THE AMERICAN OR EASTERN OYSTER 1_ I Q r? /i\ R Y JUN 31965 I W0003 HOLE. MASS. UNITED STATES DEPARTMENT OF THE INTERIOR FISH AND WILDLIFE SERVICE BUREAU OF CO/WAAERCIAL FISHERIES Circular 205 Cover Picture: Oystermen picking out legal sized oysters from material collected from public beds. Small oysters and old shells and their fragments are returned to the beds. UNITED STATES DEPARTMENT OF THE INTERIOR Stewart L. Udall, Secretary Frank P. Briggs, AxsMant Secretary jor Fish and Wildlije FISH AND WILDLIFE SERVICE, Clarence F. Pautzke, Commissioner Bdrea0 of Commep.cial Fisheriks. Donald I.. McKenian, Director The American or Eastern Oyster By VICTOR L. LOOSANOFF Circular 205 Washington, D. C. March 1965 CONTENTS Page Introduction j Environment . Anatomy and physiology 2 Growth g Reproduction Gonad development and spawning 8 Eggs and larvae 12 Effects of tennperature on eggs and larvae 14 Effects of salinity on eggs and larvae 15 Effects of turbidity on eggs and larvae 16 Food and feeding of larvae 17 Metamorphosis, or setting, of larvae 17 Diseases of larvae 17 Oyster enemies Diseases and parasites 17 Predators 20 Competitors 25 Oyster industry 28 Sanitary control 33 Selected references 34 The American or Eastern Oyster By VICTOR L. LOOSANOFF. Senior Scientist Bureau of Commercial Fisheries Tiburon, Calif. INTRODUCTION The American or, as it is more often called. Eastern oyster is the oyster of commerce of our Atlantic and Gulf of Mexico coasts and is also sold in small quantities on the Pacific coast. Its scientific name is Crassostrea virginica, and it is a true oyster, being a member of the Phylunn MoUusca, Class Pelecypoda, and Family Ostreidae. True oysters are distinguished by having dissinnilar lower and upper shells, by attaching the left shell to a substratum, and by having no traces of foot and byssus in adults. Their shell liga- ment is a band between the two valves which may be of triangular shape. Altogether, more than one hundred living species of oysters have been described, but only a few are of economic importance. Most oysters occur between tidal levels or in shallow waters of estuaries, but some species live in depths of several thousand feet. They are en- countered along the temperate and tropical coasts of all continents. In addition to the Eastern oyster, two other commercial species are cultivated in this country. One is the Japanese or, as it is now called. Pacific oyster, Crassostrea gigas, grown on the west coast, principally from im- ported seed: the other is the Olympia oyster, Ostrea lurida, a native of the Pacific coast. In 1949, the European flat oyster, O. edulis, was introduced into New England and now is found occasionally in Maine waters. Recently, small numbers of hatchery-grown oysters of this species have been used in planting experi- ments in California. The Ajnerican oyster (fig. 1) is ^videly dis- tributed and in some areas is extremely abundant. It is found from Massachusetts south along the eastern coast of the United States and also along the Gulf of Mexico coast. Some groups still live in the waters of Maine and New Hampshire where they were considerably more abundant several decades ago. The Annerican oyster is also cultivated in Canadian waters, principally in the shallow southern part of the Gulf of St. Lawrence. The Pacific oyster is imported as seed from Japan and grown in large quantities in Puget Sound, Willapa Bay, and Grays Harbor in Washington, and also in Humboldt, Tomales, and Drakes Bays of California. Small quantities of Pacific oysters are also grown in certain protected inlets along the Oregon coast. It was first imported to the United States on a commercial scale in 1905, but during the last 3 decades its production has rapidly increased and now constitutes about 15 percent of the total annual yield of oyster meats in the United States. It is a large, rapidly growing moUusk, anatomically and in general appearance closely resembling the Eastern oyster. The Olympia oyster, a native of the Pacific coast, is found from Charlottestown, British Columbia, to San Diego Harbor, Calif. It occurs in greatest numbers in Washington, especially in the lower part of Puget Sound. ENVIRONMENT The American oyster (hereafter referred to only as oyster) is adapted to live in waters with considerable variations in salinity and temperature. Its optimum salinity range is roughly from 10.0 to 28.0 parts per thousand (p.p.t.) or, in other words, in ■water containing about 1.0 to 2.8 percent sea salt. The oyster can survive in the open ocean for some time, but usually it does not reproduce or grow well there. It also can survive periods of spring floods or heavy rains when the salinity of the water is abnormally reduced. In such