 5.9). Through the latter a stream enters the inhalant chamber formed by the opposed tentacles of the lophophore, and escapes into the exhalant chamber by passing between the filaments of the spiral arm. Water currents are created by beating of cilia lining the exhalant chamber and spiral arm. Heavy part- icles in the food stream are forced past the filaments into the exhalant Rejection Rejection current current Diaphragm carryin vestibular —Ventra! lip of mouth Fic. 5.7. CILIARY FEEDING IN AN ENboproctT Loxosomella (a) Lateral view of animal, showing direction of feeding currents; (4) ciliary currents, and direction of beat of lateral cilia on tentacles (tiny arrows) (from Atkins, 1932.) chamber, while light particles accumulate on the brachial membrane and are thrown into a brachial groove by the action of cilia on the brachial lip. From the groove the food particles are carried to the mouth. Selection of particles, accordingly, is on the basis of size and weight (87). Molluscs. Ciliary feeding devices are highly developed in lamellibranchs and certain gastropods, in which they have independently evolved. They have been intensively studied, especially in lamellibranchs, and for detailed accounts the reader is referred to papers by Orton, Yonge and Atkins (see 117). Ciliary feeding in prosobranch gastropods has developed independently in at least six families. Of these the Calyptraeidae (Calyptraea and Crepidula) are the most specialized, and feed by sifting out diatoms and other fine plankton material by means of a modified ctenidium. In the sedentary slipper-limpet Crepidula fornicata, the mantle cavity is divided into two lateral chambers, one ventro-lateral, the other dorso-lateral, by a Ing6ing current Exhalant chamber { = —— Inhalant chamber (h) Base of gill filament zy PSO oe — THE ae as “aN Ce) RRR Buccal lip ‘Inner edge of lophophore Fic. 5.8. FEEDING IN BRACHIOPOD Crania (a) View of feeding animal. (b) Diagram of currents in the mantle cavity (longitudinal section through animal). Larger arrows show the course of the main currents; dotted arrows below represent the course of larger particles falling out of the main stream. (c) Section through lophophore to show ciliary currents: 1, direction of beat of current- producing cilia on gill filament; 2, direction of beat of frontal cilia. (From Orton, 1914.) NUTRITION AND FEEDING MECHANISMS PAI) long filamentous gill. The gill filaments are attached to the left side of the mantle cavity and extend across it so as to form a continuous membrane. On the sides of the filaments are lateral cilia which create a water current; this current enters the ventral chamber, passes through the gill, and leaves the dorsal chamber as an exhalant stream (Fig. 5.10A). The current entering the mantle cavity passes through a mucus-filter produced by a filter gland. This filter strains out mainly the larger particles, which are carried towards a food pouch near the mouth. Here the particles Brachial Mouth Valve Oral disc region Brachial membrane Fic. 5.9. LOPHOPHORE AND FEEDING CURRENTS OF THE BRACHIOPOD Neothyris lenticularis View with pedicle valve removed. The filaments have been cut from one side of the spiral arm and the dorsal region of a lateral arm. (From Richards (87).) are worked up into mucous pellets to be eaten or rejected. The gill filaments are covered by mucus, which is secreted by an endostyle at their base. Besides lateral cilia, the filaments bear frontal and abfrontal cilia which beat towards their tips. Fine particles in the inhalant stream are caught on the gill filaments, and are transported by frontal cilia across the ventral surface of the gill to a food channel. This is a ciliated groove placed in the left side of the mantle cavity and roofed above by the tips of the gill filaments. In it the collected food particles, embedded in mucous strings, are carried forwards towards the mouth where they are seized by the radula and ingested. The method of filtering is essentially the same in other ciliary-feeding prosobranchs. Mucus used for entangling food particles is secreted by a 214 THE BIOLOGY OF MARINE ANIMALS pedal gland in Vermetus, and by the propodium in Capulus. The latter structure is an anterior extension of the foot, to which food particles are conveyed from the mantle cavity and from which they are collected by a grooved proboscis. An interesting variation is the development in Stephopoma of a sweeping mechanism for supplementing collection of particles within the mantle cavity. The long anterior filaments of the gill are extended and drawn through the water like a sweep-net, and any particles encountered are trapped in the mucus which coats the filaments (522 76,1105 115). Shelled pteropods (opisthobranchs) are another group of gastropods making use of ciliary feeding (Fig. 5.108). In Limacina, for example, Food pouch Outward tract Endostyle’ Ciliated and ciliated field groove Visceral mass V:sceral Mass Shel| B Fic. 5.10A. FEEDING MECHANISM OF THE SLIPPER-LIMPET Crepidula fornicata Animal removed from its shell and mantle turned over to the left. From the endostyle at the base of the gill, mucus and food particles are lashed on to the gill filaments. (From Orton, 1914.) Fic. 5.10B. FEEDING MECHANISM IN A THECOSOMATOUS PTEROPOD Cavolinia in- flexa (x 34) Arrows on wings indicate directions of ciliary currents. (After Yonge, 1926.) feeding currents are created by the beating of cilia in the mantle cavity. Food particles falling out of the stream become entangled in mucus and are carried by ciliary tracts to the mouth. Some collecting is also carried out by the foot. Pallial feeding has been lost in higher pteropods, and feeding is carried out by ciliated fields on the wings and lateral and median lobes of the foot (Cavolinia, Cymbulia). These animals feed on smaller members of the plankton, e.g. diatoms, protozoa, crustacean larvae, etc. (70, 77). Lamellibranchs. The majority of lamellibranchs feed on particulate matter, which they filter by means of ciliary mechanisms on gills and labial palps. The ctenidia typically take the form of folds of the body wall, which are suspended in the mantle cavity and which divide the latter into two chambers (Figs. 4.4, 5.11). Water currents created by cilia on the gills enter the ventral or inhalant chamber, pass through slits in the gills and NUTRITION AND FEEDING MECHANISMS O¢ ES) proceed posteriorly as an exhalant stream in the dorsal or exhalant chamber. As the current slackens on entering the large inhalant chamber, coarse particles fall out of the stream on to the mantle surface. Fine particles become entangled in a mucus-sheet covering the gill surface. The filtered particles are then carried to food grooves at the bases and free margins of the gills and thence towards the mouth. In the majority of lamellibranchs (filibranchs, eulamellibranchs) the gills are responsible for collecting food particles. Some species, e.g. Ostrea and Pecten, are without siphons for regulating the passage of water currents. In Ostrea the inhalant current is restricted to a ventral area; in Pecten water is drawn into the mantle cavity along the whole ventral and part of the anterior region, but chiefly in two restricted areas (ventral and anterior). Water leaves the body posteriorly in an exhalant Fic. 5.11. DIAGRAMS SHOWING THE ORGANIZATION OF THE MANTLE CAVITY IN (a) A PROTOBRANCH Nucula, AND (b) A TYPICAL LAMELLIBRANCH SUCH AS Ostrea Transverse view across the body, with exhalant chamber above, and inhalant chamber below the gills. Direction of water currents indicated by arrows passing through the gills; direction of food streams by arrows on surface of gills; forwardly directed streams to mouth indicated by x. (From Yonge, 1928.) region (Fig. 5.12 (4)). In many other lamellibranchs, in- and excurrents enter and leave the mantle cavity by way of siphons at the posterior end of the body, elsewhere the mantle folds being largely fused. Water currents are created by the lashing of lateral cilia on gill filaments or leaflets, as illustrated in Fig. 5.13. In protobranchs the gills consist of a series of flat leaflets on either side of the body. In other lamellibranchs the gills on either side usually consist of two lamellae or demibranchs, each of which is made up of a series of ascending and descending filaments united at their free extremities. The lateral cilia cause a current to flow through the narrow slits between the gill filaments or leaflets, from the inhalant into the exhalant chamber. On the outer surface of the filaments are frontal cilia which are chiefly responsible for collecting food particles. In proto- branchs the frontal cilia transport particles to the mid-line, whence they Siphonal \ tentacle (a) See Exhalant ne 4 Mouth chamber \:-:-. eee Gill fe ii Incurrent chamber Left mantle fold Fic. 5.12. FEEDING CURRENTS IN LAMELLIBRANCHS (a) A protobranch Yoldia, with right valve and mantle fold removed. Arrows added to indicate directions of ciliary currents. (b) Ostrea, right valve and mantle fold removed. Within the shell plain arrows denote ingoing currents, and feathered arrows outgoing currents. ((a) after Kellog, 1915; (4) after Yonge, 1926.) tatero— frontal c tateral cilia fC ‘cilia Vg & YANN ASK Ca, (ZS CELE CEERAB RATT CE MMII Henn teat es toa Food Gr Stroove Gee i © © ©) ws @ Cree O = ae Lateral (2) Fic. 5.13. (a) LATERAL VIEW OF A GILL FILAMENT OF Mytilus edulis. LC, FC, DIRECTIONS IN WHICH LATERAL CILIA AND FRONTAL CILIA LASH. (6) GILL FILA- MENT OF Heteranomia squamula IN TRANSVERSE SECTION. ((a) from Orton, 1912; (b) from Atkins, 1936.) NUTRITION AND FEEDING MECHANISMS PA are conveyed anteriorly in two streams towards the mouth. In other lamellibranchs the frontal cilia usually beat towards the lower free margin of the lamellae where there is a food groove in which particles move anteriorly towards the mouth (Figs. 5.13, 5.14). In addition, the gill filaments of many lamellibranchs possess large latero-frontal cilia, which strain the feeding currents and throw particles on to the frontal face of the filaments where they come under the influence of the frontal cilia. A certain amount of mechanical selection of food particles takes place on the gills. In Pecten the gill lamellae are thrown into folds, and the frontal Gill axis . ‘| Inner demi branch Fic. 5.14. TRANSVERSE SECTION THROUGH GILL FILAMENTS OF Preria hirundo Solid circles, orally directed currents. The arrows indicate directions of currents created by frontal cilia: ventral currents are due to coarse cilia; dorsal currents to fine cilia. (From Atkins, 1936.) cilia in the grooves between the folds beat towards the base of the lamellae, where there are forwardly-directed ciliary tracts. On the ridges of the folds the frontal cilia beat normally towards the free extremities of the filaments, again provided with ciliated tracts. But when particulate matter becomes very heavy it evokes a profuse secretion of mucus, and the mucous strings which result are conveyed by frontal cilia on the ridges to the ventral ends of the lamellae, to be dropped off on the mantle wall. Muscular movements of the gills are also used in getting rid of large or irritant particles. Such movements cause the gill grooves in Ostrea and Pecten to contract, and throw their contained particles upon the ridges, whence they drop upon the mantle surface. Heavy particles also tend to fall out of the ventral food grooves upon the surface of the mantle, whence they are removed. 218 THE BIOLOGY OF MARINE ANIMALS In protobranch bivalves the gills are relatively small and function chiefly in the creation of a water current, food being collected principally by the large labial palps (Fig. 5.12 (a)). These are provided with a long appendage or proboscis which is grooved and ciliated; as this moves over the substra- tum it collects fine particles which are carried along the ciliated groove to the base. Here they are subjected to sorting by the palp lamellae, suitable material being carried by cilia to the mouth. In other lamellibranchs the labial palps sort out food material passed to them from the gills. There is a pair of palps in front of the gills on either side of the mouth. The inner surface of the palps is ridged and ciliated, and conveys food particles from the ciliated tracts on the demibranchs to the mouth (Fig. 5.15). The ciliation of the palps is complicated, the whole Upper margin of palps RAR Ze SS Proximal oral groove Lateral oral groove Fic. 5.15. VIEW OF LABIAL PALPS AND JUNCTION WITH THE GILLS IN THE OYSTER Ostrea edulis (x 5%) Arrows indicate directions taken by particles; XY, point where material is rejected from the palps. (From Yonge, 1926.) forming an extremely efficient sorting mechanism. A tract of especially large cilia on each labial fold drives small particles across the palps towards the mouth. Large particles or mucous masses tend to be drawn down into furrows on the palps by other ciliary tracts. The cilia in the furrows beat towards the upper or anterior margins of the palps, where there are pos- teriorly directed ciliated tracts carrying material to the mantle to be rejected. Muscular action is also of importance in the degree of sorting which takes place, retraction of the palps, for example, opening up the furrows and causing particles to fall into the outwardly directed ciliary tracts. Habits and modes of feeding of lamellibranchs are many and varied. Most are dependent upon fine detritus, bacteria and plankton. Mytilus and Ostrea filter particles down to a lower limit of about 2u. The optimal particle size for most efficient filtering is 7-8u in Mytilus, and particles NUTRITION AND FEEDING MECHANISMS 219 of that size can be completely retained or let through at will. An estimated diet sheet for bivalves (Jivela, Mytilus) is given in Table 5.1, and data for filtering rates are listed in Tables 4.5 and 4.6 (p. 169). Quantities of water pumped by bivalves are surprisingly large, up to 34 l./hour in the oyster, for example. Under suitable conditions the oyster feeds almost continuously, and stomachs are nearly always found to contain 100d) (6; 7a, 17,28, 52, 58,.59,-77a, 97a, 110, 114,116). TABLE 3.1 MATERIALS INGESTED BY CILIARY-FEEDING LAMELLIBRANCHS Digested Ingested Dead Living (as detritus) a. Phytoplankton 1. Dinoflagellates, 32,000 per 1., 12 g* 2. Diatoms, 32,000 per 1., 1:5 g* 3. Bacteria, 45,000,000 per 1., 0-05 g* 4. Other unicellular and multicellular algae 5. Zoospores and other reproductive cells b. Zooplankton and Nekton 1. Flagellates 2. Rhizopods and ciliates (including tintinnids) 3. Reproductive products of invertebrates Ova and larvae Spermatozoa 4. Invertebrates and vertebrates c. Benthic algae and eelgrass — — d. Benthic invertebrates a = e. Inorganic and refractory 0 I+ +++ I+ | EI+ | ++ lerelare -| {- | al | | ott+t+ +4 +4444 I+ Sign + indicates generally; -- commonly, but only small individuals or species; — rarely or not at all. * Quantity of dry organic matter potentially ingested by one adult animal per year. Based on computa- tions for a single mussel (Mytilus californianus) 70 mm in length, filtering 2:5 1. of water per hour. (From Coe (17).) Protochordates. Besides the groups we have just examined, ciliary feeding mechanisms are widely utilized by protochordates. Although pharyngeal gills are often involved, considerable diversity exists in the functional patterns concerned with feeding in these forms. Pharyngeal gill-bars form a straining apparatus in Amphioxus and other cephalochordates. These animals lie buried in sand, with the oral region at the surface, and draw a feeding current into the mouth, over the gill- bars and out through the atrium. Large particles are strained off by buccal cirri and smaller particles by the gills (Fig. 5.16). The feeding current is created by lateral cilia on the pharyngeal bars, where particles become entangled in mucus secreted by the ventral endostyle, and these mucous masses are driven dorsally by frontal cilia on the pharyngeal surfaces of the gill-bars. In the dorsal wall of the pharynx the food material enters a dorsal ciliated groove and is moved posteriorly to the oesophagus and stomach. There is also some minor collecting of food in the buccal cavity 220 THE BIOLOGY OF MARINE ANIMALS by the ciliated wheel organ and Hatschek’s pit, where particles become entangled in mucus and whence they are carried to the dorsal groove via peripharyngeal ciliated bands (22). In balanoglossids the anterior end of the body consists of a proboscis and collar which together form a simple ciliary mechanism for collecting particulate matter and detritus. The proboscis secretes mucus which traps particles and the resultant mucous strings are moved towards the mouth under the impetus of cilia on the proboscis. A respiratory current created by pharyngeal cilia assists in drawing in the mucous strings. Most entero- pneusts are burrowers(e.g. Saccoglossus, Balanoglossus), inhabiting muddy sand, and appear to ingest the surrounding soil for the sake of contained organic matter without much selection. Some species are said to protrude Gill bar Velum Fic. 5.16. ORAL Hoop AND ANTERIOR REGION OF AMPHIOXUS, SHOWING FEEDING CURRENT (INDICATED BY ARROWS). PARTICLES OF DETRITUS ARE TRAPPED ON THE ORAL CirrRI. (From Dennell (22).) their proboscis from the burrow on occasion, and gather up detritus from the surface of the substratum. In any event, the gut of freshly collected animals is swollen with sand and the casts consist largely of sand grains. The pharynx is perforated by gill slits and the pharyngeal bars bear strongly developed lateral cilia which create an outwardly directed respiratory and feeding current. Particles are prevented from passing through the gill-slits by the action of a sorting mechanism dependent upon latero-frontal cilia, which carry such particles ventrally into the lumen of the pharynx. Ciliary tracts lining the pharynx move particulate matter and mucous strings posteriorly into the oesophagus, while surplus water is strained off by the gill-slits and oesophageal pores (8, 54). In Cephalodiscus all parts of the body and stalk are covered with cilia which transport food particles to the arms. These bear short cilia which carry some particles distally to be discharged, and they are also provided with a broad and shallow groove containing longer cilia which move food NUTRITION AND FEEDING MECHANISMS 221 particles basally towards the mouth. Tentacles arising from the arms are ciliated as well, and exhibit active jerking movements which may be concerned with procuring and selecting food particles (31). Ascidians are provided with a branchial sac perforated by many small apertures. By the beating of cilia on the sides of the gill-bars, sea water is drawn into the branchial siphon and passes through the fine meshwork of the pharyngeal basket on which fine particles are caught. These food particles are collected by cilia on the pharyngeal surface of the gill-bars and on their papillae, and are carried across the branchial surface towards the dorsal lamina. In simple ascidians this process is aided by transverse waving of the longitudinal bars. The filtering process is assisted by the production of mucus-sheets, which are secreted by the endostyle and transferred to the inner surface of the pharynx by endostylar cilia. On reaching the dorsal lamina the food-laden mucous masses are carried towards the posterior end of the branchial cavity and thence to the oesophagus. Ascidians feed on plankton and detritus. On exposed shores the food consists almost entirely of plankton, often enriched by a considerable quantity of gametes and algal spores. But in protected waters and estuaries it is made up largely of suspended matter and detritus, together with some unicellular algae. Ciona intestinalis is capable of filtering off particles down to 1—2u in size, and this is in large part due to the fine porosity of the mucus-sheets produced, since when mucus secretion is in abeyance, small particles of this size are no longer retained. In solitary ascidians ciliary activity is augmented by rhythmical squirting, caused by quick contractions of the body wall. The steady ciliary current of Phallusia mammillata amounts to 60 c.c. per hour, while spontaneous squirting moves some 300 c.c. of sea water per hour in a fasting animal. In a hungry animal, rhythmic squirting increases greatly in frequency, whereas the addition of food restores the frequency to normal (Fig. 5.17). Squirting renews the sea water in branchial and atrial chambers, brings a fresh lot of food particles into the branchial sac and periodically renews the water about the animal (47, 52, 64). In thaliacians (Sal/pa, Doliolum) the endostyle secretes a mucous net of entangling threads into the pharyngeal cavity. Food particles caught in the mucus are transported to the dorsal pharynx by peripharyngeal cilia, and thence to the oesophagus by other ciliary tracts. In doliolids the through- current is produced by cilia of the branchial stigmata; in salps, by rhythmic contractions of circular muscles of the body wall (13, 52). The small appendicularians (Larvacea) have a most remarkable method of feeding. These animals are pelagic in habit and dwell in a gelatinous house which is secreted by the animal and forsaken from time to time, when a new one is formed (Fig. 5.18). The house is essentially a filtering appara- tus for straining off fine particles and nannoplankton on which the animal depends for food. Water currents, created by lashing of the animal’s tail, enter through a pair of dorsal funnels provided with a fine grating for BD THE BIOLOGY OF MARINE ANIMALS excluding larger particles (> 30u in Oikopleura). Within the cavity of the house is an elaborate collecting apparatus containing paired wings divided into dorsal and ventral chambers. Water enters the ventral division on each Fic. 5.17. SPONTANEOUS SQUIRTING BY AN ASCIDIAN Phallusia mammillata The animal was starved for 14 hours, and given food (flagellate culture) continuously for 6 hours. Food added at first arrow, and stopped at second arrow. Time scale in hours. (From Hoyle (47).) Tube t Septa Houth Coarse | i/ter Exit Feeding \ fil Bie Tail Body Fic. 5.18. Oikopleura albicans IN 1TS HOUSE The arrows show the feeding currents. (From Lohmann.) side, and on passing through the dorsal division it is strained of all suspended matter by many fine septa. This collected food material is sucked into the pharynx by ciliary action. When the pressure inside the ~ house rises sufficiently, it forces open a spring door at the posterior end. Ejection of water through this opening propels the animal forward, NUTRITION AND FEEDING MECHANISMS 223 but the main function of the house and water current is feeding and not locomotion. The house quickly becomes clogged with particles and is abandoned in the matter of a few hours; a new one is constructed in 15-30 min. CILIARY FEEDING BY LARVAE. Free-swimming planktonic larvae of many animals feed by ciliary devices, often quite different from those of the adults. Among such larvae are numbered the pilidium of nemertines, the trochophore of annelids, veliger of molluscs and successive larval stages of echinoderms. A veliger larva—that of the oyster for example—bears a conspicuous ciliated disc or velum, which is used for feeding and locomo- tion (Fig. 5.19(a)). Arising from the velum is a crown of large cilia which Fic. 5.19. FEEDING DEVICES OF LARVAE (a). Veliger larva of the oyster. Large arrow above shows direction of movement; smaller arrows in figure show direction of ciliary feeding currents; 1: digestive diverti- cula; 2: adductor muscle; 3: stomach; 4: style sac; 5: midgut; 6: rectum; 7: anus; 8: mantle cavity; 9: oesophagus; 10: foot; 11: mouth; 12: ciliated tract at base of velum; 13: velum (from Yonge, 1926). (6). Tornaria larva showing fields and furrows on sur- face, and paths taken by feeding currents; 1: apical plate; 2: antero-dorsal sulculus; 3: mid-dorsal field; 4: lateral sulcus; 5: locomotive girdle; 6: sub-dorsal sulculus; 7: lateral bay; 8: oral sulcus; 9: mouth; 10: pre-oral sulculus; 11: pre-oral field (from Garstang (30).) collect and throw particles on to a ciliated tract around the base, where they become entangled in mucus and are carried back to the mouth (19). In echinoderm larvae, such as the auricularia and bipinnaria of star- fishes, food particles are collected by longitudinal ciliated bands bordering grooves or sulci, along which they are conveyed to the dorsal border of the stomodaeum. The great extension of the longitudinal ciliated band which results from folding and development of the larval arms greatly increases the effective food-collecting area. The feeding process in tornaria larvae of enteropneusts is remarkably similar to that of echinoderm larvae. The planktonic tornaria likewise possesses longitudinal bands of cilia bordering a system of sulci which lead to the mouth (Fig. 5.19(b)). The cilia beat transversely, sweeping particles into the gutters, along which they are conveyed to the mouth 224 THE