instances, however, the temperature is an extremely important factor in survival because the lower the temperature, the longer the oysters can live in water of low salinity. For example, experiments have shown that when the water temperature is only about 50° F. many oysters can survive exposure to a salinity of 3.0 p.p.t. for 30 days. Oysters in Long Island Sound, where salinity of the water is about 28.0 p.p.t., fed even when placed in water of reduced salinity, sometimes Figure 1. — American or Eastern oyster, Crassostrea virginica, photographed to show its upper and lower shells. New shell growth is clearly seen along the edges. The white tubes on the shells are some of the fouling organisms normally associated with oysters. as low as 5.0 p.p.t,, although under such conditions their behavior was often abnormal and they did not grow. However, in water of 7.5 p.p.t. the oysters grew, but much more slowly than at higher salinities. The lowest salinity at which spawn of Long Island Sound oysters will develop normally is about 7.5 to 10.0 p.p.t. However, some oysters with ripe gonads which developed at an optimum salinity of about 25.0 p.p.t. spawned when placed in a salinity of only 5.0 p.p.t. Experiments also have shown that these oysters can withstand sudden changes from low to high salinity, and vice versa, without serious injury. They fed and expelled true feces within a few hours after the change. In general, oysters accustomed to low salinities stop feeding and close their shells at lower salinities than oysters conditioned to live in water of higher salinity. As a rule, changes in salinity of oyster body fluids closely follow changes in salinity of the surrounding water. The temperature range that oysters can tolerate also varies greatly. For example, in Long Island Sound the winter water tem- perature over the oyster beds may drop below 32° F., while during the warm part of summer it rises to about 73° F. in deep water and as high as 78° F. in inshore areas. Oysters in certain shallow water areas of Chesapeake Bay withstand seasonal temperature variations from the freezing point in winter to 90° F. in summer. In the Gulf of Mexico the annual range of temperatures over the oyster beds is from about 50° to 90° F. It is interesting that the oyster, living between tidal levels, may be frozen solid in winter; yet, if not disturbed, will thaw out and survive upon being covered by water. If a frozen oyster is shaken or dropped, however, changes occur in the cells of its body that lead to death of the moUusk. ANATOMY AND PHYSIOLOGY Various aspects of anatomy and physiology of oysters are described in hundreds of arti- cles written by different authors. This knowl- edge, however, is very well summarized in the book, "Oysters," by C. M. Yonge, which is an important general reference. Most of the following material on anatomy and physiology of oysters is based on Yonge 's summary. The oyster shell consists of two valves held together at the hinge by a complex elastic ligament. The upper valve normally is flat, while the lower is concave, providing space for the body of the oyster. The concave shell is the one by which the oyster is normally attached. The valves fit closely together, mak- ing a watertight seal when the oyster closes, provided the edges of the shells have not been broken off or otherwise damaged. The shell is principally limestone (calcium carbonate) and, therefore, is quite heavy. The formation and repair of the shell are functions of an organ called the mantle, which surrounds the body of the oyster (fig. 2). The umbo, at the hinge end, is the oldest part of the oyster shell. As the oyster grows, the mantle secretes successive layers of shell material, each projecting beyond the previous one. This secretion finally results in a succession of concentric lines marking the external surface of the oyster shell. Basically, the oyster shell consists of three layers. The thin outer layer of organic nature, known as the periostracum, protects the cal- careous shell during its formation. Because it is thin and sometimes quite eroded, this layer is often overlooked. The inner epithelium of the outermost fold of the mantle edge secretes the oyster's periostracum. Under the periostracum are second and third layers of shell. Both have an organic matrix but are largely calcareous. The second is the prismatic layer, which consists chiefly of crystals of calcium carbonate, a linnelike substance. The prismatic layer, secreted by the outer epithelium of the outer fold of the mantle edge, also shows definite concentric markings of successive periods of growth. The third or innermost layer is sometimes called the nacreous layer and consists prin- cipally of thin sheets of calcium carbonate covering the inner shell surface. This surface usually is smooth and white except for an area of purple scar where the shell muscle is attached. The pearly inner layer of the shell is laid down by the entire mantle surface and, as a result, is lustrous and smooth, showing no growth rings. The ligament, which is formed by a spe- cialized gland at the edge of the mantle, consists of the same three shell layers but with the periostracum always worn away and the other two layers not calcified. It is con- tinually being eroded above and being added to below, the latter process being the greater so that it thickens with age. The chemical composition of oyster shells may vary from area to area and sometinnes with individuals of different ages. In general, however, the oyster shell contains from 93 to 95 percent calciunn carbonate and about 0.5 percent organic matter. Magnesium carbonate and calcium sulfate also are present m small quantities. Old oyster shells are used for a variety of purposes, including chicken feed. Most of them, however, are saved and replanted later on the oyster beds to "catch" the new generation of oysters. The main sections of the soft part of the oyster can be seen if one of the shells is removed (fig. 2). A thin, creamy, membrane- like organ called the mantle lies against the inner sides of both valves. The two principal functions of the mantle are to protect the more vital organs and to secrete the shell. Pearls are produced by the oyster mantle, but in the Annerican oyster they are valueless because they lack lustre and usually are mis- shapen. Pearls are small deposits formed around a nucleus, such as the cyst of a para- site, a particle of broken shell, or even a grain of sand. Usually pearls are embedded in the mantle or are found just under the sur- face of the meat; however, they are occa- sionally formed as blisters attached to the inside surfaces of the shell. Sometimes thin layers of greenish or brown- ish color are found embedded in the calcium material of the inner shell surface. These layers, which differ in size, are of organic nature, being composed of material known as conchiolin. These layers may be secreted by the oysters as a defense against the intrusion of boring sponges, worms, or other enemies attacking the shells. These layers, however, may be found in oysters not infested with these forms, A common characteristic of the oyster shell is the occurrence of soft "chalky" material embedded in the harder inner layers. These deposits may be used by the oysters merely as a measure of economy in smoothing out the inner surfaces of the shells, because depositing "chalky" layers requires only one- fifth as much material as is needed to build the same volume of shell of the usual hard, subnacreous layers. Normally, these deposits are laid down in summer and later in the season are covered \vith harder layers. The mantles of oysters have brown, black, or orange margins. The margins regulate the flow of water entering through the shell valves when they are open. The edges of the mantle have many tentacles that perform a variety of functions, including straining the water to keep coarse particles from entering the delicate filtering system of the oysters and warning the oyster of chemical changes inthe surround- ing water. The tentacles are also sensitive to light, enabling the oyster to detect an approach- ing predator when its shadow falls across the mollusk. Near the center of the body is a large ad- ductor muscle, attached to both valves, which controls the closing and opening of the shell. HEART RECTUM MUSCLE GILLS HINGE MOUTH PALPS MANTLE B Figure 2. — Photograph (A) and diagram (B) showing main features of the anatomy of the oyster lying In Its left shell and having the right shell and right half of the mantle removed. Detailed description of anatomy is given in text. When it contracts the shell closes, but upon its relaxation the elastic properties of the ligament located at the hinge force the valves apart. Even superficial examination of this muscle shows that it consists of two different parts (fig. 2): a sennitranslucent part near the hinge, and an opaque, white part surround- ing the first part. Microscopic examination of the tissues composing these two parts of the muscle also shows that their fibrils differ in size and structure. Physiological studies of the muscle have shown clearly that the two parts perform distinct functions. The translucent area can contract very quickly but, on the other hand, it soon tires and is compelled to relax. There- fore, it could be called appropriately the quick muscle. The opaque portion, however, con- tracts relatively slowly but can remain con- tracted for long periods while expending