BIOLOGY OF MARINE ANIMALS opening. The adoral bands of echinoderm larvae are represented in the tornaria by ciliary patches above and below the mouth: the sub-oral appears to be inhalant, driving particles inwards over the ventral lip; the supra-oral is exhalant, driving surplus water outwards. Echinoderm and hemi- chordate larvae are generally regarded as showing phyletic affinities (30). Tentacular Methods A few animals depend largely or entirely upon freely movable tentacles for collecting fine food particles. The best examples are provided by certain holothurians, such as Cucumaria, Thyone and Psolus. These animals, which live in crevices or buried in mud, bear a crown of sticky tentacles about the mouth. When extended they entangle plankton and other fine particles. At intervals the tentacles are thrust one after another into the mouth and adhering material is wiped off and ingested. There are also many tubicolous and burrowing polychaetes (terebellids, spionids, etc.) possess- ing extensile cephalic tentacles, which move over the surface of the bottom, collecting particles and small organisms. This material is transferred to the mouth by ciliary action (p. 207). Mucus-traps Secretion of mucus-sheets for entangling food particles is commonly associated with ciliary feeding, as we have seen, and we have instanced one form in which a mucus-bag is used as an alternative mode of feeding. This animal, Chaetopterus, produces a mucous sieve, and draws water through it by the pumping action of parapodial fans (p. 207). Very similar is the feeding habit of the echiuroid worm Urechis caupo. This animal inhabits a U-shaped burrow through which it pumps water by means of peristaltic movements of the body wall (Fig. 14.6, p. 558). Near the anterior end of the worm there is a ring of girdle-glands, which secrete a mucus-bag or funnel. The mouth of this bag is fastened to the wall of the burrow, while the lower end remains attached to the body wall. Water flowing along the burrow passes through this net, which filters out particles down to 40 A in diameter. Periodically, when the net becomes clogged, it is slipped over the head, caught by the muscular proboscis and swallowed. Urechis feeds intermittently, periods of pumping alternating with long periods of quiescence lasting from 20-60 min. During pumping periods the animal is not always feeding, and a shift to feeding activity is accom- panied by an increase in ventilation rate, from 11 to 29 c.c./min. Observa- tions show that a worm produces a new feeding bag about once per hour, and that the time spent in feeding amounts to 13 min in each hour (average values). The metabolic rate of Urechis is known, but observations are not at hand to relate these data (pumping rate and O, consumption) to actual food intake under normal environmental conditions. Feeding funnels, produced by parapodial glands, are also used by Nereis diversicolor in filter-feeding (36, 40, 63, 65). An interesting example of a mucus-trap is provided by a sessile gastropod NUTRITION AND FEEDING MECHANISMS 229 Vermetus gigas. By means of a large pedal gland this animal secretes long mucous strings, which extend up to 30 cm away from the shell, and which entangle fine plankton material. At intervals these threads are drawn back towards the mouth, to be seized by the radula and swallowed. In addition Vermetus gigas is able to capture small organisms which come within range of its radula. The ctenidia are small and the ciliary current slight in this form, and are of minor importance in securing food. Certain other filter-feeding gastropods also possess a supplementary mode of feeding by means of thread-like mucus-traps (5, 76, 118). Feeding Mechanisms Involving Setae Crustacea, in common with other arthropods, lack cilia, but many species nevertheless feed on minute particles which they strain from the Fic. 5.20. VENTRAL VIEW OF Calanus finmarchicus, TO SHow WATER CURRENTS WHEN THE ANIMAL IS SWIMMING SLOWLY Large incoming whirls to right and left; smaller feeding whirls on ventral surface. Other arrows indicate direction of water currents towards and away from the animal. (From Cannon, 1928.) surrounding sea water with the aid of fine setae occurring on appendages. Feeding by copepods, the most important members of the zooplankton, will be described first, before considering other filter-feeding crustaceans. Copepods. When swimming slowly and steadily, pelagic copepods such as Calanus finmarchicus often feed automatically by straining off phyto- plankton, although they are capable of selecting larger food items. Swimming movements are due to rapid vibrations of the anterior appen- dages, and some of the water drawn towards the animal is caught in a vortex created by the activity of the maxillipeds and maxillules (Fig. 5.20). This current passes through the stationary maxillae which bear long setous filters, and food particles which are filtered off are passed to the mandibles. Because of their importance in the economy of the sea, copepods have been studied intensively, particularly Calanus finmarchicus, which is widely M.A.—8 226 THE BIOLOGY OF MARINE ANIMALS distributed and often abundant. Ca/anus feeds upon the microplankton; its food includes diatoms, flagellates and other unicellular algae, protozoa and small crustaceans. The filtering apparatus is very efficient, straining off minute flagellates from the nannoplankton down to a few micra in size. The actual food intake varies somewhat with the habits of the animal. The majority of the Ca/anus at the surface are feeding at all hours of the day. At deeper levels the proportion of individuals containing food is distinctly less, in correlation with decrease in density of phytoplankton. There is some evidence for a diurnal feeding rhythm with greater activity at night, which may be significant in terms of daily movements to and from the surface. When the animals execute vertical diurnal migrations, feeding takes place mostly at the surface and during the hours of darkness (68, 69). Ostracods. There is much variety in the feeding habits of different ostra- cods, some like Asterope and Cytherella being purely filter feeders, others like Cypridina feeding on detritus and large food masses. While feeding, Asterope remains buried in the mud and abstracts minute food particles from the feeding-current which it passes through its burrow. In feeding, a current of water is drawn through the valves of the shell by the vibratory activity of the maxillae, while the first trunk limbs bear valves which allow the passage of water in an antero-posterior direction only. Food particles are caught in a setous filter on the maxillule, are combed off by the maxilla and are transferred to the mouth by long setae on the maxillules. The mechanisms of filter-feeding are essentially the same in other filter-feeding ostracods, although the various processes may be carried out by different limbs (12). Cirripedes feed largely on small marine crustacea which they garner by means of casting movements of cirri. These are really the thoracic legs which terminate in a pair of rami armed with long hairs. The food collected by the cirri is deposited on the mouth parts, where it is ground up by the mandibles and worked into small masses to be swallowed. Amphipods. Several benthic amphipods are particle feeders. Ampelisca lives in tubes or pockets of sand grains and mucus. When feeding, the pleopods are kept in constant motion, drawing in water over the head and mouth parts, and driving it outwards over the telson. Food particles brought in by this current are seized by the gnathopods and mouth parts; setae on these structures probably serve for straining and selecting minute particles. A rather different method of feeding is used by Haustorius. This animal is found on sandy beaches, where it burrows into the sand. It possesses a filter-mechanism by which it feeds on small food particles suspended in the water in the sand. The maxilla acts as a pump, producing an anteriorly directed current, and also performs the function of a sieve plate, filtering off food particles. These are removed by the maxillipeds and passed on to the mouth parts (20). Mysids such as Hemimysis and Praunus have two methods of feeding, one for dealing with large food masses and another for filtering off sus- pended particles. We are here concerned with the latter process. Water NUTRITION AND FEEDING MECHANISMS 227 currents responsible for locomotion and feeding are produced by rotary movements of the thoracic limbs (Fig. 5.21). As these whirl around they draw water towards them from all directions and pass it into an area of low pressure near the base of the limb. The separate water streams join to form a forwardly directed current beneath the mid-ventral body wall. Anteriorly the food stream is sucked forwards by vibration of the maxillae. Food particles are collected by setous combs on the maxillae and are pushed on to the mandibles, where the food mass is ground up and sucked into the oesophagus by peristalsis. Less palatable material is thrown out laterally by the mouth parts. Food may be filtered directly from the sea water as Fic. 5.21. FEEDING CURRENTS PRODUCED BY Hemimysis Dorsal view of an animal swimming freely in the water (x 4.8). (From Cannon and Manton, 1927.) the animals swim about, but when live plankton is sparse the animals swim to the bottom where they stir up particles from bottom deposits. In this way mysids are able to increase the amount of material in suspension and can feed upon it (98). Leptostraca. This group includes sedentary, burrowing and pelagic species. Nebalia is a littoral form which burrows into mud and lives just beneath the surface. Food particles are filtered off a stream of water which enters the carapace anteriorly and makes its exit at the posterior end. This feeding current is produced by oscillatory movements of the thoracic limbs, and food particles are filtered off by rows of setae on these same appendages. The collected food is then brushed on to proximal (gnatho- basic) setae and passed forwards to the mouth parts (11). 228 THE BIOLOGY OF MARINE ANIMALS Cumaceans are bottom-dwelling animals which burrow in sand or mud. In Diastylis a filter current is created by movements of maxillae and maxilli- peds, and this is augmented by the respiratory current. The water currents created by the animal pass through the setous mouth parts (maxillae, makxillipeds), where large particles become deposited and filtered off (21). Euphausiids are pelagic in habit and feed upon detritus and small planktonic organisms. The chief food of the antarctic krill Euphausia superba consists of diatoms. That of Meganyctiphanes norvegica is pre- dominantly organic detritus, supplemented by diatoms, flagellates and small crustaceans. Euphausiids appear to feed in a manner similar to mysids (q.v.). The filtering apparatus or food basket is formed by the thoracic limbs which are provided with fringes of setae. Feeding currents are created by lateral movements of the thoracic limbs and by beating of pleopods. Food particles caught in the thoracic basket are transferred to the mandibles to be triturated and swallowed. Meganyctiphanes has also been observed to feed upon suspended matter on the bottom, which is stirred up by the pleopods (61). Decapods. These animals are typically omnivores or carnivores, feeding on large food masses, but filter-feeding has been adopted by a few forms, often in conjunction with specialized habits. The peculiar gall crabs Hapalocarcinus and Cryptochirus live in chambers within corals and are dependent upon plankton and suspended matter drawn into the chamber with the respiratory and feeding current (p. 662). The oral region in these animals is screened by a sieve of setae fringing the maxillipeds, while the mandibles, apparently, are also used for sifting food and creating water currents. Spider crabs (Jnachus) also consume fine particles. Essentially deposit feeders, they pick up fine material in their chelae and hold it in front of the mouth where it is brushed over the maxillipeds. Filter-feeding has been acquired by several groups of anomurans. The hermit crab Eupagurus feeds to a large extent upon bottom detritus and small organisms which are swept up by a terminal brush of setae on the third maxillipeds. In addition the small chela is used to scrape bottom deposits and pick up small masses which are passed to the maxillipeds and thence to the mouth parts. Feeding methods are essentially the same in Galathea which collects finely-divided material from the substratum by sweeping movements of the maxillipeds, but larger pieces of food may also be seized by the chelae and maxillipeds and passed to the mandibles. Porcellana is a particulate feeder and possesses a filter in the form of a fringe of long hairs on the third maxillipeds (Fig. 5.22). These perform regular casting movements, thus entangling suspended particles which are transferred by the second maxillipeds to the mouth. Another method of filter-feeding is used by the mud shrimp Upogebia. This animal lives in a burrow through which it draws a feeding current by fanning its swimmerets. The first two pairs of limbs are heavily fringed with hairs so as to form a basket, and as water passes through this filter, detritus and plankton are strained off. The collected food material is NUTRITION AND FEEDING MECHANISMS 229 brushed off the basket by the maxillipeds and transferred to the mouth. Finally, we may mention here the parasitic pea*crabs (Pinnotheres), some Fic. 5.22. ANTERIOR VIEW OF PORCELAIN-CRAB Porcellana longicornis WHILE FEEDING, TO SHOW THE DIRECTION OF THE WATER CURRENTS DRAWING FOOD IN SUSPENSION TOWARDS THE ANIMAL (from E. A. T. Nicol (81).) of which inhabit the mantle cavity of lamellibranchs. These animals collect mucous food-strings from the gills of their hosts with the aid of setous fringes on their claws (62, 81, 84). Efficiency of Filter-feeding among Invertebrates The food available to filter feeders consists of minute zooplankton, phytoplankton, protozoa, bacteria and organic detritus. This organic matter varies greatly in particle size, from several hundred micra in certain algal cells to sub-microscopic colloidal dimensions in the case of detrital matter. Animals probably differ in the efficiency of their straining appara- tuses: those with structural sieves—namely sponges, bivalves, copepods and ascidians—retain particles down to | ym in size; mucous net feeders, on the other hand, filter out material down to 40 A in size. The food intake is a function of the efficiency of the filter, amount of food present in the surrounding sea water and pumping rate. Absolute filtering rates, of course, vary greatly—from less than 100 ml/day in Calanus finmarchicus to 10 1./hour in Ostrea virginica. Jorgensen, however, finds that filtration rates of different animals are of about the same order when expressed as litres of sea water per millilitre O, consumed. Calculated values are as follows— Sponges Grantia compressa 13 1./ml O, Halichondria panicea 14:3 him; Echiuroid Urechis caupo 20 1./ml O, Lamellibranchs Ostrea virginica 16 1./ml O, Mytilus edulis 14-15 |./ml O, Copepods Calanus finmarchicus 8 1./ml O, Centropages hamatus 8 1./ml OO. Ascidians Ciona intestinalis Molgula era 10-20 I./ml O; 230 THE BIOLOGY OF MARINE ANIMALS It is interesting to test the calculated values against actual measurements of food stuffs available. One ml of O, will burn about 0-8 mg of organic matter. For an animal filtering 15 1./ml O,, this is equivalent, in terms of energy consumption, to 0-05 mg of organic matter per litre. Allowing two-thirds of the energy absorbed for growth, we calculate the total food requirements for growth and respiration as about 0-15 mg of organic matter per litre. Only part of the organic matter ingested is utilized, however; in Calanus finmarchicus, for example, it is estimated that half the nitrogen ingested is lost in the faeces. Actual amounts of organic matter, in detritus and phytoplankton from different waters, range from 0-14-2-8 mg/l. These values appear to be of about the order necessary to satisfy the nutritional requirements of the filter-feeding animals concerned. All the calculations presented involve many variables, which make them rather unsatisfactory (44, 52, 59). Filtering Devices in Vertebrates Despite great disparity in size, certain adult vertebrates utilize filtering methods of feeding fundamentally not unlike those already described in certain invertebrate forms. In all these cases, whether they concern fish, fowl or whale, there is some form of filter or sieve which strains off plank- ton from the sea water. These animals really by-pass one or several in- termediate links in the food chain by feeding on planktonic crustaceans; because of their size and food requirements they must clear large volumes of water to obtain sufficient food organisms, and at least for feeding purposes they are restricted to areas of high planktonic density. Fish. Various pelagic fish are plankton feeders. Huge basking sharks Cetorhinus and whale sharks Rhineodon feed exclusively upon plankton. These creatures are provided with numerous closely set gill-rakers, which strain off the myriads of small crustaceans from the water which enters the mouth (74). Among teleosts, mackerel and herring possess large thin gill-rakers which project across the pharyngeal openings and prevent the escape of planktonic organisms. Copepods predominate in the food of young herring. Adults feed largely on a non-crustacean diet in spring (mostly sand eels) and shift to a crustacean diet in summer. Birds. Although many marine birds feed on plankton (see p. 246), there is only one group which is anatomically specialized for sifting out floating animals, namely prions or whale birds (Pachyptila). In certain species the upper mandible bears two rows of comb-like lamellae, strik- ingly analogous to the baleen plates of whalebone whales (Fig. 5.23). The resemblance is heightened by the presence of a large fleshy tongue. Whale birds are denizens of subantarctic waters. They feed from the surface on crustacea by submerging the head, and scooping up food with the laminated bill (78). Whalebone Whales (Mystacoceti). These animals show highly specialized adaptations for securing plankton. The filtering apparatus consists of NUTRITION AND FEEDING MECHANISMS 231 whalebone or baleen, which is a collective term for horny plates attached to the roof of the mouth and hanging down into the buccal cavity. The plates are arranged transversely to the long axis of the jaws and are very numerous, over 300 having been counted in the right whales. On the inner side the plates bear fine hair-like fringes which form an efficient filtering apparatus. As the whale swims about at or near the surface, with mouth open, planktonic organisms are strained off by the hair-like fringes of whalebone and the water escapes through the sides of the mouth. When the Fic. 5.23. HEAD AND BILL OF THE BROAD-BILLED PRION Pachyptila forsteri (Left) a palatal view of the upper mandible, showing the baleen-like maxillary lamellae. (Right) view of the head, showing the extensible pouch. (From Murphy (78).) lower jaw is raised and the tongue elevated, water is forced out of the mouth cavity. The planktonic organisms which have been filtered off are left stranded on the tongue and are swallowed. The principal food of whalebone whales consists of larger species of plankton (krill). In the Antarctic, blue, fin and humpback whales (Ba/aenop- tera and Megaptera) feed heavily and almost exclusively on the immense shoals of Euphausia superba which abound there; this is in the summer season. The majority of whales eat little in the winter, when they draw upon their reserves of fat. These are supplemented by small quantities of crusta- cea and fish, captured in warmer waters of the Southern Hemisphere during the winter months. The staple food of blue and fin whales (Ba/ae- noptera) in the Northern Hemisphere is Meganyctiphanes norvegica during the summer. During the winter, fin and humpback whales consume some fish (clupeids (66)). MECHANISMS FOR DEALING WITH LARGE PARTICLES OR MASSES In this section are described methods for dealing with inactive food; for seizing prey, and mechanisms for scraping and boring in connexion with feeding. Ingestion of Inactive Food There are many benthic animals which swallow, with little selection, sand, mud or other bottom deposits, from which they extract organic 252 THE BIOLOGY OF MARINE ANIMALS material for nourishment. Such animals possess gullets, tentacles and similar structures which can be pushed through the ground, whereas organs of mastication are absent. Annelids. Many polychaetes fall into this group, such as Arenicola, Ophelia, Notomastus and others. Arenicola prefers muddy sand in which it forms a U-shaped burrow. This consists of head-shaft, gallery and tail- shaft (Fig. 5.24). Usually only the latter is open to the surface, while the head-shaft is blocked by loosened sand. When the worm finds a suitable location it may stay there for weeks on end, showing a very regular pattern of activity, at least when the tide is in. Sand is loosened in the head-shaft by regular irrigation movements, and is swallowed by the muscular pro- boscis which is frequently extended and withdrawn. Feeding activity is Fic. 5.24. LUGWorRM Arenicola marina IN ITS BURROW The cross-lines lie at the boundaries between head shaft (/eft), gallery (below) and: tail shaft (right). Yellow sand above, black sand below. Solid arrows show water move- ments through burrow; broken arrows, settling of sand in the head shaft. (After Wells, 1945.) rhythmic, showing a periodicity of about 7 min in A. marina (Fig. 4.7, p. 148) (101, 102). Sipunculoids also ingest sand and mud by means of a muscular in- trovert and utilize the contained organic material. Ingestion of bottom deposits is more refined in Cirratulus. This animal burrows in mud and sand but is able to select diatoms and other algal and organic material from the non-nutritive mass. Tentacles are present but are exclusively respiratory. Sensory flaps on the anterior pharyngeal walls permit the passage of only the finest particles, which apparently are sucked into the gut. Echinoderms include many benthic and burrowing species which feed on bottom deposits. Holothurians push mucus-bound aggregations of bottom materials into the mouth with the buccal tentacles. Heart urchins (spatangoids) burrow in sand or mud and maintain communication with the surface by a mucus-lined canal (Fig. 4.6, p. 147). Through the latter a respiratory current is drawn, while small rosette feet of the buccal region NUTRITION AND FEEDING MECHANISMS 233 collect sand and food particles, the small circumoral spines pushing the food into the gut. Somewhat similar habits are displayed by burrowing ophiuroids (Amphiura, Ophiopsila, etc.), which move particles and detritus along the arms to the mouth by means of tube feet. Crustacea. Many littoral amphipods feed on organic detritus, largely vegetable matter. Corophium, a burrowing form, sometimes filters off fine particulate matter, but to a much greater extent feeds by selecting particles from the mud in which it lives. When behaving as a selective deposit feeder, Corophium scoops up and sifts small quantities of mud with the gnathopods. Larger particles are conveyed to the mandibles, where they are crushed by molar processes and swallowed. Smaller particles are retained by a fringe of setae on the gnathopods, sifted and then transferred to the mandibles to be swallowed. The Cumacea are small burrowing animals which employ filter-feeding (p. 228) or feed on small micro-organisms occurring in the soil detritus. Cumopsis collects food by cleaning off sand grains and other small objects. These are picked up by the first pereilopods and manipulated and cleaned by the maxillipeds. The food is then passed to the mouth parts (24). Scraping and Boring Here are included devices which enable animals to bore into hard materials, the fragments of which are swallowed and digested; or to scrape off encrusting material and organisms; or to rasp and bore into living prey or dead animals. Invertebrates which feed in this manner include various echinoderms, molluscs and crustacea. Certain fishes are also included in this category. Echinoderms. Sea urchins possess a set of strong teeth forming a biting and scraping apparatus known as Aristotle’s lantern. With this structure, rock-dwelling forms such as Echinus are able to scrape off and masticate encrusting organisms; bottom material is also conveyed to the mouth by tube feet. Molluscs. Especially suitable for scraping is the radula of chitons and gastropods. This is a horny ribbon covered with many rows of small recurved teeth (Fig. 5.25). The radula lies on the ventral side of the buccal cavity and frequently works in conjunction with the palatal plate or jaws. Growth of the radula is continuous during the life of the animal and takes place in a ventral diverticulum known as a radular sac, in which proliferat- ing tissue gives rise to transverse rows of cells (odontoblasts), forming new teeth, and other cells forming the horny base of the ribbon. As a result of posterior growth the radula is pressed forwards and a new surface constantly replaces that worn away. The radula is supported by cartilaginous masses providing attachment for protractor and retractor muscles by which the odontophore apparatus is protruded from, or withdrawn into, the buccal cavity. Rasping movements of the radula are brought about by action of another set of protractor and retractor muscles, by which it is drawn backwards and forwards over M.A.—8* 234 THE BIOLOGY OF MARINE ANIMALS its supporting cartilage, as over a pulley. Since the radular teeth slope backwards the effective stroke is executed on withdrawal, and this accords with the greater size of the radular retractor. In herbivorous chitons and gastropods radular teeth are well developed. Representative browsing forms are Chiton and Patella, which scrape Fic. 5.25. PROBOSCIS OF A CARNIVOROUS GASTROPOD Natica millepunctata Radula in mouth opening, boring gland below. (From Ankel (5).) encrusting algae and other small organisms off rocks. Pieces of seaweed are also eaten, being seized with lips and palate and scraped with the radula. Aplysia (an opisthobranch) browses on green algae, which are grasped by lips and jaws and rasped by the radula. Eolids feed on hydroids and sea anemones, breaking off pieces with the jaws and passing them back with the radula. They appear to be physiologically specialized in some manner for inactivating the nematocysts of their prey. Many of the dorids browse on sponges, using the radula as rasp and scoop. Others, such as NUTRITION AND FEEDING MECHANISMS 2355 Acanthodoris, attack ascidians and polyzoans: they cut into their prey with the radula and suck out semi-liquid food by means of a buccal pump (27, 92). In many carnivorous gastropods the radular apparatus is carried on the end of a long extrusible proboscis which can be inserted into the prey. Thus Cerithiopsis feeds on siliceous sponges by thrusting its long proboscis into the osculum, or through adventitious apertures, to reach the softer parts within. Sycotypus attacks oysters by stealth, waiting until the latter opens up, when it thrusts its shell between the oyster’s valves and pushes its proboscis into the soft parts. Whelks (Buccinum, Busycon) force open the valves of lamellibranchs and remove the soft parts of the prey with the aid of the proboscis (Fig. 5.26). Other gastropods bore Fic. 5.26. Sycotypus, A GASTROPOD, OPENING AN OysTER. (From Ankel (5).) through the shells of lamellibranchs by mechanical or chemical means. Urosalpinx and Nucel/a drill an opening with the radula; Natica liberates a certain amount of free acid (H,SO,) from a gland on the proboscis for dissolving a hole in the shell of the prey (Fig. 5.25). In either event the proboscis is pushed through the aperture so made and extracts the soft parts of the prey (14, 15, 29, 32, 51). Shipworms Teredinidae obtain much of their nourishment from the wood in which they bore. The wood is rasped away by movements of the Shell valves and the scrapings are carried into the mouth by ciliary action (p. 660). In addition a certain amount of plankton is collected by gills and palps. Analyses of amino-acids show that Teredo acquires its dietary-N from both the wood and suspended nannoplankton (33, 56). Crustacea. The gribble Limnoria is an isopod which tunnels into wood and feeds on wood particles. Pure wood has a very low protein content, and wood-destroying fungi contribute much amino-N to the diet of the gribble (88a). The habits of these marine borers are further described in Chapter 15 (p. 661). Ligia, the sea slater, browses on sea weeds which are cut up by the mandibles; Jdotea, another isopod, is an omnivorous scavenger and scrapes and bites food masses with the mouth parts (79). Some of the littoral gammarids feed on sea weeds, tearing off pieces with the maxillipeds and crushing them with the mouth parts. 236 THE BIOLOGY OF MARINE ANIMALS Caprellids, often found clinging to stems of hydroids, possess large palm- like claws, with which they scrape off diatoms and debris or even attack living hydroid zooids. Fishes. The majority of fishes are active carnivores and relatively few species feed upon plants or organic debris. Some exceptions are the grey mullet Mugil and Mulloides, which feed upon sea weeds, bottom mud and detritus. The gill-rakers of the mullet form a sieve-like apparatus prevent- ing fine particulate matter from reaching the gills. The jaw teeth are micro- scopic, pharyngeal grinding teeth are presentin Mulloides, and both animals possess strong pyloric gizzards. Some of the parrot-fishes (Scaridae) feed on vegetation or pieces of coral. In these animals the jaw teeth are fused into shearing plates and the pharyngeal teeth form a flat grinding pave- ment. Trunk fishes (Ostraciidae) also feed on bottom algae (2, 42, 57, 82). Methods for Seizing Prey In this section we shall be dealing with feeding mechanisms principally of carnivores which seize and devour living prey, but we shall have occasion to refer to certain omnivores which are partially scavenging in habit. Yonge shows how such mechanisms may be classified into those concerned with seizing, with seizing and masticating, and with seizing followed by external digestion. These are considered together on a phyletic basis. Protozoa. Many protozoa are raptorial, feeding on other protozoans, phytoflagellates, diatoms and even small crustaceans. Amoebae capture small, slow-moving prey which they engulf in a food-cup formed by pseudopodia. In raptorial ciliates the mouth is usually located at the an- terior end and can be widely distended for engulfing large prey. Some species possess special devices for seizing prey, such as proboscides, scoops formed of undulatory membranes and suctorial tentacles. The latter are found in certain parasitic holotrichs which use them to suck out the con- tents of epithelial cells of their host. In suctorians, which feed upon other protozoa, the prey is captured by sticky tentacles which release a paralysing secretion and suck out the contents of the prey (53). Coelenterates and Ctenophores. Members of these groups are carnivores, apart from those sedentary species dependent on symbiotic algae (p. 612). With the exception of species making use of ciliary feeding currents, coelenterates usually capture their prey by means of tentacles armed with cnidae capable of discharging adhesive and penetrating filaments. The latter are capable of paralysing small animals. Both hydroids and medusae (Hydrozoa) feed on small crustacea, worms, eggs, larvae and small fish. In the feeding reaction of tubularians, for example, the proximal tentacles bearing food bend towards the mouth while the manubrium, in turn, bends to meet them. In colonial forms the food is shared among the mem- bers of the colony. The individual polyps initiate digestion of the prey which they capture, and a constriction at the entrance to the stalk allows only the smallest particles and dissolved material to gain ingress into the branches and common stem of the colony, where absorption takes place. NUTRITION AND FEEDING MECHANISMS 237 Corymorpha, a solitary hydroid living on soft bottoms, feeds on detrital matter and has characteristic feeding movements which are repeated in quiet water about twenty times a minute. When feeding, the stalk bends over, mouth and distal tentacles touch the mud, after which the stalk straightens out and food material adhering to the tentacles is conveyed to the mouth. In hydromedusae, food is grasped by the marginal tentacles which respond to chemical and tactile stimuli. In the subsequent feeding reaction the stimulated margin bends towards the manubrium (Phialidium) or, when the manubrium is long, the latter structure bends towards the margin (Stomatoca). The food is seized by the lips of the manubrium and swallowed. Some species display a fishing behaviour in which they swim to the surface and then float downwards with tentacles fully extended (49). Siphonophores are colonial animals, entirely oceanic and pelagic in habit. Special polyps known as dactylozooids, bearing long tentacles, capture and digest the prey. As the animals drift through the water, their long trailing tentacles act like nets, capturing animals which strike against them. The tentacles are muscular and highly contractile, and when the prey is paralysed by nemotocyst-action it is drawn up to the mouths of the gastrozooids by contraction of the tentacles (9, 106). Feeding behaviour in Scyphomedusae differs in detail but usually involves stinging and manipulating the prey with tentacles and manubrium, after which the food is transferred to the mouth. The food consists of small planktonic animals—small crustacea, worms, small medusae and the like. In Chrysaora the tentacles, laden with food, contract and the food particles are swept off by the lips which form a temporary receptacle beneath the stomach. Food material collected by the bell of Aurelia is licked off by the oral arms and conveyed by cilia to the mouth and gastric pouches. Experiments have shown that the arms respond to mussel juice, proteins, peptones and amino-acids but not to carbohydrates. Cassiopeia, a sedentary form, lies on the bottom with oral surface upwards. Pulsations of the bell produce a current of sea water from which planktonic organisms are seized by the oral arms. The food, entangled in mucus, is swept by ciliary action into the numerous mouths which lie along the arms (91). Actinians (sea anemones and corals) are exclusively carnivorous. In less specialized corals the collection of food is reserved for the tentacles. These paralyse their prey with nematocysts and convey it to the mouth by muscular action. In some other corals, however, the general ectoderm participates in the capture of food, and ciliary tracts transport particles to the mouth (112, 113). In sea-anemones the presence of suitable food evokes an orderly series of feeding reactions. When a piece of meat is placed on the tentacles there is first a discharge of cnidae. The tentacles then clasp the food and bend towards the mouth, which turns towards the food and opens. The food is gradually thrust in and swallowed, and the tentacles subsequently return to their normal feeding position. The feeding response is initiated by both 238 THE BIOLOGY OF MARINE ANIMALS mechanical and chemical stimuli. Owing to rapid adaptation mechanical stimuli rarely induce a complete response, but the intervention of proper chemical stimuli usually results in acceptance of foodstuffs. Of a range of chemical substances tested, the most active are proteins and their deriva- tives, including peptones and various amino-acids. Certain lipoid extracts are also effective, but not carbohydrates. This selective sensitivity is obviously closely related to the purely carnivorous habits of these animals (85). All ctenophores are carnivorous in feeding habits. Tentaculate forms, exemplified by Pleurobrachia and young Mnemiopsis, capture small plankton organisms with their tentacles. These are provided with sticky lasso cells known as colloblasts, which hold on to the food. After making a successful capture the tentacle contracts and conveys the food to the mouth. In adult Mnemiopsis a complex ciliary and tentacular mechanism is em- ployed. Extending along the sides of this animal are four auricular grooves into which food particles are carried by beating cilia. At the bottom of a groove the particles become entangled on small tentacles: these bend over into a labial trough and food particles are conveyed down the latter channel to the mouth. Non-tentacular beroids capture their prey by means of the extensible mouth rim and can ingest relatively large animals such as crustaceans and other ctenophores (48, 67). Turbellaria. These animals possess a muscular pharynx, which can be protruded for capturing food. This consists of a variety of small animals— protozoa, nematodes and small crustaceans. Cycloporus (a polyclad) feeds on colonial tunicates, and sucks out zooids individually (50a). Nemertines are entirely carnivorous when adult, feeding on a variety of prey. Immature and small animals feed on protozoa. Larger benthic and littoral species capture small crustacea, worms, molluscs and even small fish, living or dead; pelagic nemertines subsist on small crustacea. Food is captured with the aid of a muscular proboscis which can be everted for some distance in front of the head, and in some species it is actually as long as or longer than the animal’s body. When the proboscis is shot forth it entwines itself around the prey, which is retained by tenacious mucus or quietened by means of immobilizing secretions. Moreover in some species the proboscis is armed with sharply pointed stylets (Hop- lonemertea, e.g. Amphiporus). The food is then conveyed to the mouth to be swallowed entire. A large Cerebratulus, for example, can swallow an annelid nearly equal to its own diameter. An aberrant form, Malacob- della, is commensal in the mantle cavity of bivalves (Se/iqua), where it feeds on plankton filtered off by the host (16). Annelids. Many polychaetes have muscular introverts armed with small teeth, e.g. Aphrodite, Lumbriconereis, Nephthys, Glycera, etc. The introvert is used for capturing prey, which consists usually of living animals such as worms, molluscs and small crustaceans. Nereis virens, an errant carnivore, also feeds on dead animals and algae. Tomopteris, a voracious planktonic form, swallows entire Sagitta and larval herrings. Certain syllids, e.g. NUTRITION AND FEEDING MECHANISMS 239 Autolytus edwardsi, attack hydroids. They cut off the tentacles or pene- trate the coelenteron by means of pharyngeal teeth, and suck in hydroid tissue and fluids through a protrusible proboscis (34, 83). Crustacea. Excluding the filter-feeders these animals are generally omnivorous, although many species are chiefly dependent on living prey or carrion. Food material is grasped by head or thoracic appendages and masticated by the mouth parts before being swallowed. Some species capture large particles and prey to supplement filter-feeding. Mysids, for example, seize small animals (crustaceans, arrow worms) with thoracic limbs and tear them up by means of mandibles and maxillules. Isopods are frequently carnivorous or scavenging in habit. Chiridotea, for example, seizes carrion with its gnathopods and bites off pieces with the mandibles (98). Chelae and chelipeds are used by decapods for seizing, manipulating and shredding food. In the prawn Pa/aemon the chelipeds convey pieces of food to the maxillipeds, which hold them while fragments are torn off by the mandibles and other mouth parts. In lobsters (Nephrops, Palinurus) food is held by the mandibles, while it is torn up by the action of the third maxillipeds prior to being swallowed. Similarly in the shore crab Carcinus maenas the food is shredded and torn before it is swallowed. Pieces seized by the chelae are transferred to the mandibles, which hold them while they are being torn into fragments by the other mouth parts. Algae have little or no food value for larval prawns (Palaemonetes), which need animal food for survival (10a). Molluses. Many gastropods are carnivorous in habits, feeding on living prey or carrion. Some species swallow their prey whole. Tectibranchs, such as Scaphander and Bulla, swallow entire lamellibranchs, which they grind up in a muscular gizzard. Special predatory habits are also en- countered among nudibranchs. Ca/ma, for example, feeds on the eggs of shore fishes, which are slit open with the radula and the egg contents extracted. Other gastropods masticate the food to some extent before swallowing it. Thus Pleurobranchus grasps pieces of carrion with its mus- cular proboscis and rasps off bits by means of the radula. It is likely that many of the carnivorous gastropods secrete protease from salivary glands and this assists the radula in breaking up the food. Gymnosomatous pteropods are also carnivorous in habit and feed largely on thecosomes. These animals possess an eversible proboscis provided with various devices for seizing prey, namely hooks, suckers and sticky secretions. Supplement- ing these devices are jaws and powerful radulae (70). Among bivalves one group, the septibranchs, have become carnivorous in habits. Cuspidaria and Poromya are burrowing forms which keep the siphonal openings at the surface. In this position they draw in small animals, living or dead, which chance to be in the vicinity, through the large inhalant siphon. This is accomplished by aspiratory movements of a transverse muscular septum which replaces the branchiae of other lamelli- branchs, and which divides the mantle cavity into upper and lower chambers 240 THE BIOLOGY OF MARINE ANIMALS (Fig. 5.27). Perforating the septum are small pores provided with valves and sphincters. Normally the septum lies quiescent with open pores, through which a slight current is maintained by lateral cilia. Several times each minute, however, the septum is lowered slowly, pores are closed, then the septum is quickly lifted, causing water to be expelled through the exhalant siphon and water and food to be sucked in through the inhalant siphon. The food is retained in the infra-septal cavity by a large valve Byssus groove Anterior palps ? (1) Fic. 5.27. SEPTIBRANCH BIVALVES (a) Ventral view of Poromya granulata with mantle lobes drawn back to expose the septum. Large arrows indicate direction of food; small arrows, water currents through the branchial sieves. (6) Movements of septum in Cuspidaria, (i) position of septum at rest and prior to descending; current of water through pores indicated by upward arrows; downward arrows indicate initial septal movement; (ii) position of septum at end of downward movement, pores closed; septum now moves upwards as indicated by arrows; (iii) position at completion of upward movement, pores still closed. (From Yonge, 1928.) edge Anterior and posterior branchial sieves Vi (11) (iit) (b) guarding the opening from the inhalant siphon and is pushed into the mouth by small muscular palps. Cilia are greatly reduced and are con- cerned with the removal of fine particles from the mantle cavity (111). Cephalopods capture prey by means of arms and tentacles bearing sucking discs. Cuttlefish and octopus feed on fish and decapod crustacea. The prey is conveyed to the mouth by the appendages, torn by the horny jaws and rasped by a radula. Several salivary glands discharge into the mouth region, namely the sublingual, anterior and posterior salivary glands. The latter two are generally regarded as poison glands. The secre- NUTRITION AND FEEDING MECHANISMS 241 tions contain nerve poisons capable of paralysing prey, and a proteolytic enzyme (p. 255) (100, 105). Chaetognaths are small voracious carnivores, chiefly planktonic in habits. They are provided with a pair of chitinous hooks on either side of the mouth, and with these they seize their prey, usually swallowing it whole. Sagitta feeds on copepods, young fish and other arrow worms. Spadella is.a bottom-dwelling chaetognath which attaches itself to the substratum and lunges at passing prey (85a). Echinoderms. Many ophiuroids are carnivorous, such as Ophiura and Ophiocoma. Their prey consists of small polychaetes, molluscs and crusta- ceans: these are captured by the arms and transferred to the mouth to be swallowed whole. Asteroids show diversified carnivorous habits. Those with pointed tube feet, such as Astropecten, live in sand and feed upon small lamellibranchs. Others, with sucker tube feet, attack larger bivalves, which they pull open and devour, e.g. Asterias, Pisaster. Snails, barnacles, echinoids, even decapod crustacea are attacked. Starfish have the remark- able habit of everting their stomach over the prey if this be too large to be swallowed whole, and digesting the prey before swallowing it (4, 38). Carnivorous Habits in Vertebrates Marine vertebrates are predominantly carnivorous and display much variety in feeding habits, enabling them to exploit manifold sources of food. Fishes. Cyclostomes are semiparasitic in habits. Lampreys fasten them- selves to the bodies of other fish by means of a funnel-shaped sucker which surrounds the mouth. Thus attached, they suck the blood and rasp off the flesh of their prey with horny teeth which are borne on a piston-like tongue. As the teeth wear away, they are replaced by new ones which form underneath. Hagfishes are similarly armed with a powerful tongue and lingual teeth, and soon reduce to a bag of skin and bones fish to which they are attached. Among gnathostome fishes there is great diversity in food and feeding habits. The kind of prey captured by a fish is dependent upon the structure and habits of the fish, as well as the predatory species available. We classify carnivorous fish into pelagic and benthic feeders, and note various methods of locating and seizing prey in these two categories. Pelagic foragers hunt by sight, scent or touch. These animals are usually provided with a well-developed strong dentition of pointed, cutting or sometimes grinding teeth, which are renewed as they age or wear. In sharks the older teeth in front of the jaw are shed and are replaced by forward movement of more posterior teeth. In teleosts new teeth are formed at the bases of the old, or in the spaces between. As examples of active pelagic foragers which depend upon sight for hunting other fish, we may mention mackerel, tunny, bluefish (Pomatomus) and barracudas (Sphyraena). The jaws are armed with sharp teeth but are otherwisejun- specialized, and agility and speed are used in pursuing the prey. Many 242 THE BIOLOGY OF MARINE ANIMALS sharks sight their prey and some have peculiar feeding habits, such as the thresher shark (A/opias) which herds shoals of small fishes into compact masses by threshing the water with its tail, before rushing in to devour them. The rough dogfish Scyliorhinus hunts chiefly by scent, mostly but not exclusively near the bottom, and feeds on anything which comes its way (50°93). Fishes from deeper pelagic waters are frequently much modified in connexion with feeding, but naturally the habits of these animals are subject to inference. Teeth are often long and fang-like and the jaws flexible and distensible, so that very large prey can be captured (Chauliodus, Chiasmodon, etc., Fig. 5.28). The mechanics of these distensible jaws are described in some detail by Tchernavin (99). Conditions of feeding are certainly peculiar in the dark, sparsely populated waters of the deep sea: Anglers (ceratioids), which have a luminous fishing lure, are believed to attract their prey within reach by this device (Fig. 13.8, p. 547). Long barbels, occurring for example in Eustomias, may provide tactile appreciation of prey (Fig. 13.18, p. 556). Very large gape and distensible stomach are significant adaptations to few and infrequent meals. Carnivorous fishes living on or near the sea bottom may be classed as active foragers, stalkers and purely sedentary forms which sit and wait for prey to come near. Active foragers which depend on sight hunt only by day, at least in shallow waters. The cod, for example, is a roving fish which snaps at anything within its reach on or near the bottom. Foraging is largely visual but is aided by a barbel which is employed as a tactile or gustatory organ. A strictly bottom dweller is the dragonet Callionymus. This fish swims along near the bottom and comes to rest at intervals with the body poised on the large pectoral fins. In this manner it explores a wide area of the sea floor and captures such slow-moving bottom forms as crustacea, echinoids and worms. The lemon dab Microstomus kitt hunts mainly tubicolous polychaetes. Coming to rest at intervals with head raised, it scans its neighbourhood with movable eyes and, sighting a worm, suddenly pounces upon it. Certain other bottom fish depend largely on tactile sense when foraging. The sole Solea solea, for example, has a dense mass of tactile villi on the lower cheek. When feeding it creeps slowly over the bottom, exploring with its snout and feeling objects in its path with the sensitive cheek villi. Its food consists of errant polychaetes, small crustaceans, molluscs and ophiurans. The gurnard Trig/a lineata is another form which crawls over the bottom by means of long pectoral filaments (Fig. 5.29). As the fish creeps along, the filaments are kept in constant movement, exploring the bottom: whenever anything promising is encountered, the fish suddenly whirls along and swallows it or subjects it to further examination. The filaments are richly provided with sensory cells acting as taste receptors. Rays are thought to feed largely by scent. On encountering small fish and crustacea they dart over it, cover it with body and pectoral fins, and devour it at leisure (42, 82, 89, 93, 