little energy. Furthermore --and this is important-- when the opaque portion of the adductor muscle contracts, it becomes locked as if by a ratchet: hence, it is known as the catch muscle. This long- sustained contraction may be caused by a continuous discharge of nerve impulses. The ability of the oysters to stay closed for several days or even weeks is extremely useful be- cause it helps them survive unfavorable condi- tions, such as freshets or temporary pollution of the water. Shell movements of an oyster caused by contraction or relaxation of the adductor nnuscle can be recorded easily by several means. One method consists of recording the shell motions on a kymograph (an apparatus that has a recording p>en and paper mounted on a drum which rotates at a constant speed). Studies of shell movements are now commonly used to observe behavior of oysters under normal and abnormal conditions. For example, records obtained on shell movements of oysters exposed to different concentrations of pol- lutants in sea water showed the response of the oysters to their presence and the level of tolerance of these substances. Usually, in the presence of pollutants or during unfavorable physical changes in the environnnent, the rh^-thm of opening and closing of the shells is changed, and movements of the valves become irregular. If the concentrations of harnnful substances become too strong, the shells of the oysters remain closed. The main body of the oyster lies between two folds of the mantle which is attached to it. Under the mantle at the anterior end of the body, nearest to the hinge, are four thin lips, or palps (fig. 2), After the lips are four rows of sickle-shaped organs known as the gills, arranged one below the other like pages in a book. The gills extend almost from the mouth, which IS hidden under the palps, to about two- thirds of the distance around the body. At their end the mantle lobes of the two sides are united, and this union divides the nnantle cavity into a large, inhalant chamber containing the gills, and a much smaller, or exhalant, chamber. The water enters the inhalant cham- ber and leaves the oyster body by the exhalant chamber. Therefore, the gills form a com- plete partition dividing the mantle cavity. The only passage from one chamber to another is through the fine interstices or openings between the cross-connected filannents of the gills. As Yonge indicates in his book, the precise region where the water enters the body of the oyster is controlled by the margin of the mantle. Usually this margin separates only in the central region of the inhalant chamber so as to concentrate the inflowing water and increase its speed. The gills of an oyster, and of many other bivalves, are complex organs principally con- cerned with respiration and feeding. In gen- eral, these gills may be compared to a fine sieve. The openings of the sieve, called ostia, are surrounded by structures resembling mi- croscopic whips, or cilia, which beat inward in an orderly manner, producing a current of water that passes through the pipelike struc- tures inside the gills and is finally discharged through the exhalant or cloacal chamber. Surfaces of the gills also are covered with cilia arranged in definite rows. They lash continuously, creating a current of water, and push the microscopic algae and other small particles toward the edges of the gills. The material that settles on the gill surfaces is entangled in mucus secreted by special cells of the gills and is carried gradually toward the mouth. Before the material is swallowed it is sorted, first on the gills themselves but especially in the region of the palps, A portion of the material may be rejected in the form of pseudofeces, which consist of mucus secretions producedby oyster gills in which small marine algae, detritus, and particles of turbidity-causing materials are embedded. When food organisms or tur- bidity-creating particles, such as silt, are too numerous, the oyster may reject as pseudo- feces the largest portion of material collected on its gills. In general, oysters, like many other bivalves, feed most effectively in rela- tively clear water. Scientists have found that the rate of water pumping is affected by several factors, in- cluding temperature, salinity, turbidity, and the presence in the water of various chemical substances. Most Eastern oysters stop feeding and hibernate when the water temperature decreases to about 41° F. Some adult Long Island Sound oysters, however, pump water at temperatures as low as 34° F, but, as a rule, the rate of pumping remains low as long as the temperature of the water is below 46° or 47° F, Within the range of about 47° to 61° F. the rate rapidly increases, but between 61° and about 