94). NUTRITION AND FEEDING MECHANISMS 243 Fishes which stalk their prey are often provided with peculiar mouth parts. In tube-mouthed fishes the snout is prolonged in the form of a tube with a small mouth at the end. Pipe-fishes (Syngnathidae), for ex- ample, feed largely on small crustacea, which they actively seek in crevices, Fic. 5.28. Chauliodus (above) AT REST; (below) SWALLOWING A FISH. (From Tchernavin (99).) among vegetation, etc. The tube-like beak is used after the manner of a syringe, small crustaceans being sucked in by inflating the cheeks. The John Dory Zeus is provided with a protractile mouth and stalks small fish. Gradually approaching its victim, it shoots its jaws forward with great rapidity and engulfs the prey. 244 THE BIOLOGY OF MARINE ANIMALS Finally we may note a few examples of sedentary benthic forms which lie and wait for prey to approach. Perhaps the best-known is the angler-fish Lophius piscatorius, which simulates the bottom on which it lies remarkably closely in shade and pattern. The first dorsal spine of the angler consists of a movable spine with a bait-like tag at the end, and this can be erected as a fishing lure. When a fish is attracted by the lure, this is cast down in front of the mouth, and as the fish follows it the angler opens its mouth and takes in its victim with a sudden gulp. Angler-fish feed preferably on gadoids, clupeoids and other soft-finned fishes. Of a similar nature are the habits of stargazers Uranoscopus. The mouths of these creatures open towards the dorsal surface of the head and they bury themselves to a large extent in the sand, with only the mouth and eyes at the surface. At intervals a small > > 2-2 Fic. 5.29. Trigla lucerna FEELING ITS WAY ALONG THE SEA FLOOR BY MEANS OF LONG PECTORAL FILAMENTS. (From Steven (93).) lure is protruded from the mouth and caused to wriggle on the sand so as to simulate a worm or other small invertebrate, and thus attract small prey to within reach of the stargazer’s jaws. The electric fish Torpedo, which is an inactive benthic form, is believed to stun or kill other fish by means of electric shocks. This animal captures round fishes, some of them of fair size. Individuals, observed in captivity, respond only to living prey. At the approach of a fish the electric ray leaps upwards and attempts to envelop it with its pectoral fins and snout. Galvanometer recordings obtained during this manoeuvre show that the ray discharges an electric shock at the moment that it folds its head and wings over the prey. The shock apparently is used to stun the prey while it is being swallowed (82, 104, 107). Marine Birds. Marine birds can be grouped into several communities: as littoral species confined to shores and beaches, e.g. plover, sandpipers; inshore species which do not range beyond sight of land, e.g. cormorants, NUTRITION AND FEEDING MECHANISMS 245 ducks, the majority of gulls and terns; offshore species which range out to the continental edge, e.g. gannets, auks, certain gulls; and pelagic species, notably the tubinares (petrels, shearwaters, albatrosses) and penguins. The food of the various species is determined by availability in the regions frequented but each species has its own inherent feeding habits (Fig. 5.30). Along shores the sandpipers and plovers feed on small crustacea, in- sects, worms and molluscs secured at the surface, by turning over stones or weed or by probing into the ground. Curlews, willets and phalaropes have Fic. 5.30. HEADS OF SOME MARINE BIRDS (a) Oyster catcher Haematopus; (b) sooty tern Sterna fuscata; (c) brown pelican Pele- canus occidentalis; (d) cormorant Phalacrocorax; (e) gannet Sula bassana, (f) puffin, Fratercula; (g) dovekie Alle alle; (h) razor-billed auk Alca torda. long thin bills which serve for probing in sand and mud. Oyster-catchers (Haematopus) and turnstones (Arenaria) use their chisel-like bills for jabbing shellfish or knocking limpets off rocks. In high latitudes of the southern hemisphere are found peculiar littoral birds known as sheath-bills (Chionididae). The sheath-bill is an omnivor- ous scavenger but also feeds in the inter-tidal zone on small fishes and invertebrates. Another antarctic scavenger is the giant fulmar (Macro- nectes giganteus). At sea it feeds on crustacea and squid, but it spends much time on land, where it eats offal and attacks other birds. Gulls have varied feeding habits. They gather shellfish, worms, crusta- 246 THE BIOLOGY OF MARINE ANIMALS cea, etc., on the shore when the tide is out, and also fish and play the role of scavengers in coastal waters. Terns are mainly coastal birds of warmer waters, although some species migrate far north to breed, notably the arctic tern Sterna paradisea. Their food is mainly small fish which they secure by plunging from the wing into the sea. To a minor extent small cuttlefish, crustacea and pelagic molluscs are also taken. Conspicuous fishers in coastal waters are cormorants and shags (Phala- crocoracidae). Their food consists chiefly of fish and crustacea, obtained by diving from the surface. Cormorants possess hooked mandibles, and employ feet and wings for propulsion under water. Pelicans (Pelicanidae) frequent coasts and estuaries, where they dive for fish in shallow waters. Frigate-birds (Fregata) are tropical and completely aerial in habits. They feed on fish, molluscs, jellyfishes, etc., picked up from the surface, and they also force other birds such as gulls and terns to release their catch. Characteristic coastal and offshore birds of the continental shelf in the northern hemisphere are the auks, guillemots and puffins (Alcidae). Their food consists of planktonic crustacea and fish, which are captured by diving from the surface. Gannets and boobies (Sulidae) are also fish eaters, which plunge from the wing and pursue their prey beneath the surface. These birds generally have lance-shaped bills suited for catching fish and larger crustaceans. In the southern hemisphere the role of the Alcidae is filled by penguins and diving petrels. Some penguins are pelagic in the non-breeding season, others sedentary in habits. Their food consists almost entirely of fish, cuttlefish and crustacea, obtained by diving. King penguins consume squid and fish; smaller penguins, such as the Adélie and Gentoo, feed extensively on krill, especially Euphausia. The diving petrels of the southern hemisphere (Pelecanoididae) are usually found in coastal regions, although some species are partly pelagic in habits. Their food consists largely of small fish and crustacea, obtained by diving. Among the characteristic pelagic birds are the Tubinares—petrels, shearwaters, fulmars and albatrosses. Most of these birds spend much of their lives far beyond sight of land. Their food consists to a large extent of surface plankton, including crustacea, jellyfish, molluscs, as well as squid and small fish. They frequently glean their food from the surface as they skim over the waves, but they sometimes settle to swim about and feed when food is concentrated, or dive short distances below the surface. With the large-scale exploitation of marine resources now practised by man, namely whaling and trawling, some sea birds, notably fulmars, obtain a substantial amount of their food from the offal thus afforded. Indeed Fisher (26) advances evidence for the thesis that the phenomenal increase in numbers of the fulmar (Fu/marus glacialis) in the North Atlantic during the past three centuries is the result of the additional food provided for these birds by the activities of whalers and, later, steam trawlers. Other pelagic birds have diets as follows: penguins (fish and plankton feeders); kittiwakes and swallow-tailed gulls (fish, crustacea, molluscs, NUTRITION AND FEEDING MECHANISMS DAT offal); tropic birds (fish and squid) (1, 18, 23, 26, 41, 55, 60, 75, 78, 88, 95, 96, 108, 109). Mammals. Carnivorous marine mammals include whales, seals, sea otters and even bats. The toothed whales (Odontoceti) are hunters and exploit many forms of nekton. The sperm whale Physeter feeds on fishes and especially cepha- lopods. It is believed that squid are caught during deep dives. Porpoises and dolphins are voracious feeders on small fish. The killer whales (Orca) are powerful rapacious animals and are the only whales that attack other cetaceans. They eat whole porpoises, seals, and kill walruses and large whales. Seals differ greatly in feeding habits. Some seals are planktonic feeders. Ringed seals (Phoca hispida) and harp seals (P. groen/andica) in the Canadian Arctic eat mostly planktonic crustacea, at least in summer. Crab-eating seals (Lobodon carcinophagus) of the Antarctic are selective feeders, con- suming krill. Fish plays an important part in the diet of eared seals (Callorhinus ursina). Elephant seals (Mirounga leonina) of South Georgia capture cephalopods; walruses (Odobenus) dive after bivalves. In antarctic waters the large leopard seal, besides eating fish and cephalopods, attacks penguins and other seals (25, 37, 72, 73). Among other carnivorous marine mammals we may note the interesting sea-otter (Enhydra lutris), colonies of which occur off the west coast of North America. Their food consists of hard-shelled invertebrates—clams, sea-urchins, abalones, ete.—which they collect on the bottom. A curious return to the sea has been made by certain peculiar piscivorous bats (Noctilio, Pizonyx) which fly offshore and capture fish at the surface, using their hind legs for that purpose (3). MECHANISMS FOR TAKING IN FLUIDS OR SOFT TISSUES These mechanisms are especially characteristic of parasites. Some parasitic polychaetes are described in Chapter 14. /chthyotomus sanguinarius is a blood-sucker, attacking eels (Myrus). It cuts into the flesh of the fish by means of a pair of stylets and pumps in blood with its pharynx. An anticoagulant is secreted by glands near the mouth (Fig. 14.9, p. 595). Marine leeches, such as Pontobdella which attacks rays, also have piercing jaws and secrete an anticoagulant hirudin from salivary glands. Certain gastropods, free-living and parasitic, fall into this category. There are nudibranchs, such as Hermaea, which slit open algae (Codium, etc.) with their radulae, and suck in the fluid contents of the cells. Dendro- doris attacks compound ascidians, sucking in the soft tissues. Among parasites may be mentioned ectoparasitic Aglossa with well-developed suctorial proboscides (p. 596). There is also a semiparasitic lamellibranch Entovalva, which lives in Synapta, and which apparently depends on fluid matter for nutriment. Parasitic crustaceans often depend upon piercing and sucking to obtain 248 THE BIOLOGY OF MARINE ANIMALS nourishment. Parasitic copepods found in fish and invertebrates usually have suctorial mouth parts which allow only liquid or semiliquid food to be ingested. Ectoparasitic Epicaridea possess stylet-like mandibles en- closed in a suctorial oral cone. Pycnogonids are partially sucking, partially raptorial in habit. They are armed with a complicated proboscis for sucking in soft tissues and fluids. They show a preference for hydroids and anthozoans, but also attack _ many other soft-bodied animals, including tunicates, holothurians, etc. Hydranths are seized with the chelae and gradually forced into the mouth. When feeding on larger animals, e.g. actinians, the proboscis is thrust into the host and the tissue juices sucked out. In the hind part of the proboscis there is a filter of chitinous hairs which strain out coarse matter so that only juice and fine particles reach the mid gut (45). 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CHAPTER. 6 DIGESTION For many divisions there are in the stomack of severall animals: what number they maintain in the Scarus and ruminating Fish, common description, or our own experiment hath made no discovery. But in the Ventricle of Porpuses there are three divisions. In many Birds a crop, gizzard, and little receptacle before it; but in Cornigerous animals, which chew the cudd, there are no less than four of distinct position and office. SiR THOMAS BROWNE, Garden of Cyprus INTRODUCTION IN the. preceding chapter we have reviewed various feeding methods encountered among marine animals and we now turn to a consideration of how foodstuffs are digested and absorbed. The earliest holozoic denizens of the seas were probably unicellular forms that ingested their food by phagocytic action. This level of organization is represented by marine rhizopods and flagellates among the protozoa. In this group there is diverse specialization within the boundary of a single cell-equivalent, and intracellular digestive mechanisms are present. Digestion in sponges 1s essentially the same as in protozoans, since they lack a true gut and capture food particles by means of flagellated choanocytes, which resemble choano- flagellate protozoans. In metazoans a true gut is present, except in certain degenerate and parasitic forms, and it is here that the food is processed and broken down preparatory to assimilation by the animal. In the following account attention will be focused on digestive processes in invertebrate metazoans and lower chordates. Comparative reviews of digestion in these animals have been prepared by Yonge (72), Vonk (66), Barrington (9), Prosser (57) and Mansour-Bek (43). FUNCTIONAL DIVISIONS OF THE GUT The foodstuffs utilized by animals and the digestive mechanisms which deal with the ingested food show extraordinary diversity throughout the animal kingdom. Every conceivable kind of organic food is exploited and consumed in the sea. In general it may be said that the form of the alimen- tary canal and the nature of the digestive process are correlated with the mode of feeding and character of the food. From a functional viewpoint Yonge (72) has proposed a classification recognizing the following five regions in the gut: (a) reception; (6) conduction and storage; (c) digestion and internal trituration; (d) absorption; (e) formation and transport of faeces. 254 DIGESTION 25 INGESTION, STORAGE AND TRITURATION OF FOOD Reception The region of reception includes the mouth, buccal cavity and pharynx, together with those diverse ancillary structures employed in feeding— for example, ciliated fields, biting mouth parts, radulae, jaws and sucking apparatuses. The work of these structures in gathering or seizing food has already been described (Chapter 5). Digestion of foodstuffs takes two forms: the food particles are taken in by cells and broken down intracellularly; or they are attacked by digestive enzymes in the gut cavity and the soluble products are absorbed. Food material has to be selected of a size that can be swallowed; or, if of excessive size for the gape, it must be reduced to suitable dimensions. The food of filter and detritus feeders is preselected and consists of fine particles. Scrapers and borers, by means of mechanical aids, obtain their food in particulate condition. Carnivores and omnivores break down prey or food masses by tearing them apart with appendages or mouth parts; grinding them up in the mouth or gizzard; or by subjecting them to chemical action. In any event the food has to be rendered particul- ate in order to permit phagocytosis, or to provide maximal surface for enzymatic action. In the anterior gut region there are teeth in the buccal cavity of verte- brates; radulae and jaws in chitons, gastropods and cephalopods; and jaws on the eversible pharynx of certain errant and carnivorous polychaetes. The muscular pharynx of the polychaete Aphrodite serves as a gizzard for crushing the food. In some animals preliminary chemical action is used to attack and break down large food masses preparatory to swallowing. Certain carnivorous gastropods which lack a gizzard—e.g. Dolium, Cassis—secrete free acid from the buccal glands and use it for dissolving calcareous matter in their food (consisting of other molluscs and echinoderms). A poisonous secre- tion is produced by the buccal glands of certain gastropods (Toxiglossa) and by the posterior salivary gland of the octopus. In the latter animal the gland produces amines having powerful effects on the nervous system, such as tyramine, octopamine and hydroxytryptamine. These toxic secretions are used to immobilize the prey (5a, 27). Equally specialized are the salivary glands of blood-sucking ectoparasites such as leeches and the polychaete Ichthyotomus, which produce an anticoagulant (p. 595). The secretion of the salivary or sublingual glands of lampreys also prevents coagulation of the blood of fishes on which the lamprey feeds. Proteolytic secretions are poured over the food in some instances to reduce it to semi-liquid form, a process termed extra-intestinal digestion. To cope with large food masses starfish evert the stomach and pour proteases over the food, and Portuguese men-of-war discharge ferments through the gasterozooids which adhere to the prey. A polyclad Leptoplana initiates digestion outside the body by exuding protease through the everted pharynx over the food mass. The enzyme in these instances is secreted by 256 THE BIOLOGY OF MARINE ANIMALS the stomach or coelenteron. The octopus predigests its prey by discharging a protease into it. This is said to arise in the posterior salivary gland, along with toxic substances (27, 36a). Salivary glands are frequently present in the anterior gut region. The glands are given topographical names according to their location, e.g. buccal glands opening into the buccal cavity, sublingual glands discharging under the tongue or radula and pharyngeal glands opening into the pharynx. The primitive function of these glands is that of secreting mucus for lubricating the food—for example in triclad turbellarians, gastropods such as Patella, and cephalopods. Unicellular mucus glands are of general occurrence in the gut epithelium as well. In certain animals which have acquired extracellular digestion, the salivary glands secrete digestive ferments. The kinds of enzymes produced by the salivary glands differ according to the animal’s diet. Thus, in many carnivorous gastropods the secretion effects a preliminary digestion of protein, and in herbivorous opisthobranchs it attacks carbohydrates. Secretion of special substances for use outside the body has been mentioned (vide supra). Conduction and Storage Food is conducted to the digestive chambers by the oesophagus. In certain animals the oesophagus is dilated into a crop for storage purposes. In leeches, for example, which depend upon large meals of blood at in- frequent intervals, the crop forms a large part of the gut. A crop is present in herbivorous gastropods such as Patella, Aplysia and Haliotis. The large gastric cavity in fishes may be considered a storage as well as a digestive chamber; when a stomach is wanting, the anterior intestinal region is similarly enlarged (e.g. Chimaera, Fundulus). Some preliminary digestion often takes place in the crop, by enzymes regurgitated from more posterior regions (herbivorous gastropods). Trituration in Gizzards Following the oesophagus, the anterior region of the gut is frequently specialized as a grinding organ or gizzard for reducing food to particles small enough to be further manipulated by the digestive apparatus. A gizzard lined with chitinous teeth is present in various opisthobranchs (e.g. Aplysia) and pteropods. It is particularly well developed in Scaph- ander (Bullidae), where it contains several tough plates capable of crushing shells which are swallowed whole. Septibranchs (Lamellibranchia) are scavengers, and possess a powerful crushing gizzard capable of breaking up large food masses into particles small enough to be ingested by cells of the digestive diverticula. The pyloric stomach of certain bottom-feeding fishes which ingest sand and mud is bulbous and highly muscular (e.g. Mulloides). Trituration is aided by sand taken in with the food (1, 24, 31353149). The stomach of crustacea is a capacious organ, ectodermal in origin and lined with chitin. Here the food is broken up and mixed with digestive DIGESTION D575 enzymes secreted in the midgut. The stomach is relatively simple in the Entomostraca. In the Malacostraca, especially in decapod crustaceans, a powerful gastric mill is present, provided with calcareous teeth which grind up the food. Sorting Mechanisms The stomach is often the site of special mechanisms for sorting out the finely divided food and passing it on to other regions where digestion is completed. In many crustacea these filtering mechanisms are very complex, and guard the entrance to the hepatopancreas in which absorption of lateral food stream Cardiac foregut Midgut filter < (1.931): NICHOLLS, J. V. V., ““The influence of temperature on digestion in Fundulus heteroclitus,’’ Contr. Can. Biol., 7, 45 (1931). NicoL, J. A. C., ““Autonomic nervous systems in lower chordates,” Biol. Revi l27/1 (1952): Norris, E. R. and ELAM, D. W., “‘Preparation and properties of crystalline salmon pepsin,” J. Biol. Chem., 134, 443 (1940). Norris, E. R. and MATHIES, J. C., ““Preparation, properties and crystalliza- tion of tuna pepsin,” J. Biol. Chem., 204, 673 (1953). OwEN, G., “Observations on the stomach and digestive diverticula of the Lamellibranchia,” Quart. J. Micr. Sci., 97, 541 (1956). Prosser, C. L., ““Feeding and digestion,” in Comparative Animal Physiology (London, Saunders, 1950). Ray, D. L. and JuLIAN, J. R., ““Occurrence of cellulase in Limnoria,” Nature, 164, 32 (1952). SAWANO, E., “Studies on the digestive enzymes of Ostrea circumpicta,” Sci. Rep. Tohoku Imp. Univ., Ser. 4, 4, 328 (1929). SAWAYA, P., “Contribuicao para a fisiologia do aparelho de apreensao dos alimentos e da glandula do intestino médio de Ostracodo,”’ Bol. Fac. Filos. Ciénc. S. Paulo, Zool., No. 6, p. 107 (1942). STERN, J. A. and Lockuart, E. E., “A study of the proteolytic enzyme activity of the pyloric caeca of redfish,” J. Fish. Res. Bd Can., 10, 590 (1953). 62. 63. 64. 65. 66. OF: 68. 69. 70. ce 12, U3: 74. DIGESTION 279 Stott, F. C., “The food canal of the sea urchin Echinus esculentus and its functions,” Proc. Zool. Soc. Lond., 125, 63 (1955). TAKESUE, K., ““Studies on the change of the digestive enzyme systems with growth of the fish, Plecoglossus altivelis,’ J. Shimonoseki Coll. Fish., 3, 89 (1954). THOMAS, J. G., Pomatoceros, Sabella and Amphitrite, L.M.B.C. Mem. 33 (Liverpool Univ. Press, 1940). ULLMAN, A. and BOOKHOUT, C. G., ““The histology of the digestive tract of Clymenella torquata,’ J. Morph., 84, 31 (1949). VonkK, H. J., ““The specificity and collaboration of digestive enzymes in Metazoa,”’ Biol. Rev., 12, 245 (1937). WEEL, P. B. VAN, “Processes of secretion, restitution, and resorption in gland of midgut of Atya spinipes,” Physiol. Zool., 28, 40 (1955). WELLS, G. P., ‘““Defaecation in relation to the spontaneous activity cycles of Arenicola marina,” J. Mar. Biol. Ass. U.K., 32, 51 (1943). YONGE, C. M., “Studies on the comparative physiology of digestion. 2. The mechanism of feeding, digestion, and assimilation in Nephrops norvegi- cus,” J. Exp. Biol., 1, 343 (1924). YONGE, C. M., “Structure and physiology of the organs of feeding and digestion in Ostrea edulis,’ J. Mar. Biol. Ass. U.K., 14, 295 (1926). YONGE, C. M., “‘Digestive processes in marine invertebrates and fishes,” J. Cons. Int. Explor. Mer, 6, 175 (1931). YONGE, C. M., “Evolution and adaptation in the digestive system of the metazoa,” Biol. Rev., 12, 87 (1937). YONGE, C. M., “‘Recent work on the digestion of cellulose and chitin by invertebrates,’ Sci. Progr., 32, 638 (1938). ZACKS, S. I. and WELSH, J. H., “Cholinesterase and lipase in the amoebo- cytes, intestinal epithelium and heart muscle of the quahog, Venus merce- naria,” Biol. Bull., 105, 200 (1953). CHAPTER EXCRETION Les corps ammoniacaux sont toxiques, ils doivent étre éliminés rapide- ment, ou étre transformés en corps moins toxiques (urée, acide urique) pour éviter leur accumulation dans l’organisme. H. DELAUNAY, 1931 INTRODUCTION EXCRETION, in general, refers to the elimination of waste or poisonous substances from the organism. In this sense it obviously embraces a multiplicity of processes and functions and requires closer definition. On occasion the loose use of the term has resulted, at best, in loss of precision and, at worst, in much confusion in zoological literature. This is apparent in treatment both of excretory organs and excretory processes. As with many other functions, excretion is best known among vertebrates, es- pecially mammals. Among invertebrates our knowledge of excretion is still fragmentary. Reviews of certain aspects of the subject are available, notably those of Delaunay (8), Scheer (44), Prosser (39) and Baldwin (2a). As the result of processes of growth, metamorphosis, tissue maintenance and metabolism, an animal periodically or continually produces and accumulates waste materials, obsolete tissue and metabolic end-products which are eliminated in various ways. The nature and amount of these products depend upon the animal’s way of life, its diet, activities and environmental conditions. Let us consider these first in terms of materials which are eliminated. The foodstuffs taken in by the animal contain a certain amount of indigestible material, and this is eliminated in the faeces, together with mucus and any other products which may be discharged into the aliment- ary canal. All animals, except possibly in some instances of suspended animation, are engaged in metabolizing organic materials, either breaking them down to provide energy, or building them up into specific products or tissues. Of the foodstuffs which are metabolized by the animal, carbohydrates and fats are oxidized to water and carbon dioxide, and are readily elimin- ated as such. The nitrogenous compounds, predominantly proteins and nucleic acids, are degraded into various nitrogenous end-products, which are eliminated in several ways according to the species. A large part of the nitrogen (about 90%) is derived from the a-amino-N of amino-acids which are split off from proteins through the action of proteolytic enzymes. Amino-acids are deaminated for the most part to ammonia. Other 280 EXCRETION 281 nitrogenous waste products from protein catabolism include unchanged amino-acids, and urea, uric acid and trimethylamine oxide. Nucleic acid metabolism yields purines, which may be excreted in this form, or undergo further deamination leading to uric acid, urea or finally ammonia. Por- phyrin metabolism yields still further nitrogenous end-products, which are sometimes conspicuous because of their coloration. Nearly all animals show some degree of ionic regulation, and maintain the internal milieu constant by processes of differential absorption and excretion of specific ions. Most marine invertebrates are isosmotic with sea water, but in estuarine environments species with hyperosmotic body fluids may have to pump out excess water which tends to flow into the organism, in addition to that produced by the oxidation of foodstuffs. Marine vertebrates are usually hypo-osmotic to sea water (except elasmobranchs), and can utilize some of this metabolic water to reduce anisosmotic hazard. Animals are normally efficient in conserving monosaccharides produced by the hydrolysis of more complex carbo- hydrates. There are instances in which the animal casts off part of its body at intervals. In the Polyzoa the polyp periodically degenerates, forming a compact brown body which is evacuated through the anus when a new polyp is regenerated. Although sometimes considered a device for getting rid of accumulated excretory products, it must be confessed that the significance of this behaviour is unknown. Crustacea moult at intervals, casting off their exoskeleton, with resultant loss of chitin. NITROGEN EXCRETION In this chapter we shall be concerned chiefly with nitrogen excretion. The principal nitrogenous end-product among marine invertebrates is ammonia, a highly toxic substance which must be eliminated rapidly. In these animals the problem of nitrogenous excretion is greatly simplified by the existence of an abundant circumambient medium for carrying away waste materials. In littoral invertebrates which are partially terrestrial in habit, and in fishes whose phyletic history has involved a return to salt water, specialized mechanisms have developed in conjunction with osmotic Stress: In aquatic animals some part of the excretory nitrogen is lost across the general body surface, especially through thin gill membranes where these are present. Part is discharged through special excretory organs, kidneys and nephridia, which often have other important functions as well as nitrogen excretion, namely ionic regulation and osmoregulation. Nitrogen excretion presents three facets: the way in which end-products are produced (biochemistry); the manner in which excretory products are discharged to the exterior (physiology); and the relation between the mode of nitrogen excretion and the environmental conditions in which the animal lives (ecologic aspects). 282 THE BIOLOGY OF MARINE ANIMALS Survey of Nitrogenous End-products among Marine Animals Ammonia is formed by deamination of amino-acids, and sometimes of purines. To avoid toxaemia ammonia must be excreted rapidly or converted into a less toxic compound. Ammonia is very soluble in water, diffuses rapidly and is eliminated readily by aquatic animals. Ammonia tolerance varies among animals but the concentration in the blood is always low. Some recorded values (as mg NH.-N per 100 c.c. or 100 g blood) are: crustaceans, 0-4—2:5; cephalopods, 1-4—4-8; selachians, 1-4~—2-5; and tele- osts, 0-3-5-5 mg%) (Table 7.1). Animals which eliminate a high proportion of nitrogenous waste as ammonia are termed ammonotelic (Needham). These include actinians, polychaetes, sipunculoids, crustaceans, sublittoral gastropods, lamelli- branchs, cephalopods and echinoderms. Teleost fishes also excrete much ammonia (Table 7.2). Bacterial oxidation of ammonia in the sea is discussed by Spencer (50a). Urea is derived from amino compounds, namely amino-acids and purines. Like ammonia it is very soluble and diffusible but much less toxic. Urea forms a much smaller proportion of the total nitrogenous excretion than ammonia in marine invertebrates. It is excreted in small and variable amounts in all the major groups examined, namely coelenterates (actinians), annelids, sipunculoids, molluscs, crustaceans and echinoderms (Table 7.2). Animals in which the principal excretory end-product is urea are termed ureotelic and are found among vertebrates; ureotelism seems never to have been exploited by an invertebrate group. Marine elasmobranchs are highly ureotelic and excrete more than four-fifths of non-protein nitrogen as urea. Uric acid is a relatively non-toxic substance of low solubility, which can be excreted in solid form. It is an important excretory product in birds, reptiles, insects and possibly certain gastropods. In some of these animals the ammonia produced by degradation of proteins is largely converted into uric acid, and such forms are termed uricotelic. Uric acid is also formed by the oxidative deamination of purines and is excreted in traces or small amounts by many marine invertebrates and fishes. The occurrence of ureotelism and uricotelism has special significance in conjunction with environmental conditions and breeding habits, and is discussed in a later section. Purines: when nucleic acids are hydrolysed in the organism, purine and pyrimidine groups are liberated. The purine bases so formed are adenine and guanine. These substances are excreted unaltered by some animals, whereas others degrade them to a greater or lesser extent and excrete them as uric acid, allantoin, allantoic acid, urea or ammonia, depending on the degree of breakdown of the purines. Guanine or guanine- like substances are utilized by certain animals (cephalopods, crustacea, fish) as reflecting agents in chromatophores, sometimes in association with pterines (55). Little is known about the fate of pyrimidine bases. EXCRETION 283 NH, NH—CO CH, | | | \ co CO C—NH, CH,—-N=0 | Pe ie S60 y NH, NH—C—NH CH, UREA URIC ACID TRIMETHYLAMINE OXIDE HN—CO N=C—NH, em | NH;—C C—NH HC C—NH | ll CH | | /CH NG NZ ae one GUANINE ADENINE NH, NH, NH, | | | CO CO—NH CO COOH CO | | ° Sco || | NH—CH—NH NH—CH——NH ALLANTOIN ALLANTOIC ACID Trimethylamine oxide is a nitrogenous base found in marine teleosts, elasmobranchs and in several invertebrate groups, especially molluscs and crustaceans. Levels in blood and tissues of these animals are shown in Table 7.3. The characteristic odour which arises from marine fish after death is due to the liberation of trimethylamine from the oxide as the result of bacterial activity. Trimethylamine oxide is a soluble non-toxic substance with neutral reaction. It forms a considerable proportion of the waste nitrogen excreted by marine teleosts (30°% or more), and smaller amounts are excreted by selachians (Table 7.2). Trimethylamine oxide is possibly produced by methylation of ammonia, but the process is poorly under- stood. Mammalian experiments have shown that trimethylamine can be converted into urea (1). Creatine and creatinine: creatine (as creatine phosphate) plays an im- portant role in muscle metabolism of vertebrates and also occurs in some protochordates and echinoderms (Chapter 9). It is always present in small amounts in the excreta of vertebrates, and is an important con- stituent of urinary nitrogen in some teleosts (Table 7.2). Some creatine may be converted into creatinine, and both occur in the urine of some species (53). Nitrogenous pigments are produced by breakdown of haemochromogens and other substances. The metabolism of haem in vertebrates gives rise to bile pigments which are excreted in the urine and faeces. Little precise information is available about the transformation of nitrogenous pigments among invertebrates. 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A protoporphyrin is found in the integument of starfish (Asterias), and is regarded as a by-product of chlorophyll excretion. Porphyrins (uropor- phryin and coproporphyrin) are deposited in the shells of testaceous molluscs; in species without shells or with uncalcified shells (Duvaucelia and Aplysia), uroporphyrin occurs in the integument (24, 25). MODES OF NITROGEN EXCRETION Ammonotelism Aquatic animals are usually ammonotelic, and produce ammonia as the major end-product of protein catabolism. Small amounts of ammonia may also arise from the breakdown of purines and pyrimidines. No species is restricted entirely to one excretory product, however, and as Table 7.2 shows, ammonotelic animals excrete other nitrogenous compounds, sometimes in traces, often in relatively large amounts. Actinians, for example, are predominantly ammonotelic but they also excrete a little urea, and small amounts of uric acid and tetramine (tetramethyl ammo- nium chloride) are found in the tissues of some species, the latter substance probably being formed by the methylation of ammonia (52a). The principal nitrogenous excretory product of polychaetes is ammonia, but Aphrodite gives off small amounts of urea and uric acid, and small quantities of urea and uric acid have been reported in the coleomic fluid of Arenicola, and - uric acid in Chaetopterus (Tables 7.1 and 7.2). Other predominantly ammonotelic groups are sipunculoids, amphipods, isopods, decapod crustaceans, echinoderms, lamellibranchs, cephalopods, many marine gastropods and ascidians. Nearly all species examined excrete small quantities of urea, and frequently uric acid as well. Concentrations of urea and uric acid in the blood and body fluids of various species are given in Table 7.1. The small amounts of urea and uric acid produced by am- monotelic animals are derived largely from degradation of nucleic acids (8,10: 15015a, 34). Some species, although predominantly ammonotelic, excrete consider- able quantities of amino-acids. Thus, echinoderms (starfish, urchins and cucumbers) lose amino-acids in quantities that may equal the ammonia excreted. Crustacea excrete surprisingly large amounts of amino-acids, up to 10% of total non-protein nitrogen in some species. Part of the amino- acid nitrogen is discharged through the excretory organs proper, but there is a certain amount of leakage across the body surface as well. Excretion of amino-acids represents loss of potentially useful metabolites, and points to inefficiency of conservation mechanisms (2). Uricotelism in Vertebrates Uric acid is the main nitrogenous end-product of protein metabolism in two groups of higher vertebrates, namely Squamata (lizards and snakes) and birds. This mode of excretion is a specialization adapting these EXCRETION 289 animals to dry terrestrial conditions both in the egg stage (they lay en- closed, cleidoic eggs) and after hatching. Because of its low solubility, uric acid in solution does not cause toxaemia, and by excreting it in solid form water is conserved for other metabolic functions. The same factors favour uric acid as an excretory product in embryonic stages, when it can be stored as solid material until the time of hatching. Uricotelism must have proved advantageous to those species of birds which have re-invaded the seas. Pelagic maritime birds, of course, have no access to fresh water for drinking purposes. Since the salt content of their blood is only about one-third that of sea water, maintenance of water balance is an important factor in their physiology and, for all practical purposes, presents demands as acute to maritime avian species as those obtaining in arid desert regions. Uricotelism, by reducing the water required in excretion, has been a favourable character under conditions of overt or concealed water scarcity. The osmotic relationships obtaining in marine reptiles probably can be bracketed with those found in birds but, unfortunately, very little is known of the physiology of those forms. Among sea-snakes (Hydrophiidae), some are oviparous (Laticaudinae), others viviparous (Hydrophiinae), and one species, Pe/amis platurus, is wholly pelagic in habit. Presumably these forms, of recent evolutionary origin, have retained the uricotelism of terrestrial ophidians. The green turtle Che/onia mydas excretes both urea and ammonia as major nitrogenous end-products, and possibly has undergone little alteration in its mode of nitrogen excretion during pro- gression towards a maritime existence (Table 7.2). Terrestrial forms, on the other hand, appear to be in the process of evolving towards uricotelism at the present time. Marine turtles bury their eggs on sandy shores and during development the production of urea predominates over uric acid, as in the adult. Total nitrogen actually diminishes, and it appears that the turtle egg absorbs water from the moist sand in which it lies and gives off end-products of nitrogen metabolism through the shell (22, 26, 32, 33, 50). Ureotelic Fishes and Excretion of Trimethylamine Oxide A second mode of nitrogen excretion which has appeared among vertebrates is ureotelism. Teleosts tend to be ammonotelic but many species produce appreciable quantities of urea, amounting to a tenth or a fifth of total excretory nitrogen in some marine forms. A striking peculiarity of marine teleosts is found in the large amounts of nitrogen excreted as trimethylamine oxide (TMO). Trimethylamine oxide is found in muscles, blood and other tissues: concentrations are highest in marine species, and much smaller amounts occur in freshwater teleosts. It also occurs in molluscs and crustacea (up to 300 mg % in muscles of the lobster Homarus), but marine teleosts alone excrete it as an important nitrogenous end- product (Tables 7.2, 7.3). Variations in concentration of TMO are reported in different species of teleosts. Several factors affect TMO content, among which may be enumerated environment, season, size, age, etc. Thus arctic species of teleosts tend to have higher concentrations of M.A.—10 290 THE BIOLOGY OF MARINE ANIMALS TMO than species from the North Sea, and seasonal changes have been noted in the herring (46). The burden of palaeontological and physiological evidence indicates that teleosts have made a secondary return to the sea. The ancestors of extant marine species were originally freshwater inhabitants, and we find that all marine teleosts today still retain a blood osmotic pressure which is much less than that of sea water. This condition imposes a strong os- motic gradient which the fish must combat to maintain physiological homeostasis, and it is reasonable to suppose that the synthesis of TMO, by substituting a highly soluble non-toxic substance for ammonia, is an adaptation linked with the existence of osmotic stress and the necessity of conserving water. Part of the trimethylamine oxide possibly arises from detoxication of NH;, but a certain amount is exogenous in origin, originat- ing in food. Chinook salmon (Oncorhynchus tshawytscha), for example, lack TMO when in fresh water, although it is present in sea-run adults. Salt water does not influence deposition of TMO but feeding Pecten muscle (which contains TMO) causes rapid accumulation of the sub- stance in salmon flesh. It has also been suggested that the trimethylamine oxide occurring in the blood and tissues of marine teleosts may have osmoregulatory significance, but the small quantities present in the blood and the ease with which it crosses the gill membranes show that its osmotic effect must be small (4, 35). Elasmobranchs are unique in that they combine a high level of uraemia with ureotelism. Urea levels in the blood of selachians lie around 2-2-5 % (Table 2.7), and this substance forms the principal nitrogenous excretory product (80-90% of the total nitrogen excreted). The gills are relatively impermeable to urea and the kidneys regulate the amount of urea ex- creted so as to maintain the blood concentration at a high level. As a consequence of the high urea content, the blood of marine elasmobranchs is slightly hypertonic to sea water although the salt content is actually less than that of the outside medium (Chapter 2). It has been argued, therefore, that modern elasmobranchs, like teleosts, are derived from freshwater ancestors, and that by acquiring external membranes imper- meable to urea they have been able to turn it to use in an osmoregulatory role. The regulation of osmotic pressure during development in selachians 1s achieved in one of two ways, depending on the method of reproduction in the species. Oviparous forms lay cleidoic eggs which are impermeable to urea. The ammonia which is produced by the embryo during develop- ment is converted into urea, and its accumulation provides a source of osmoregulatory material for the young fish on hatching as well as solving its major excretory problem. In viviparous forms, on the other hand, the urea requirements of the embryo are met by the mother fish which provides the necessary quantities of urea. In addition to urea, considerable quantities of trimethylamine oxide occur in the blood and tissues of elasmobranchs, ranging from 0-25—1-43 % (Table 7.3). It has been reported that the concentration of TMO in the TABLE 7.3—CONCENTRATION OF TRIMETHYLAMINE OXIDE IN BLOOD AND TISSUES OF SOME MARINE ANIMALS (mM per 1,000 g moist tissue or 1,000 c.c. fluid;) Group and Animal Sea anemones: Hexactiniae Polychaeta Terebellidae and Nereidae Brachiopoda Terebratalia transversa Echinodermata Asterias vulgaris Strongylocentrotus franciscanus Cucumaria miniata Mollusca Cryptochiton stelleri Littorina sitkana Ostrea japonica Mytilus edulis Macoma inquinata Cardium californiense C. corbis Pecten hericius P. hericius Octopus apollyon Loligo opalescens Arthropods Copepods, mixture of species Balanus nubilus Amphipods, sand-fleas Pandalus danae Homarus vulgaris Pagurus ochotensis Cancer productus Cyclostomes Myxine glutinosa Fish Squalus acanthias ditto ditto ditto Scyliorhinus stellaris Raja laevis Hydrolagus colliei ditto Sebastodes sp. ditto ditto Scorpaenichthys marmoratus ditto Pseudopleuronectes americanus ditto Gadus callarias Clupea harengus Scomber scombrus Gadus virens ditto Oncorhynchus tshawytscha (s.w.) Sources: Hoppe-Seyler and Schmidt (21); Hoppe-Seyler (19, 20); Kutscher and Ackermann (29); Tissue Entire animal ditto ditto ditto ditto ditto ditto ditto ditto ditto ditto ditto ditto Muscle Entire animal ditto ditto Entire animals ditto ditto ditto Muscle Entire animal ditto ditto Muscle Blood serum Liver Muscle press juice Blood serum Muscle press juice Muscle Blood serum Muscle Heart Blood serum, etc. Muscle Blood serum Muscle Blood serum Muscle Muscle press juice ditto ditto Muscle ditto Beatty (3); Norris and Benoit (35); Dyer (11). s.w., sea water. Content TMO 5:7-27-0 nil negligible 14-2 negligible ditto negligible 46 negligible 50 negligible 292 THE BIOLOGY OF MARINE ANIMALS urine of elasmobranchs is only 10% of that in the blood, and this suggests that it is actively retained by the fish, presumably as an osmoregulatory ascent (11125 172 28). Conditions in Littoral Invertebrates As previously noted the mode of nitrogen excretion has adaptive signific- ance in connexion with osmotic stress and the water relations of the organ- ism. Littoral animals might be expected to show specialized trends in nitrogen excretion corresponding to the degree of exposure and desiccation to which they are subjected, but unfortunately only a few species have been investigated. Gastropods show some tendency for gradation from ammonotelism to uricotelism when species are ranked from aquatic to terrestrial forms. At the one extreme are sublittoral and lower littoral marine gastropods, such as Aplysia, which excrete much ammonia and little uric acid. At the other are the strictly terrestrial snails and slugs, the excreta of which contain more uric acid. Terrestrial and freshwater gas- tropods show a tendency to uricotelism. The excretion of uric acid in lieu of ammonia is of value to animals living under conditions of restricted water supply and producing cleidoic eggs. The uric acid content of the nephridia of a long series of molluscs examined by Needham shows nice correlation with habitat (Table 7.4.) Uric acid levels, low in sublittoral and low-littoral lamellibranchs and gastropods, are higher in littoral periwinkles and show maximal values in terrestrial snails and slugs. Further analyses of the partition of non- protein-N in the excreta of marine molluscs are required to substantiate these correlations (cf. Table 7.2). The different species of periwinkles listed in Table 7.4 show marked zonation on the shore. Two species, the flat periwinkle Littorina littoralis and the common periwinkle L. /ittorea, are found on the lower half of the shore. Occurring at higher levels, from the middle shore to high-tide mark, is the rough periwinkle L. saxatilis, while the small peri- winkle L. neritoides inhabits crevices above high-tide mark. In apparent agreement with the degree of exposure which these different species encounter is an increase in the amount of uric acid excreted, which reaches its maximum in L. neritoides, the species subjected to greatest exposure. There appears to be no correlation between breeding habits and uric acid excretion, as has been suggested, however. Thus, both L. neritoides and L. littorea produce free-swimming larvae; L. littoralis produces large eggs in which the larvae develop to the crawling stage; whereas L. saxatilis re- tains eggs and larvae in a brood pouch. A pulmonate Onchidella celtica, which has returned to the sea and which seeks shelter in moist crevices during low tide, contains little uric acid. Potamopyrgus jenkinsi, an estuarine species which has invaded fresh water during recent historic times, produces little uric acid (Table 7.4). Aquatic and terrestrial amphipods and isopods are essentially am- monotelic, and more than 50% of the total non-protein-N of the excreta EXCRETION 293 consists of NH3. The level of nitrogen excretion is considerably lower in terrestrial than in freshwater, littoral and marine species (Table 7.5). Urea and uric acid, at most, form trivial end-products, and amino-acids seldom TABLE 7.4 Uric AcID CONTENT OF NEPHRIDIA OF SNAILS AND SLUGS (mg/g dry weight) Terrestrial High littoral Helix pomatia 700 Onchidella celtica 1-7 H. aspersa 64, 167 Limax maximus 205 Estuarine and freshwater L. flavus 31 Potamopyrgus jenkinsi 0-1 Littoral Lower littoral and sublittoral Littorina neritoides 25 Buccinum 4 L. saxatilis 5 Gibbula 2 L. littoralis 2°5 Nucella 4:5 L. littorea 1-5 Monodonta 0:56, 2:8 26 summer Nassarius 2:9 44 winter TABLE 7.5 NITROGEN EXCRETION IN ISOPODS AND AMPHIPODS Mean values for total non-protein nitrogen excreted Species Habitat mg N/10g/24 hours Gammarus locusta Marine littoral 4-9 Marinogammarus marinus Marine littoral and estuarine 1-1 M. pirloti Marine littoral 2:9 Gammarus zaddachi Estuarine 6-0 Orchestia sp. Semi-terrestrial 2:0 Ligia oceanica ditto i Oniscus asellus Terrestrial 0-3 Gammarus pulex Freshwater 23 Asellus aquaticus ditto 2°6 (From Dresel and Moyle (10)) exceed 10°%. In these groups excretory adaptation to terrestrial conditions has taken the form of reduction of nitrogen metabolism rather than of transformation of NH, to other less toxic products (9, 10). Among decapod crustaceans the shore crab Carcinus maenas shows no increase in the relative amounts of urea and uric acid excreted over sub- littoral species. Excretion in high-littoral and terrestrial species has not been investigated. ENZYMES INVOLVED IN NITROGEN EXCRETION Protein Degradation Protein degradation and synthesis of particular excretory products are catalysed by enzymes at all stages. Amino-acids are deaminated with the 294 THE BIOLOGY OF MARINE ANIMALS formation of ammonia by enzymes known as deaminases. However, as we have seen, not all groups excrete the nitrogenous end-products of protein metabolism as ammonia; some species convert the latter into urea or uric acid. In mammals, ammonia is synthesized into urea by the Krebs (ornithine) cycle, according to the following schema. Arginine Ammonia Urea Citrulline Ornithine » en ee + Carbon dioxide In this process arginine is converted into ornithine and urea through the catalytic action of arginase present in the liver. The resulting ornithine interacts with ammonia and carbon dioxide, and is reconverted into argin- ine in a repetition of the cycle. Urea may also be formed directly from dietary arginine by the action of arginase. The elasmobranch fishes, in which a high degree of uraemia is a natural condition, contain arginase and synthesize urea in all tissues of the body except the brain and blood; the liver in particular is rich in arginase. It is not known, however, whether the ornithine cycle operates in this group. Teleost fishes show much specific variation in liver-arginase content, but this never attains the high level of selachians. The ornithine cycle is reported to be absent from bony fishes, and the urea excreted is probably formed from dietary arginine. The ornithine cycle does not appear to be present in any invertebrate group. Most marine invertebrates are ammonotelic, and in these animals arginase is absent or occurs only in very small quantities. Thus in a series of marine molluscs investigated by Baldwin, the hepatopancreas (liver) was found to be free of arginase (2). Purine Metabolism From studies of excretory products and enzyme complements, certain tentative conclusions have been drawn about the course of purine meta- bolism in invertebrates. Purine bases which are released by the hydrolysis of nucleic acids are sometimes excreted unaltered. But some animals possess specific enzymes, adenase and guanase, which deaminate the purine bases adenine and guanine, converting them to hypoxanthine and xanthine. These, in turn, may be oxidized to uric acid by xanthine oxidase. Some or all of these compounds are excreted in various proportions by different groups of animals. Various invertebrates possess a further com- plement of enzymes capable of breaking down uric acid through a series of steps to ammonia. An outline for the course of purine degradation is as follows — EXCRETION 295 Purine Bases adenase Adenine | ——-—> | Hypoxanthine | | xanthine guanase y oxidase | Guanine | ee Xanthine | a | Uric acid uricase [at ip ae Polat ohioME Sd < \ Allantoin | allantoicase urease Allantoic | _ ——= | Urea |——-—->| Ammonia | acid In the few flatworms and annelids which have been examined, enzymes capable of deaminating purines are lacking, and purines are excreted. Many other invertebrates possess uricolytic enzymes, namely actinians, sipunculoids, molluscs, crustaceans and echinoderms. All four enzymes capable of decomposing uric acid to ammonia—uricase, allantoinase, allantoicase and urease—have been identified in certain species: such include Sipunculus, Mytilus and Homarus. A small but significant propor- tion of the excretory products shown in Table 7.2 is derived from purine as distinct from protein metabolism. Data relating to processes of purine deamination among invertebrates are very fragmentary, and more bio- chemical information is desirable. Fish degrade purines part or all of the way to urea. The livers of selach- ians (Raja) and of certain teleosts (scombrids ef a/.) contain uricase, allan- toinase and allantoicase; others (salmonids, anguillids, pleuronectids) lack allantoicase. Urea is the final end-product of protein degradation in selachians and it forms a significant proportion of the nitrogenous excreta in some teleosts. Now urease converts urea into ammonia, and its absence in marine fish in conjunction with the conservation of urea in selachians and excretion of urea in teleosts, is significant for osmotic reasons which have already been discussed. It has been pointed out by Florkin that the degradation of amino-acids and purines in many animals tends to terminate in a common end-product. In crustaceans, for example, the end-product of both protein and purine catabolism is ammonia; and in elasmobranchs, urea. Uricolytic enzymes show a rather patchy distribution among animals. Many invertebrates, including some relatively simple groups, possess the full complement of enzymes capable of degrading purines to ammonia. This appears to be a primitive condition, and in the course of evolution certain groups of animals have dropped various enzymes in the series. allantoinase ELIMINATION OF NITROGENOUS EXCRETA: RENAL AND EXTRARENAL ROUTES Invertebrates. Primitive invertebrate groups, including the protozoa, sponges and coelenterates, possess no specialized excretory organs and 296 THE BIOLOGY OF MARINE ANIMALS excretion takes place across the general body surface. Owing to simple organization no tissue in these animals is far removed from the external medium. Ammonia is the principal end-product of protein catabolism and is readily eliminated by diffusion. Echinoderms and tunicates are other groups lacking excretory organs. Most metazoans, however, possess some kind of tubular structures which discharge fluids to the exterior, and on the basis of their morpho- logical appearance such structures are termed excretory organs. Several types can be distinguished, the simplest and most primitive being nephridia. These are found in flatworms, nemertines, certain annelids and cephalo- chordates. A nephridium consists of a hollow flame-cell or solenocyte lying in the parenchyma and a tubule leading from the solenocyte to the exterior (Fig. 7.1). In the intracellular cavity of the solenocyte there are cilia or flagella, the apparent function of which is to drive fluid to the exterior by the creation of a gradient of hydrostatic pressure. In some instances the nephridium may open into the coelom by a ciliated funnel (nephridiostome). Another tubular structure encountered in many metazoans is the coelomoduct. This is a duct which opens into the coelom by a ciliated funnel (coelomostome) and leads to the exterior of the body. The nephri- dium is regarded as an excretory organ, whereas the coelomoduct, primi- tively, is a genital duct. In many polychaetes nephridia and coelomoducts become fused in various ways so as to form a conjoint mixonephridium with excretory and genital functions (Fig. 7.1). Molluscs and crustaceans possess kidneys of more specialized structure. In gastropods and lamellibranchs there are one or two excretory organs, derived from coelomoducts, which open into the pericardium by ciliated reno-pericardial apertures (Fig. 7.2). The central portion of the tubule is usually enlarged as a renal sac, often very extensive and bearing lamella- tions and diverticula. Distally a renal duct leads to the pallial cavity. In addition there are pericardial glands surrounding the auricles in lamelli- branchs. These structures are believed to move waste material from the haemolymph into the pericardial fluid. The renal apparatus of cephalopods is similar to that of other molluscs, and consists of a multi-chambered renal sac, connected by small apertures with the viscero-pericardial coelom, and opening to the exterior by paired renal papillae. The excretory organs of crustaceans consist of two pairs of antennal and maxillary glands, which open at the bases of the corresponding append- ages. Usually only one pair is functional in the adult, and in the Malacos- traca it is the antennal gland. The kidney of decapods arises in an end-sac (coelomic sac), which communicates with an enlargement known as the labyrinth by reason of its convoluted structure. From the latter an excre- tory duct leads to a bladder which opens to the exterior. There is much specific variation in the structure of these organs. The bladder is sometimes greatly enlarged and the excretory duct very short in marine forms (30a) (Fig. 7.3). It must be confessed that very little is known for certain about the Pe el AS. CTRL Rot Fe. RTA my 92K re 1 > EXCRETION 297 physiology of nephridia in lower animals, and any role that they may have in voiding nitrogenous excreta still awaits quantitative analysis. Other suggested functions are osmoregulation and ionic regulation, and in the earthworm it has been discovered that a urine is secreted which is strongly hypotonic to the body fluids (42). In marine gastropods and lamellibranchs ammonia is an important constituent of nitrogen excretion, but various species, notably Mytilus, have an appreciable amino-acid fraction (Table 7.2). No urine analyses are Coelomo- ' stome KY | RARE Dorsal a ieee |p Nephridial Y FEE canal BEE i= Ventral SUT ‘/ SIPOUDA AO AA SNAT ‘OD ‘aJ0poyjAYd AO AKT “A “DIUDZUDY AO SANTINO ‘Vv “SALAWHOATOd AO SHAY “p'Q “OI 2) q U0 © . Seah : Fic. 8.5. OCELLI AND EYES OF GASTROPODS (a) Patella; (b) Haliotis; (c) Murex. (After Plate (121).) Fic. 8.6. POSITION OF EYE-PIGMENTS IN OMMATIDIA OF Palaemonetes UNDER VARIOUS CONDITIONS A, from an eye in the light-condition. B, from a dark-adapted eye. c, from an experi- mental animal which, after being adapted to darkness, was injected with eye-stalk extract prepared from light-adapted specimens. bm, basement membrane; c, cornea; dp, distal pigment; pp, proximal pigment; rh, rhabdome; rp, reflecting pigment. (From Kleinholz (83).) 312 THE BIOLOGY OF MARINE ANIMALS cephalopods the eye has an external cornea, closed over in some forms, a lens, iris-folds and retina (Fig. 8.7). Enclosing the eye is a cartilaginous sclera perforated by fine holes through which pass the optic nerve fibres. The visual cells of the retina bear long rods and contain retinal pigment. Numerous small muscles are attached to the exterior of the eyeball and can move the eye to some extent in all directions. In some forms, e.g. Sepia and Octopus, there is an external eye-fold which closes the eye. There are many peculiarities in the eyes of pelagic cephalopods, the significance of which is poorly understood. One of these, the telescopic eye, found in Toxeuma for example, is an adaptation for weak illumination. A mechanism exists for accommodation in the eye of the cuttlefish (Sepia). In accommodating for distant vision the ciliary muscles contract Fic. 8.7. CAMERA EYES (Left) Sepia. (Right) a teleost. L, lens; G, optic ganglion; O, optic nerve; E/, eyelid; Rc, retinal cells; GL, ganglion layers. (After Plate (121).) and draw the lens towards the retina. It has been found that stimulation of the sclerotic muscles causes the lens to be displaced forwards, and this displacement may be a means of accommodating for very near vision (141). VERTEBRATE Eyes. These are all built on the same fundamental pattern and, except when degenerate, are provided with a lens capable of throwing an image upon the photosensitive retina (Fig. 8.7). The latter is inverted and from front to rear shows the following layers: a feltwork of optic nerve fibres; two layers of ganglion cells (tertiary and secondary), separ- ated by a synaptic layer; a basal layer of rods and cones, the photo- receptors proper. The rods and cones, by virtue of their inherent sensitivities or nervous connexions, are adapted for photoreception over different intensities of illumination. Rods are highly sensitive visual cells, functioning at low in- tensities, whereas the cones operate in bright light. Corresponding to these differences in function are differences in photosensitive substances (p. 329). Neural connexions in the retina are very complex. Cones are con- nected to one or a few nerve fibres, arrangements which promote visual SENSORY ORGANS AND RECEPTION 313 acuity. In contrast, many rods feed via secondary neurones into a single fibre. The latter arrangement, which results in a high degree of summation of the more sensitive photoreceptors, enhances sensitivity at the expense of visual acuity. Probably no other organ in the vertebrate body shows such fine adapta- tions to the functional needs of the animal as does the eye. Only a few factors relating to vision in marine fishes can be considered, and for fuller treatment the reader is referred to Walls (146). The eye in most elasmobranchs and teleosts possesses the normal vertebrate structure. Some deep-water selachians and teleosts have degen- erate eyes and are blind, e.g. Benthobatis (ray) and Ditropichthys (teleost). The majority of bathypelagic and bathybenthic fishes, however, have functional eyes and this is probably due to the widespread occurrence of luminescence in deep-water animals (Chapter 13). The eyes of deep-sea fishes are believed to be the most sensitive in existence and contain enor- mous numbers of rods per unit area of retinal surface (103). Pure rod retinas are found in most elasmobranchs and deep-sea teleosts. This factor, combined with a high degree of summation, makes for in- creased visual sensitivity. The former animals are largely nocturnal in habits, and the latter live in dimly-lit or dark waters below the photo- synthetic zone. A few selachians are known to possess cones, e.g. Mustelus, which is diurnal, and My/iobatis, which is pelagic in habits. Retinae with rods and cones are characteristic of teleosts from well-lighted waters. The eyes of mesopelagic fishes are often relatively large and have wide pupils and large lenses, factors related to the dim light of the regions which they inhabit. Some species—e.g. the hatchet-fish Argyropelecus—have tubular (so called telescopic) eyes, in which the lens is enlarged relative to the size of the eye. The retina in the equatorial region appears thin and degenerate, and the functional retinal surface is confined to the fundus. Because of these optical features a small, bright image is thrown on the retina. In lower vertebrates accommodation is usually accomplished by dis- placement of the lens. The lens at rest is adjusted for near vision (myopia) or distant vision (hypermetropia), according to the animal. In a myopic eye, movement of the lens backwards adjusts for distant objects by ad- vancing the image and bringing it into focus on the retina. A converse process takes place in hypermetropic eyes capable of accommodation. Teleosts are myopic and the eyes at rest are set for near vision. Attached to the lens ligament is a small retractor lentis muscle (Campanula Halleri), which is capable of displacing the lens backwards and accommodating to some extent for distant objects. Nervous control is mediated by the oculomotor nerve. In the tubular eyes of deep-sea teleosts little or no lens movement is possible. Such eyes possess an accessory retina lying on the cylindrical walls near the lens, whereas the main retina lies at the back of the fundus. The accessory retina takes care of distance vision, and 314 THE BIOLOGY OF MARINE ANIMALS the main retina perceives near objects. The eyes of selachians are hyperme- tropic at rest but are capable of some accommodation for near vision. The ciliary body bears a small protractor lentis muscle, which is so oriented that it moves the lens outwards towards the cornea on contracting, and so focuses for near objects (16, 40, 146). Regulation of Light falling upon the Retina Devices exist in various animals for controlling the intensity of light reaching the retinal cells. This may be achieved at the pupillary entrance by movable lids or iris; or by movement of retinal and chorioidal pigments. Migration of pigments in the eye, as will be noted later, has other physio- logical consequences. Cephalopods. The iris of the cephalopod eye is pigmented and highly muscular, and by contraction and dilation it regulates the amount of light entering through the pupil (Sepia, Octopus). Strong illumination causes the pupil to close, darkness produces opening. The two pupils react to light independently of each other. The pupillary reaction to illumina- tion is areflex whose centrelies in the suboesophageal ganglion. Afferent and efferent pathways pass through the optic peduncle, section of which pro- duces maximal dilation. The reflex is susceptible to control by higher centres as revealed by the fact that when an octopus is excited the pupil becomes dilated. An additional pathway, in the superior ophthalmic nerve, inhibits closure of the pupil (153). The retinal sensory cells contain a dark pigment making excursions in light and darkness (Loligo, Sepia, Eledone). After the animal has been in the dark for 24-48 hours, the pigment becomes densely concentrated at the base of each retinal cell. On exposing the animal to light, some of the pigment remains in a basal position while the remainder becomes scattered through the retinal cell and accumulates also at its distal end. Thus, there is a movement of retinal pigment proximally in darkness, and distally in the light (118). Crustacea. The position of crustacean eye-pigments differs in the light- and dark-adapted eye, and shows a pattern characteristic of each condition. In the light-adapted eye of Palaemonetes, for instance, the distal retinal pigment envelops the ommatidium and extends inwards as far as the basal retinal pigment, which is dispersed outwards under illumination. The extent of migration depends on the level of incident illumination. In the dark- adapted eye the distal pigment migrates peripherally, while the basal pigment moves inwards and assumes a position below the basement membrane. In the light-adapted eye, therefore, each ommatidium is enclosed in a light-absorbing sleeve of retinal pigment, the separate ommatidia are screened from each other and the retinular cells are stimu- lated only by light entering that ommatidium (appositional eye). But in the dark-adapted state, as the result of pigment dispersion away from the centre of the ommatidium, the separate elements attain optical continuity and may become exposed to light rays passing through neighbouring SENSORY ORGANS AND RECEPTION a1 ommatidia as well (superpositional eye) (Fig. 8.6). The reflecting white pigment bordering the retinular cells migrates proximally beneath the basement membrane in daylight, and extends distally about the retinular elements in darkness. In the latter arrangement it forms a functional tapetum or reflecting layer (Palaemonetes). The positional changes which the retinal pigments undergo show much variation in different species. In Palaemonetes, as noted, all three pigments migrate, whereas migration is limited to the proximal retinal pigment in the lobster Homarus (86, 90, fel eer 18), In addition to pigment movements evoked directly by changes in Light Dark At day es At night (a) (b) Fic. 8.8. INFLUENCE OF DIURNAL RHYTHM ON THE POSITION OF IRIS (RETINAL) PIGMENTS IN THE COMPOUND EYE OF A DECAPOD CRUSTACEAN (a) Shows the pigment systems in light-adaptation during the day and during the night, respectively; (b) shows the migration of the pigment in the dark-adapted eye under the influence of the diurnal rhythm in daytime and at night. c., cornea; h.c., hypodermis cell; cr.c., crystal cone; i.f., iris tapetum; /.p., iris pigment; rh., rhabdome; r.p., retinal pigment; m.f., membrana fenestra; ¢., tapetum; e.s., eye-stalk. (From Henkes (68).) environmental illumination, there are rhythmically occurring diurnal migrations which take place independently of any changes in light in- tensity, and persist even when environmental conditions remain constant (Figs. 8.8, 8.9). The occurrence of persistent diurnal rhythms in the migration of eye pigments has been noted in many crustacea (Palaemonetes, Portunus, Homarus, etc.). In Palaemonetes the diurnal rhythm continues for months in animals which are kept under conditions of constant darkness and temperature. Persistent rhythmical movements of retinal pigment can also be observed under conditions of constant illumination, but are of much smaller magnitude (68, 85, 113, 130, 154, 155). It has been discovered that the eye-stalks of various decapod crustaceans contain substances capable of influencing the position of the eye-pigments. 316 THE BIOLOGY OF MARINE ANIMALS One of these substances is a pigment-dispersing principle or hormone. When extracts prepared from the eye-stalks of light-adapted prawns (Palaemonetes) are injected into dark-adapted animals retained in darkness, the distal and reflecting eye-pigments move proximally into the positions characteristic of the light-adapted state (Fig. 8.10). Eye-stalk extracts taken from light-adapted animals are more effective in causing pigment migration than extracts prepared from dark-adapted specimens (Fig. 8.6). This result indicates a higher content of pigment-dispersing hormone in the sinus glands of light-adapted animals (vide p. 445). A pigment- M N M N M N M N M N M N M WN 22 23 24 25 26 ih 28 29 30 Fic. 8.9. DtURNAL MOVEMENTS OF EYE-PIGMENTS OF A NOCTURNAL SHRIMP Anchistioides antiguensis Records of the movements of distal pigment cells of four animals kept in constant illumination (hollow circles), except for the last two days, and of four animals kept in con- stant darkness (solid circles). N, M, noon and midnight. In the sketch of an ommatidium (Jeft), A is the extreme peripheral or night position of the distal pigment cells; B, the inner or day position, in constant darkness. Plotted points refer to distance from cornea to outer boundary of the pigments (each unit = 10 yz). (From Welsh (154).) dispersing principle has also been recognized in the eye-stalks of many brachyurans (Cancer, Uca, Libinia, etc.). Even a brief exposure to light (5 min for Palaemonetes) suffices to release enough stored dispersing-hormone to produce light-adaptation of the distal pigment cells. Light-adaptation continues for a further 10 min after the animal is returned to darkness, indicating continued release of dispersing-hormone during that period, and its accumulation in the blood. Other work points to the existence of a second hormonal factor involved in dark-adaptation. It appears that some controlling centre is activated by photic stimula- tion, and the degree of activation, in turn, determines the relative pro- portions of the two retinal pigment hormones which are secreted. The SENSORY ORGANS AND RECEPTION S7 persistent periodicity in retinal pigment migration, observed in many crustaceans, depends upon regular rhythmicity in the release of the regulatory hormones. In Palaemonetes the production of the dark- adapting hormone reaches its maximum about midnight, and is minimal at dawn; during the daylight hours the hormone is stored in preparation for the nocturnal phase of the cycle. In Palaemonetes the eye-stalks are the chief source of the retinal pig- ment-dispersing hormone, but lesser amounts occur elsewhere in the c.n.s. (brain, connectives and ventral ganglia). Similarly, stores of dis- 0-20 0-15 ° S Distal Pigment Index S DH 0 60 120 180 240 Time (minutes) Fic. 8.10. RESPONSE OF THE DISTAL RETINAL PIGMENT OF DARK-ADAPTED PRAWNS (Palaemonetes) TO INJECTION OF EYE-STALK EXTRACTS Maximal dispersion of pigment (light-adaptation) occurs 30-60 min after injection. (From Brown et al. (13).) persing hormone have been found in the brain, optic ganglia and other nervous centres of grapsoid crabs (10, 11, 13, 14, 83, 84, 89, 90, 90a, 130, | oo tome Ce Pre oD Photomechanical Changes in Fishes. In fishes three mechanisms are concerned in regulating the amount of light which reaches the retina, namely alteration of pupillary aperture, migration of retinal pigment and movement of visual cells. These adaptations are found in diurnal species of the neritic zone. PUPILLARY MOVEMENT. According to their habits, three groups of selachians can be distinguished. These are diurnal (day-feeding) sela- chians such as Mustelus, having pupils wide open in the daytime; nocturnal (night-feeding) forms, e.g. Scyliorhinus, Raja, whose pupils close almost completely in daylight; and deep-sea forms such as Spinax, with large eyes having wide pupils and weak iris musculature. The sphincter iris muscle 318 THE BIOLOGY OF MARINE ANIMALS contracts in direct response to illumination and is not under nervous control, but the movement is much more marked in nocturnal species (Fig. 8.11). The dilator muscle is under control of the oculomotor nerve, stimulation of which causes the pupil to open. Pupillary responses occur in some benthic teleosts (eel, star-gazer, angler fish, flat fishes), but are the exception rather than the rule, since the iris shows little movement in the majority of teleosts. The pupil of the eel (Anguilla) is capable of wide changes in diameter, but control is mainly by direct response of the sphincter muscle to incident light. The pupil of the isolated eye constricts when illuminated and re-expands in darkness. The well-developed pupillary reaction of other teleosts (Uranoscopus, Red Light Movement 5 ears 10 Minutes Fic. 8.11. MOVEMENT OF THE DORSAL MARGIN OF THE PUPIL IN THE ISOLATED EYE OF Scyliorhinus stellaris, (N RESPONSE TO ILLUMINATION (from Young (168).) Lophius) is reflexly controlled by antagonistic nerves. Sphincter muscles are supplied by the sympathetic system, stimulation of which causes constric- tion of the pupil. Dilator fibres originate in the ciliary ganglion, and stimu- lation of the oculomotor nerve produces dilatation of the pupil (16, 167, 168, 169). RETINAL PHOTOMECHANICAL CHANGES. In elasmobranchs (Mustelus, Galeus, Raja, etc.), the chorioid contains a guanin layer (tapetum) which can be exposed