82° F, the increase is compara- tively slow. Between 83° and 90° F, a further increase in the pumping rate occurs, and it is within this range that the maxinnum average pumping rate of about 3-1/2 gallons per hour per oyster was recorded. Beyond 93° F., however, oysters begin to show a marked decrease in pumping rate, and their shell movements become abnormal. The maximum rate of pumping for an indi- vidual oyster was registered at a little less than 10 gallons per hour. For shorter periods of 5-15 minutes the rate of pumping of the sarne oyster exceeded 10-1/2 gallons per hour. This oyster was only about 4 inches long; it is probable that larger oysters can pump even larger quantities of water per given unit of time. As mentioned before, oysters feed most efficiently when the surrounding water is relatively warm. Under favorable conditions the oyster keeps its shell open about 11 hours during a 24-hour period. There is no corre- lation between opening and closing of the shell and the time of day. The pumping rate of oysters kept at tem- peratures below 40° F. and then quickly changed to a temperature of about 650-68° F. was virtually the sanne as the control ' oysters, thus indicating that the oysters respond and adjust to such radical changes. Oysters, there- fore, are physiologically well adapted to rapid changes which they sometimes encounter in nature, for example, when living on tidal flats, where at autumn low tides the night air may cool the water of the small pools containing the oysters almost to freezing, while during the day the incoming tide may cover the same oysters with much warmer water. Turbidity caused by various substances, in- cluding natural silt, also may affect the rate of pumping. Although very small quantities of silt sometimes stimulate the normal pump- ing activities of oysters, heavier concentra- tions significantly reduce the rate of water pumping and strongly affect the shell move- ments. Long Island Sound oysters living in relatively clear water reduced the pumping rate to about 68 percent of normal after being exposed to a concentration of silty water (one-fiftieth of an ounce of silt per quart of water). Greater concentrations of silt more strongly affected shell movements and rate of water pumping. In turbid waters the oysters discharged large quantities of pseudofeces containing silt. Shell movements of oysters kept in turbid waters were clearly associated with frequent ejection of large quantities of silt and mucus accumulated on the gills and palps. The size and shape of turbidity-producing par- ticles are important. Different turbidity-creat- ing substances, when present in the water in the same concentrations, affect experimental animals in different degrees. The oyster eats plankton, which consists of nnicroscopic plants and animals living in the water. According to some authorities, organic detritus, the product of disintegrating plants and animals, may also contribute to the oyster diet. Many kinds of marine bacteria are also ingested and possibly some nnay be digested. No definite experimental proof exists that oysters can absorb nutriments directly from sea water. Oysters gather food with their gills, and in filtering the water through the gills the oyster retains many microscopic orga- nisms, although some small, elongated forms without appendages may escape. Only 10-50 percent of the bacteria present in sea water are detained by the gills. The food particles caught on the gill sur- faces are embedded in the mucus and are pushed along the upper or lower edges of the gills to the palps, which either may direct food into the nnouth or reject it. Unwanted material is expelled by a sudden closing of the valves. The mouth of the oyster lies between the palps and opens into the esophagus, which leads to the stomach. The stomach opens into the intestine, a long, coiled tube ending in the vent or anus. The stomach is surrounded by a brown digestive gland, which in fat oysters IS obscured by the mantle and by gonad nnate- rial, but in poor oysters, especially after the spawning period, shows clearly through the surrounding tissue. This dark-colored mass of tissue is sometimes misnamed the liver, but usually biologists call it the digestive diverticula. A series of ducts unites this organ with the stomach. As has been described by many authors and so well summarized by Yonge in his book, the digestive system of the oyster possesses a most remarkable structure. Called the crys- talline style, it is a gelatinous rod and occurs only in other bivalves and in certain snails. The head of the style projects from the elon- gated style sac where it is formed and extends across the stomach cavity, pressing against an area known as a gastric