or occluded by migratory chorioidal pigmented cells (Fig. 8.12). The guanin-containing cells extend obliquely towards the retina and overlap each other like tiles. Internally, there is a layer of pigmented cells having processes which can project over the guanin cells. In bright light the migratory pigment cells expand and cover the guanin cells with pigmented processes. Consequently, the guanin cells are screened SENSORY ORGANS AND RECEPTION 319 by black pigment which absorbs incident light. In dim illumination the processes are retracted and the guanin cells, now exposed, reflect light reaching them back upon the retina. This mechanism provides more light for the rods in dark-adaptation, and provides a sharper image in the light- adapted eye. A tapetum is sometimes wanting (abyssal shark Somniosus, pelagic ray Myliobatis, etc.) (146). In the eyes of some teleosts there occur photomechanical changes consisting of migrations of retinal pigment and movements of the visual cells. These changes are most conspicuous in duplex retinae (provided Fic. 8.12. OCCLUSIBLE CHORIOIDAL TAPETUM OF ELASMOBRANCHS (Left) Section through chorioid of light-adapted eye, showing pigmented processes expanded on external surfaces of guanin plates, shielding them from light which has passed through the retina. (Right) dark-adapted condition, showing pigmented processes retracted, whereby the guanin can reflect light back through the visual cells. gp, guanin plates; cc, chorioidal capillaries; n, nucleus of guanin cell; pc, layer of migratory chori- oidal pigment cells; pe, pigment epithelium of retina (devoid of pigment); uc, unmodified chorioid; pp, process of chorioid pigment cells. (Diagram based on Mustelus, from Walls (146), after Franz.) with rods and cones) and are concerned with light- and dark-adaptation of the retina. In teleost eyes provided with retinal photomechanical mechanisms, the retina is backed by a layer of pigmented epithelium having long processes which extend between the visual cells. When the fish is exposed to bright light, pigment granules migrate down these processes, forming sleeves about the visual cells and shielding them from oblique rays. In dim light the pigment migration is reversed. The cones and rods of many teleosts are also contractile. In the light-adapted condition the cones lie outside the pigmented region or migrate away from it, while the rods move in the direction of the pigment and are partially shielded. In darkness the rods 320 THE BIOLOGY OF MARINE ANIMALS shorten and draw their sensitive portions away from the pigment towards the exterior. When mechanical changes are performed extensively and expedi- tiously, as described above, they are doubtless of great value in adjusting the retina to changes of external illumination. In the light-adapted con- dition the cones are exposed to stimulation, while the rods are partially or entirely shielded from strong light. In the dark-adapted state the rods, functional in dim light, are fully exposed, while the cones are displaced out of the way. Examples of fishes whose retinae show photomechanical changes are eels (Anguilla), scorpion-fish (Scorpaena), top-minnows (Fundulus), sticklebacks (Gasterosteus), etc. Retinal pigment is abundant in many pelagic teleosts (mackerels, tunnies, mormyrids, etc.), but it is uncertain to what extent the eye pig- ment is migratory and whether the guanin forms a functional tapetum. In bathypelagic species, such as Evermanella, the pigment epithelium con- tains guanin but no dark pigments, and the tapetum is therefore non- occlusible (16, 40, 118, 146). Kinds of Light Responses Since photic stimuli are utilized as sensory cues in so many forms of behaviour it is possible to select only certain types of photic responses for particular consideration. A well-marked category of photic responses includes orientation reflexes controlled either by intensity differences or the directional properties of light. Many animals show special behavioural responses to sudden changes in light intensity. Tubicolous polychaetes (sabellids, serpulids), gastropods (Onchidella, Chromodoris), cirripedes and others respond by contraction to a sudden decrease of intensity. Hagfishes (Myxine), some anemones (Cerianthus), Mya, Ciona and the enteropneust Saccog/ossus contract under sudden increase in intensity. Still other species react to either decrease or increase of intensity, e.g. sea-urchin Diadema. Allied to such responses are shadow reflexes and reactions to moving objects. The former occurs even in the absence of image-forming eyes, and then depends on successive temporal stimulation of photosensory cells, either dispersed or aggregated into ocelli. Other responses, non-muscular, evoked by photic stimuli are colour changes and luminescence (Pyrosoma) C75 26,e145032-°65,66, 187,007, hil 14): ORIENTATION TO LIGHT: TROPISMS. The oriented responses to light of lower animals are of two kinds: bending reactions and oriented locomotory movements. Originally applied to all oriented responses, the term tropism. is now restricted in animal physiology to the bending reactions of sessile animals. Examples are the heliotropic bending of hydroid polyps (Euden- drium) and sea anemones (Cerianthus), and the bending of sabellid tubes towards the light. Moore (111), who made a particular study of Cerianthus, found that the number of degrees through which the animal turned was proportional to the logarithm of light intensity. SENSORY ORGANS AND RECEPTION 321 Oriented Locomotory Responses. In the simplest type of locomotory response to light, an animal seeks or avoids an illuminated region by a kind of trial and error activity classified as photokinesis. In a non- directional light gradient animals displaying photokinesis congregate in light or dark regions. When there is a tendency to shun the light the animals may move more rapidly or change the direction of movement more often in illuminated areas, with the result that they remain longer or come to rest in dark regions. This kind of response is shown, inter alia, by turbellar- ians. Animals which possess suitably organized photoreceptors can make use of the directional properties of light. When placed in a horizontal light beam they move directly towards or away from the light source according to whether they are photopositive or photonegative. The simplest type of reaction to directed light, known as klinotaxis, is shown by animals which make regular swaying movements, and which are able in consequence to make successive temporal comparisons of light intensity. Examples are Euglena and the planktonic larvae of many benthic animals. The photo- receptors in these animals are simple stigmata or ocelli partially shielded by pigment and so organized that the retina is stimulated by light coming from some particular direction with reference to the axis of the animal’s body. The larvae of Arenicola, like those of many other polychaetes, are strongly photopositive after hatching, and aggregate at the lighted surface of the water by means of klinotaxis. When swimming, the larva rotates on a longitudinal axis and, if it be laterally illuminated, each eye is directed alternately towards and away from the light (Fig. 8.13). When one of the eyes is directed towards the light, the body contracts and the head turns in that direction; this soon results in orientation and the animal continues to swim towards the light source (104, 105). Animals with well-developed photoreceptors orientate directly to a light source in a straight path without pendular movements. In one form of response the animal is able to orient itself through achieving balanced and equal stimulation of symmetrically disposed paired photoreceptors (tropo-taxis). Such responses are widespread in annelids, gastropods, etc. When confronted with two light sources it proceeds at some angle between them, depending on the relative intensity and stimulating power of the two lights. More complex are those responses in which the animal moves directly towards a light source without the necessity of balanced stimulation of two receptors (telo-taxis). In this response the animal orients directly to one of two lights. When Hemimysis, for example, is exposed to a single light, it swims to and fro in line with the beam of light. But when an ad- ditional light is arranged with its beam at right angles to the first, some of the mysids remains in the first light beam, while others cross over and move along the second beam. The mysid is thus capable of selecting by some central process one of the two light beams for orientation, and is not M.A.—11 322 THE BIOLOGY OF MARINE ANIMALS dependent upon balanced stimulation of two photoreceptors. The com- pound eyes of Hemimysis are spherical and are located on movable stalks; consequently it has binocular vision over 360° and is able to orientate while proceeding towards and away from the light (7, 49, 163). Other forms of taxes are the dorsal-light reaction and the light-compass reaction. The dorsal-light reaction is found in many animals which norm- ally swim horizontally with the dorsal surface uppermost, such as Charybdea (Scyphomedusa), Tomopteris (Polychaeta) and Palaemon (Malacostraca). , ee Fic. 8.13. ORIENTATION OF THE LARVA OF Arenicola IN A LIGHT BEAM Arrows indicate direction of the light rays. The larva swims towards the light source in a spiral course. At d the light is changed from m to n, and the animal aligns itself to the new light source. (Lower right) enlarged view of the head, showing paired ocelli. (From Mast (105).) In species with statocysts, both statoreception and photoreception are involved in regulating the response. The mechanism of the dorsal light re- action of Palaemonis depicted in Fig. 8.14. Palaemonis a prawn that possesses statocysts; in normal posture its dorsal surface is kept uppermost, what- ever the direction of incident light. When an intact animal is held obliquely it makes pushing movements with its lower limbs that would tend to return it to the normal position. Quite otherwise is the behaviour of an animal which has been deprived of its statocysts. When turned over on its side so that its back is towards a source of lateral light, it maintains a symmetrical position without attempting to resume an upright posture. SENSORY ORGANS AND RECEPTION = PAS) Again, if the light is coming from the side while the animal is held in its normal position, it makes pushing movements with its legs on the side opposite the light; these movements would tend to turn the animal on its side. In the absence of statocysts, therefore, Pa/aemon orientates itself solely by phototaxis. Another prawn, Processa, which lacks statocysts, Feed Ia Tla Ib Tb ace Pua - a ae ---> os Pe a —> Ila Wa Ib ay ee ---> o— <> ---> —— ---> oN Va Vb FIG. 8.14. ORIENTATION RESPONSES OF A PRAWN UNDER THE INFLUENCE OF LIGHT AND GRAVITY I-IV, animals suspended freely; V, resting on the bottom; a, intact animals; b, animals deprived of statocysts. Broken arrows, Jeft and above, indicate direction of incident light. Curved arrows indicate sense of turning. Small broken arrows in figures indicate direction of thrust of leg. When the animal is suspended freely and held obliquely, the legs on the lower side make pushing movements which would return the animal to its normal position (back uppermost) if it were free to move (Ia—IVa). Lateral illumination does not interfere with this statocyst reaction. When an animal with statocysts removed is held suspended, while light is coming from the side, it makes pushing movements with its legs on the side opposite the light (IIIb). When resting on the bottom and exposed to lateral light, an animal lacking statocysts compromises by inclining the body at an angle of about 45°. (After Alverdes, 1926.) shows photic responses resembling those of Pa/aemon when deprived of statocysts. There are some invertebrates which show orientation to some fixed locus, and a faculty of this kind assumes importance in an animal that has some fixed home to which it returns after each excursion. Cues from several sensory fields may be used in achieving spatial orientation, in- cluding a photic response known as the light-compass reaction. In this reaction the animal need not orientate directly towards the source of light, but instead is able to maintain some definite angle between the light source and its direction of motion. Although best known in terrestrial 324 THE BIOLOGY OF MARINE ANIMALS arthropods, there is a marine nudibranch, Elysia viridis, which shows this response clearly. In a horizontal beam of light it responds by oriented movements and crawls in a straight line. When the position of the light is moved, the sea-slug responds by moving in a new direction, and by repeat- ing this procedure it can be shown that the direction of locomotion pur- sued after each shift of the light source bears a constant relation to the direction of the light. This relationship is expressed in the angle between the longitudinal axis of the body and a line extending from the animal’s eye to the light source. Each animal maintains its orientation angle con- stant, within certain limits which lie between 45° and 135°. The latter limitation has been explained on the basis of the structure of the eyes, which are pigmented cup-shaped ocelli provided with a lens, and so situated that only light falling within an angle of about 35° to 130° can reach the retina. Light-compass reactions are also reported in other marine ani- mals—periwinkles, crabs and polychaetes. This type of response enables an animal to use a light source as a sensory guide for orienting itself while foraging over a wide radius, and releases it from stereotyped progression confined to the path of the light rays (49, 113a). Vertical Migration. Many planktonic animals are known to make regular vertical migrations through the water column each day. The phenomenon has been observed in many groups: tintinnids, siphonophores, polychaetes, pteropods, crustaceans, chaetognaths, appendicularians, fish, etc. Migra- tory planktonic species usually occur at some considerable depth below the surface during the day, and in the evening they migrate towards the surface from the day-depth (Fig. 8.15). Many oceanic species, however, do not necessarily reach the surface but merely rise to a higher level. Sub- sequent temporal phasing involves: a departure from the surface at or before midnight; a return to the surface just before dawn; and a sharp descent to the day-depth when the sunlight starts to penetrate the water. The subject is conveniently reviewed by Russell (126) and Cushing (34). The depth range is sometimes very great, for example, up to 400 m or more for species of pelagic decapod crustaceans in the North Atlantic. Different species inhabit different depths during the day, but even in one species-population the day-depth is variable, depending on age, season, weather conditions, etc. The major determining factor in diurnal migration is light intensity. The depth to which light penetrates changes continuously during the day and from day to day, and it has proved possible to correlate changes in depth- distribution of a species with these fluctuations of light intensity. The daily ascent from the day-depth takes place during falling light intensity. At great depths, where the amount of daylight is always low, the ascent may begin early, in some cases even at midday. Descent in the morning occurs during increasing light intensity. The midnight sinking noticed in many species is seemingly due to a passive condition induced by total darkness. The short rise at dawn represents a return by the animals to the mean optimal light intensity for the population. SENSORY ORGANS AND RECEPTION a5 There is general agreement that vertical migration depends on changing light penetration during the day. Migratory planktonic animals aggregate in a band or region of optimal light intensity. At levels below the optimum, locomotory movements are initiated or increased; and at intensities above the optimum, movements slow down. Experimental evidence for several species reveals that there is a linear relationship between velocity of loco- motory movements and log,, light intensity. Under laboratory conditions Daphnia, a freshwater cladoceran, can be made to execute a complete cycle of vertical migration by varying cyclically Time 1400-1800 2200-0200 0600-1000 1800-2200 0200- 0600 1000-1400 First calyptopis € 50- 0 250- 100 Range of haul(metres wn 9 S 1 o >) 1000-750 NH bas) 9 ond calyptopis 50- 0 100- 50 250- 100 500- 250 rd calyptopis 50- 0 100- 50 250- 100 T 500- 250 | e of haul (metres) Se a a 3 =) > Ran So Le) o ' ~) wn QS 4 750- 500 1000- 750 nge of haul (metres) % R Fic. 8.15. DIAGRAM SHOWING VERTICAL MIGRATION IN CALYPTOPIS STAGES 1-3 oF Euphausia superba IN THE FALKLAND ISLANDS SECTOR OF THE ANTARCTIC (from Fraser, 1936.) the light-intensity. At low intensities, the movement of Daphnia is inde- pendent of the direction of the light and is determined solely by photo- kinesis; the dawn rise is a manifestation of this factor. The photokinetic response continues even in blinded animals. Superimposed on the photo- kinetic response at high light intensities is a phototactic response, in which the animal moves towards the light at reduced light intensities, and away from it when the light intensity is increased. In this way, Daphnia is able to follow a zone of optimum light intensity. One or both of these mechan- 326 THE BIOLOGY OF MARINE ANIMALS isms may be expected to operate in regulating the vertical migrations of some pelagic marine animals (60a, 605). ADAPTATIONS TO WEAK AND BRIGHT LIGHT. Adaptations of this kind are best known in arthropods and vertebrates. Attention has already been drawn to the migratory pigments found in crustacean and fish eyes, and to the occlusible tapeta of certain nocturnal fishes (p. 314). Apposition eyes of arthropods are adapted to function at high light intensities, superposi- tion eyes at low intensities. On migration of the iris pigments the apposition eye may function as a superposition eye. Retinomotor changes in the fish eye act so as to screen the rods from strong light. An occlusible tapetum is found in some nocturnal fish. The tapetum reflects light, which has already passed through the retina, back upon the sensory cells; in consequence, the sensitivity of the eye to weak light is enhanced, but at the expense of visual acuity. The two types of photosensory cells, the rods and cones, of the verte- brate eye function most effectively over different intensity ranges. The rods function at low intensities: when exposed to bright light they become in- sensitive; on return to darkness they regain sensitivity over a period of an hour in man. The cones are less sensitive and are able to function over a range of higher intensities. Diurnal fish possess duplex retinae, containing both rods and cones. Nocturnal fish and those from deep waters possess pure rod retinae. Enhanced sensitivity to weak light is brought about by an increase in the number of rods, and in the number of rods connected to each tertiary neurone. The latter arrangement, allowing greater summation of photo- receptor response, at the same time results in a decrease of visual acuity. DARK ADAPTATION. In general, animals become more sensitive to light after having been in the dark for some time. Curves for two invertebrates, showing recovery of sensitivity to light during the course of dark adapta- tion, may be seen in Fig. 8.16, derived from the work of Hecht. In man and other animals with duplex retinae, vision, during the progress of dark adaptation, is taken over by the rods. The spectral distribution of sensitivity for the rods, based on visual purple, is different from that of the cones (vide p. 329). As the eye becomes dark-adapted, the spectral sen- sitivity curve shifts towards lower wave-lengths, the phenomenon known as the Purkinje shift. There are, consequently, two spectral curves, with maxima at 554 my and 507 mw (in man). These give the distribution of spectral sensitivity in the photopic (light-adapted) and scotopic (dark- adapted) eye, respectively. Since light of longer wave-lengths is preferenti- ally absorbed in penetrating sea water, it can be argued that the Purkinje shift has biological meaning when read in terms of the light conditions obtaining in an aquatic environment. SPECTRAL SENSITIVITY. Animals are never equally sensitive to all regions of a spectrum having an equal energy content, and for many reasons it is important to know the spectral sensitivity of their photoreceptors. Deter- minations of this factor have been carried out on many species by several SENSORY ORGANS AND RECEPTION 3271 methods. Briefly these involve study of the behaviour of the animal in differently coloured lights, and measurement of the electrical activity of photoreceptors (either retinal action potentials or optic nerve impulses), when the retina is stimulated with light of selected wave-length and known intensity. The spectral sensitivity curves obtained by these methods are termed action spectra. Some spectral sensitivity curves for lower animals, based on behaviour studies, are shown in Fig. 8.17. These are all marine species with the ex- ception of the freshwater sunfish Lepomis auritus. The data for Cerianthus were obtained by measuring the reaction time for the heliotropic response to light of different wave-lengths. For Mya, the reaction time for with- drawal of the siphons was measured. Telotaxis in two differently coloured iS bo an y Re) G12 % v . © 10 Ciona 25 i) & 8 3 6 M 2 = oe = | U4 & 0 30 60 90 120 Time (minutes) Fic. 8.16. COURSE OF DARK-ADAPTATION IN A LAMELLIBRANCH Mya AND AN ASCIDIAN Ciona The curves were established from reaction times to a standard flash after various periods in the dark. Ordinates, reaction times in seconds: left, Ciona; right, Mya. (Curves from Hecht (66), redrawn and smoothed.) light beams was used to determine the action spectrum of Pa/laemonetes larvae. The behavioural response of Myxine which was utilized was movement preparatory to swimming when the animal was illuminated. For the sunfish, the visual rheotropic response was employed. Further action spectra, for Limulus and Eledone, based on electrical responses of retina and optic nerve, are shown in Fig. 8.18. Most of these curves have the same general shape, but there are significant differences in wave-length of maximal sensitivity, ranging from 490 to 550 my (58, 67, 108a, 111, 136, 159); COLOUR VISION. Colour is one of the attributes of an object which make it visually recognizable. The fundamental discrimination between wave- lengths takes place in the retina, specifically in the cones of the vertebrate eye, but colour vision occurs only when wave-length differences are recognized in the c.n.s. To demonstrate colour vision in animals necessi- tates tedious training experiments (139). 328 THE BIOLOGY OF MARINE ANIMALS Crustaceans are the only marine invertebrates in which colour vision has been postulated. The evidence is derived from: decorating-responses of spider crabs in coloured aquaria; selection of coloured shells by hermit crabs; background responses of shrimps (Crangon); and optomotor res- ponses of crabs and prawns to vertically oriented moving coloured stripes. The latter can change colour with respect to yellow and red environments independently of intensity (147, 165). There is a wealth of observations relating to colour vision in fishes. Experimental techniques have involved the preference method (e.g. 100 Lerianthus 80 mem branaceus Larvae of Palaemonetes 60 vulgaris 40 20 dD nw S 0 = 450 500 550 600 650 400 450 500 550 600 650 700 > 100 S Mya | 4 80 arenaria us — © 60 é Lepomis 40 auritus 20 0 i 450 500 550 600 650 450 500 550 500 550 600 Wave-length (mz) Fic. 8.17. ACTION SPECTRA OF VARIOUS ANIMALS, DETERMINED BY BEHAVIOUR STUDIES (Sources: Cerianthus, from Moore (111); Palaemonetes, White (159); Mya, Hecht (67); Myxine, Steven (136); Lepomis, Grundfest (58).) voluntary selection of one among several colours), learning and back- ground responses. Minnows which have been taught to associate a given colour with food subsequently distinguish blue and green from each other, and from yellow and red. On different backgrounds, or in lights of differ- ent colours, various teleosts show chromatic adjustments which depend on environmental colour independently of intensity. The most convincing proof of colour vision in fish has been secured by means of conditioned reflexes. Blennies (Blennius pholis) were given a visual stimulus associated with an electric shock. During the course of training the fish learnt to discriminate grey from other colours whenever they were contrasted, but SENSORY ORGANS AND RECEPTION 329 failed to discriminate varying intensities of grey. Blennies, it was found, possess a definite and wide range of colour vision, and were able to discriminate blue, green and red from grey (19, 69, 148). Photosensitive Pigments For light to affect photosensitive tissue it must be absorbed by some pigment and produce a photochemical change leading eventually to sensory excitation. The first of these photolabile pigments to be isolated was visual purple or rhodopsin, associated with rod function in some vertebrate eyes. Rhodopsin consists of a carotenoid retinene, (vitamin A, aldehyde) con- jugated with a protein opsin. An allied pigment, porphyropsin, is found in % 100 D S Vis (bility tz S RO oS (a) (b) 400 500 600 400 400 600 Wave-length (mu) Fic. 8.18. (a) AVERAGE VISIBILITY CURVE FOR THE LATERAL EYE OF LIMULUS. ACTION SPECTRUM DETERMINED FROM IMPULSE DISCHARGE IN SINGLE OPTIC NERVE Fispres (from Graham and Hartline, 1935). (b) SPECTRAL SENSITIVITY CURVE FOR THE EYE OF Eledone. DATA DERIVED FROM RETINAL ELECTRICAL RESPONSE (from Bliss (8), after Piper.) the rods of many fishes: its prosthetic carotenoid is retinene, (vitamin A, aldehyde). Marine fishes usually possess rhodopsin. Porphyropsins are characteristic of freshwater teleosts, but also occur in some marine forms (Labridae, Coridae) and in species which migrate to and from the sea (alewife, salmon, trout). The eel (Anguilla) and killifish (Fundulus) have both pigments, but predominantly rhodopsin (Fig. 8.19) (143). The absorption maximum of rhodopsin lies at about 500 mu, with variations for different species between 490 and 502 mu. Some visual purples from fish have peaks as follows: Petromyzon marinus, 497 mu; Squalus acanthias, Pleuronectes platessa, Trigla lucerna and Gadus polla- chius, 500 mu. Conger eels and deep-sea teleosts have golden rhodopsins, called chrysopsins, with absorption maxima around 485 mu. The absorp- tion bands of porphyropsins are displaced towards the red, with maxima aad | 330 THE BIOLOGY OF MARINE ANIMALS around 522-533 my. A visual pigment found in chicken cones is iodopsin, which shows maximal absorption at 560 my. Iodopsin contains the same carotenoid fraction as rhodopsin, but differs in its protein moiety (30, 31, 35, 39a, 39b, 54, 78, 79, 143, 144). A systematic study of invertebrate rhodopsins is only beginning. The retina of the squid (Loligo) contains a relatively stable red pigment, with maximal absorption at 493 mu. Squid rhodopsin, like that of vertebrates, contains a retinene chromophore linked to a protein, opsin. Rhodopsins with similar properties are found in the eyes of blue crabs (Callinectes hastatus) and horse-shoe crab (Limulus polyphemus). These have absorp- tion maxima around 480 mu. Other photosensitive pigments have maxima 1-0 0-8 > D> Extinction S ae 2 to 50 600 450 500 § Wavelength Fic. 8.19. ABSORPTION SPECTRA OF PHOTOSENSITIVE PIGMENTS FROM THE RETINAE OF FISH, ILLUSTRATING THE TRANSITION FROM AN EXCLUSIVELY RHODOPSIN TO AN EXCLUSIVELY PORPHYROPSIN SYSTEM The dogfish possesses only rhodopsin; eel and killifish predominantly rhodopsin; brook trout predominantly porphyropsin; white perch only porphyropsin. (From Wald (143).) (derived from difference spectra) as follows: Euphausia, 462 my; Nereis, 505 mu; Asterias, 505 mu; Homarus, 515 my (8, 9, 74a, 80, 120, 144a). The absorption characteristics of the visual pigments determine the spectral sensitivity of the retina. So long as there is not differential absorp- tion elsewhere in the optical system, the absorption spectrum of visual purple should be reflected in the action spectrum of the photoreceptor or organism, and many studies offer confirmation. The scotopic visibility curve for vertebrate eyes is a close replica of the spectral distribution of visual purple sensitivity in the species concerned. The lenses of fishes generally transmit visible light above 400 my equally with regard to wave-length, although in some species there is differential absorption in the blue at wave-lengths up to 440 my (38, 39). SENSORY ORGANS AND RECEPTION 551 Coastal waters vary greatly in their absorption characteristics, depend- ing on turbidity, but transparency is frequently greatest in the green region of the spectrum. The greatest transparency of clear ocean water is at a wave-length of 480 muy, i.e. in the blue region of the spectrum (vide Chapter 1). Absorption in both coastal and deep waters increases greatly above 550 mw. It is within this general range of 480-550 my that the rhodop- sins or visual purples of fish and invertebrates are most sensitive. It follows then that visual pigments having absorption maxima in this range will possess maximal efficiency in a marine environment. Furthermore there is now evidence accumulating that luminescent light, of animal origin, may be of considerable significance for vision in the ocean depths where there is little residual daylight, and it is of interest that the emission spectra of some species (only a few have been studied) lie in this general range (about 490-510 my). The majority of species studied hitherto have been from inshore waters, and further photochemical studies of pelagic species will prove of great interest. The A »q, of the visual pigments of Loligo and Euphausia, it will be noted, lie in the blue, below 500 my, in correlation with the greater transparency of clear ocean water to shorter wave-lengths. Electrical Activity of the Eye Two kinds of electrical activity can be recorded from the neighbourhood of the eye, namely retinal potentials and optic-nerve potentials. When electrodes are connected to front and back of the eye, a potential differ- ence between the two regions is detected. Hlumination of the eye produces a potential change known as the retinal action potential or electro- retinogram. This is in the direction of increased negativity of the free distal ends of the photoreceptor cells. In the vertebrate eye, with its inverted retina, there is an initial negative/positive response at the onset of light (a and b waves), and the potential change is completed with a positive off-effect at cessation of illumination. The components of the electro- retinogram (ERG) refer to changes taking place in the visual cells. Electroretinograms have been obtained from the eyes of various in- vertebrates. The response to light is often a simple negative wave succeeded by a sustained negative potential at a lower plateau level throughout the duration of illumination (Limulus, Loligo, Asterias, etc.) (Figs. 8.20, 8.21). The ERG recorded from the compound eye of Ligia begins with a nega- tive on-effect, quickly followed by an early positive deflexion and rapid return to base line during illumination, and ends with a positive off-effect (535652 125,2140, 1165, 166). Photic stimulation of the eye produces a train of action potentials in the optic nerve. This is well illustrated in records from single optic nerve fibres of Limulus (Fig. 8.22). When stimulated with a long light exposure, the optic fibre begins to discharge after a short latent period, initially at a high frequency, soon followed by a rather steady discharge at a lower frequency; with light off, the discharge ceases. This is an on-effect, the (b) Fic. 8.20. RETINAL ACTION POTENTIALS; (a) Octopus (Eledone moschata). (b) EYE-SPOT OF STARFISH (Asferias) Upward deflexion indicates negativity of lead towards external ends of sensory ceils. Time scale, all records, + sec. Duration of exposure shown below retinal potentials. In- terruption of upper starfish record was 3 sec. ((a) from Frohlich, 1921; (6) from Hartline et al. (63).) Ce eee ee cee ee ee ae Fic. 8.21. ELECTRICAL RESPONSES OF THE LATERAL EYE OF Limulus Action potentials of an isolated ommatidium and its nerve strand, in response to short flashes of light (20 msec) at three intensities (relative values, top to bottom, 1-0, 0-1, 0-01). Upper trace, retinal action potential; /ower trace, spike potentials. Flash indicated by black bar above time trace near beginning of each record. Time scale, 0-2 sec. (From Hartline et al. (63).) SENSORY ORGANS AND RECEPTION 355 eye responding to onset of illumination. It seems that only the eccentric cell of each ommatidium is electrically active and gives rise to conducted spikes in the optic nerve of Limulus. The function of the retinular cells and their axons still awaits explanation (150, 164, 165). Other eyes have on/off systems, the simplest of which is found in the scallop (Pecten). In the mantle eye of this animal there are two retinal layers: one, internal, discharging to onset of light; the other, external, to cessation of illumination or reduction of intensity (61). The highly complex vertebrate eye contains on/off systems, some fibres responding with an on burst, continued discharge during illumination and an off Fic. 8.22. DARK-ADAPTATION OF A SINGLE VISUAL RECEPTOR IN THE EYE oF Limulus The records show action potentials of a single optic nerve fibre in response to illumi- nating the eye with a test flash of light. The test flashes were applied at various times in the dark (indicated in the records) following a period of light-adaptation. Flash duration (8 msec) shown as interruption of white band. Time scale, 0:2 sec. (From Hartline and McDonald (62).) burst; others respond with on/off bursts only; still others show only off effects. The magnitude of the retinal action potential depends on the intensity and, for short flashes, the duration of the light stimulus. Similarly, the characteristics of nervous discharge (frequency, number of impulses) in the optic nerve depend on the conditions of illumination. These relation- ships between stimulus and response have been used extensively for measur- ing certain visual functions, namely spectral sensitivity, dark-adaptation, reciprocal relationship between intensity and duration of illumination, etc. Over a wide range the magnitude of the retinal action potential is related linearly to the logarithm of the light intensity, but the curves tend to become sigmoid at low and high intensities. The lateral eye of Limulus has been used extensively in studies of photo- 334 THE BIOLOGY OF MARINE ANIMALS reception. When an ommatidium is stimulated with brief flashes of light a train of impulses appears in the optic nerve, the number of which depends on the duration and intensity of the stimulus (Fig. 8.21). By varying these two factors it has been possible to determine the flash duration which just produces a single impulse at various intensities. The data so derived show that the reciprocity law (duration x time = a constant) holds for the production of a single impulse. This reciprocal relationship, found in the responses of photoreceptors, has been attributed to photochemical pro- cesses in the eye, and may be an expression of the Bunsen-Roscoe law of photochemistry. Recovery of sensitivity during dark-adaptation in single ommatidia of Limulus is illustrated in Fig. 8.22. The eye was stimulated by brief flashes of light at various intervals after placing the preparation in the dark, and spike potentials were recorded from a single optic-nerve fibre. The number of spike potentials is low at first, increases rapidly during the initial period of dark-adaptation and more slowly thereafter. The time course of re- covery is followed by determining the intensity of a flash necessary to elicit a single impulse, or by measuring the number and frequency of impulses evoked by a constant flash at selected intervals. The rate of recovery depends on the previous light history of the eye, varying with the intensity and duration of previous light adaptation (62). Spectral-sensitivity curves for Limulus and Eledone derived from measure- ments of retinal action potentials and optic nerve potentials are shown in Fig. 8.18. The action spectra obtained by such means generally agree with spectral-sensitivity curves obtained from behaviour studies. There is reasonably close resemblance between spectral-sensitivity curves and the absorption curves of visual pigments (rhodopsin) in those instances where data from both sources are available (63). Many arthropods are sensitive to polarized light and some species make use of this ability in orientation. Cladocerans and amphipods orientate to polarized sky light, and mysids and hermit crabs show distinct reactions to plane-polarized light vibrating in different directions. The compound eye of arthropods acts as a polarization analyser. When a single ommatid- ium in the eye of Limulus is stimulated by short flashes of plane-polarized light and spike potentials are recorded from the optic nerve, the discharge- rate is found to vary with the plane of polarization. Stimulation is maximal near 0° and 180° with respect to some particular setting, and is minimal at 90° where only half as many impulses result from a standard flash (7b, 149, 165). MECHANORECEPTION Tactile stimulation produces mechanical deformation of cellular surfaces. The corresponding sensitivity may take the form of a generalized cellular irritability, as in the independent effectors of sponges, or involve the stimu- lation of free nerve endings or specialized tactile receptors. More sensitive mechanoreceptors detect pressure waves created by vibration of distant SENSORY ORGANS AND RECEPTION 335 sources. Sound waves or high-frequency vibrations are detected by special phonoreceptors. Many animals possess tension receptors, responsive to distortion by stretch. Finally, special mechanoreceptors, the gravity and equilibrium receptors, permit the animal to orient in space. Sensitivity to Touch and Low-Frequency Vibrations The importance of tactile stimuli in the behaviour of animals requires no emphasis. Benthic species are continuously being exposed to contact stimuli of various kinds, free-swimming species respond to contact with solid objects, and many animals are sensitive in various degrees to distant vibrations. Tactile receptors may act as external proprioceptors, providing information about spatial position and movement of parts of the body and in this regard they act in conjunction with internal proprioceptors and equilibrium receptors. TACTILE SENSITIVITY. The simplest type of tactile receptors are free nerve endings lying in the skin. These endings may show generalized irritability, being sensitive to a variety of noxious stimuli, mechanical, thermal and chemical. More primitive animals, such as flatworms, anne- lids, enteropneusts, etc., possess individual sensory neurones scattered through the epidermis (Fig. 8.2). These may take the form of fusiform elements bearing sensory hairs distally, and giving rise to nerve fibres which extend toward the central nervous system (c.n.s.) (94). Crustacea have sensory hairs distributed over the surface of the body, especially at joints of the appendages. Each hair is served by a sensory neurone at its base. Free nerve endings are abundant in the skin of fish, but encapsulated endings, such as occur in higher vertebrates, are rare (160). Local tactile stimulation produces diverse responses. A gentle stimulus evokes movement in a restricted area, e.g. in a single palp or antenna. Under strong stimulation sessile and sedentary animals display withdrawal reflexes involving strong contraction of the body musculature, e.g. sea anemones, polychaetes, holothurians, phoronids, etc. Other reflexes induced by tactile stimulation are luminescence, colour changes, display of protective armament, operation of poisonous devices, etc. Contact stimuli produce orientation reflexes of various kinds. Benthic animals such as polychaetes, which crawl over the substratum, tend to come to rest in crevices or at angles between surfaces. In the absence of other suitable contact stimuli they may bunch together. This type of orientation reflex is termed thigmotaxis. When an animal is resting on the substratum it experiences continuous asymmetrical stimulation of its tactile receptors. In the absence of ventral surface-stimulation, righting reflexes are initiated, e.g. in gastropods, starfishes, etc. Another kind of orientation reflex is rheotaxis, or orientation to water currents. Visual cues are often pre- dominant in orientation, as in lobster and fish, but blind fish can orient themselves in a current when they are resting on the bottom, through frictional stimulation of contact receptors. Regional differences in tangosensitivity have been studied in various 336 THE BIOLOGY OF MARINE ANIMALS marine invertebrates. In the anemone Calliactis parasitica, the oral disc is at least 4,000 times as sensitive as the column to mechanical stimulation; also sensitivity decreases from the base of the column upwards. Now, tactile receptors of Calliactis lie in the column endoderm, the mesogloea of which increases in thickness from pedal edge towards the marginal sphincter. The decrease of sensitivity in the same direction corresponds to this increase of mesogloeal tissue which exerts a shielding effect (119). Tubicolous polychaetes are most sensitive to contact in the region of the gills. In Hydroides dianthus (Serpulidae) the order of decreasing sensitivity for various regions of the body is as follows: gills, head, thorax and ab- domen. In Ho/othuria decreasing sensitivity is shown by: tentacles, oral rim, cloacal rim, podia, anterior body region, posterior end and mid-body surface. Contact stimuli can often be localized with great precision. Foreign bodies are deftly removed from the external surface; urchins, when attacked, direct their spines towards the region affected, etc. In quiescent spinal dogfish and teleosts a localized tactile stimulus throws the body into an S-shape, the posture of which depends on the position of stimulation. A touch anteriorly causes the tail to move to the opposite side; a touch posteriorly, to the same side. Active spinal preparations of the dogfish show persistent locomotory rhythms as long as they are free from contact, while diffuse contact stimulation of the ventral surface produces inhibition of swimming movements. These various kinds of responses to stimulation are instances of reflexes involving peripheral nerve nets or central nervous systems (vide Chapter 10). The neurological basis of locomotory rhythms and reflexes in selachians is considered in detail by Lissmann (96). By recording from the facial nerve of the catfish (Ameiurus) Hoagland (73) has picked up action potentials following mechanical stimulation. Receptors (presumably free nerve endings of Gasserian origin) in lips and barbels are very sensitive to touch and water movements. The spikes are large, indicative of large axons; quite distinct from these are small spikes, produced by chemical agents and transmitted in small fibres of the geniculate ganglion. Pressure receptors in the skin of selachians are of two sorts: free nerve endings and terminal corpuscles. The latter are encapsulated skeins of nerve fibres lying in the connective tissue of the fin (Fig. 8.1). When stimu- lated by pressure they give rise to bursts of impulses in the sensory nerves. Adaptation is slow, and discharge continues for many seconds under maintained steady stimulation. The terminal corpuscles respond to fin movements as well as externally applied pressure, and thus probably act as proprioceptors as well as tactile receptors (99). PROPRIOCEPTORS Proprioceptors are mechanoreceptors that respond to stretch, bending and contraction, and provide information about the movement of body parts. Those best known are the muscle spindles of higher vertebrates, but stretch SENSORY ORGANS AND RECEPTION 337 receptors are also known to occur in fish muscles and in invertebrates. Compared with tactile receptors, tension receptors are usually very slowly adapting. An instance of a slowly-adapting pressure-receptor and proprio- ceptor, responsive to fin movements and pressure deformation in selachians, has been noted in the previous section. Stretch receptors in selachian muscle, histologically unidentified, give rise to a maintained discharge in afferent nerve fibres when a load is applied to the muscle. The frequency of discharge increases with the load, the relationship between frequency and logarithm of tension being linear. On stretching the muscle there is initially a high-frequency discharge, which declines over a period of some 20 sec, owing to adaptation, and is succeeded by a steady rhythmic discharge so long as the tension is maintained. These stretch receptors continue to function rhythmically under constant tension for over an hour. When the tension is suddenly decreased, there follows a silent period before the discharge resumes at a new frequency level corresponding to the reduced tension. In a fin at rest there is a resting discharge from the muscle receptors; bending the fin one way, and then the other, increases and decreases the discharge from a given receptor. This differential response signals the degree of muscular contraction, and the sign and magnitude of fin movements (47). Various kinds of proprioceptors have been described in different Malacostraca. These are: A. organs in the extensor muscles of thorax and abdomen (Malacostraca, except Brachyura): (a) muscle receptor organs; (6) N-cells in the ordinary thoracic muscles. B. pereiopod organs (Deca- poda): (a) rows of nerve cells ending on connective strands in the joints; (6) muscle receptors spanning the thoracico-coxal articulation; (c) in- nervated strands associated with levator and depressor muscles of the pereiopods; (d) “‘myochordotonal organs” of Barth. The joints of the legs (pereiopods) contain at least seven organs consisting of rows of nerve cells (category B (a) above). One of these organs—that occurring at the propodactylopodite joint—has been subjected to physio- logical study. It consists of many nerve cells ending in an elastic strand terminating at the joint. The connective tissue strand is in a stretched condition at all positions, and the amount of stretch increases during flexion. In the afferent nerves coming from the receptor organ there is a resting discharge which depends on the length of the organ. Sudden change in length, and vibrational stimuli, evoke bursts of impulses in the sensory axons (Fig. 8.23). The organ signals rate and extent of movement at the joint; it furthermore appears that some fibres may signal static position and direction of movement (24, 158). Muscle receptor organs occur in the dorsal body wall of thoracic and abdominal segments of malacostracans (Fig. 8.24). There are two organs on each side, lying near the dorsal extensor muscle. Typically, each organ consists of a long thread-like muscle plus sensory, motor and accessory nerve fibres. The cell body of the sensory neurone lies near the muscle, and its dendrites terminate in connective tissue intercalated in the muscle. 338 THE BIOLOGY OF MARINE ANIMALS The motor fibres innervate the muscle, and the accessory fibres form synapses with the dendrites of the receptor neurones. It has been observed that the sensory cells of the organs differ in the length and arrangement of their dendrites. The N-cells are somewhat similar sensory neurones, the processes of which are connected with ordinary muscles (those inserted on the epimeral plate of lobsters) (2, 3, 4, 4a, 48). Earlier conjectures that the muscle receptor organs are responsive to stretch have been confirmed physiologically. Stretching the isolated organ gives rise to a nervous discharge in its sensory axon (Homarus, Panulirus, Fic. 8.23. RESPONSES OF A PROPRIOCEPTOR ORGAN IN THE LEG OF THE CRAB Carcinus maenas TO VIBRATION AND PASSIVE MOVEMENT OF THE PROPODITE- DACTYLUS JOINT Recording from nerves to propodite-dactylus organ. The lower beam indicates movement of the dactylus. (a) Passive extension. (b) (c) Passive flexion at different rates. (d) Two taps on preparation box. (From Burke (24).) Cambarus, etc.). The two organs differ considerably in their physiological characteristics (Fig. 8.25). One has high threshold and adapts quickly to strong stretching in less than a minute. The other has low threshold and maintains continuous discharge for several hours under constant stretch. Furthermore, contraction of the receptor-organ muscle itself can initiate discharge in the sensory axon. The receptor muscle linked with the quickly- adapting neurone gives twitch-like contractions and has a high fusion frequency, whereas the receptor muscle connected with the slowly adapting sensory neurone gives slow contractions and has a low fusion frequency. One of the receptor organs thus acts as a phasic receptor signalling sudden flexion of the tail; the other is a tonic receptor, transmitting information SENSORY ORGANS AND RECEPTION 339 about the degree of flexion existing in each segment of the abdomen (92, 162). By intracellular recording it has been shown that excitation of the sensory cell normally starts in the distal portion of the dendrites, which are depolar- ized by stretch deformation. This potential change, a generator potential, spreads electrotonically over the nearby cell soma, reducing the resting potential of the latter. When the membrane resting potential is lowered by stretch to a certain critical level, conducted impulses are initiated in the sensory axon. The accessory nerve fibres are inhibitory and form a direct Motor fibres me é q a Ss \S , Z teren “C . E