shield, which is the only area in the stomach not covered by cilia. The crystalline style rotates continu- ously. This rotation assists in mixing the food, aids digestion, and also brings par- ticles of food in closer contact with the stomach walls. The crystalline style, inci- dentally, is the only known rotating part in any animal. The style, along with several other organs of the oyster, including the digestive diverticula and even blood cells, or leucocytes, is con- cerned with digestion. It is made of protein produced in a long style-sac which is lined with numerous cilia that rotate the style. Normally, under healthy conditions the style is added to continuously by formation of new material. As a result, the style is pushed forward while it is rotated, and its head, directed against the gastric shield of the stomach, dissolves continuously. The disso- lution of the style releases digestive enzymes that convert starches and some celluloses into the sinnple sugar called glucose and also probably break down fats. These enzymes are needed because many plant cells, which comprise the major items of oyster food, contain large quantities of starch. The nnate- rial of the dissolved style also helps to lower stomach acidity to provide optimal conditions for digestion. Thus, the style helps mix the food particles in the stomach and also pro- vides a continuous supply of digestive en- zymes. Since the particles which enter the tubes of the digestive diverticula from the stomach are small, they can be ingested into the cells and go through the so-called intracellular process of digestion, in contrast to the extra- cellular type of digestion taking place in the stomach proper. Both proteins and fats can be digested in this manner. Undigested por- tions of the particles are discharged from the cells. They are finally forced to enter the intestinal groove and are eliminated in the normal way. Blood cells of the oysters also participate in digestion. These phagocytic cells pass through the stomach walls into the stomach, where they engulf small particles such as diatoms. Digestion involves the gradualbreak- down of ingested particles, after which the blood cells move back through the stomach lining into the blood stream. Part of the absorbed food is stored in the oyster body. Oysters are considered "fat" when their meats are large and plump. This condition, however, may be caused either by development of spawn or by accumulation of reserve food material. A plump condition during breeding time results from the in- creased gonads; the fatness of the oyster after the breeding season, when the oyster feeds actively, to the reserve food. The stored material contains many substances, but it consists mainly of animal starch (glycogen), which is found in the large cells of connective tissue in most parts of the body. In oysters of Long Island Sound, accumula- tion of glycogen commences sometimes as early as August and continues until the start of winter hibernation, which is usually early in December when the water temperature decreases to about 41° F. In the warmer waters of Florida, Louisiana, and Texas, fattening may not begin until December. In all cases, the stored food reserves are nor- mally used during the following spring and summer when the oyster grows and forms its reproductive cells. The quality of oyster nneats depends prin- cipally upon its solids and amount of stored glycogen. Good quality oysters usually contain between 18 and 20 percent solids; poor ones may contain less than 10 percent. Normally, a high solids content is accompanied by cor- respondingly high glycogen storage. "Fat" oysters are normally white, and their meats fill the shell cavity. Oyster meat contains large quantities of nutritive substances that people need for a balanced diet. It is high in copper and iron, which are needed for proper composition of human blood, and because of this, oysters are often prescribed for patients with anemia. Oyster meat also contains iodine necessary for normal activities of the thyroid gland. Proteins in oyster meat are especially high in nutritive value, and the carbohydrates, in the form of glycogen, are readily digested and assimilated. Phosphorus and calcium are present in relatively high quantities. Oyster meat also contains most essential vitamins. The condition of oyster meats depends on several factors, such as location of oyster beds, quantity and quality of food present in surrounding waters, salinity of the water, and time of year. The provision that oysters should not be eaten in months that do not contain the letter "r", while valid for the flat European oyster which incubates its young during summer, does not apply to American oysters, which can be and are eaten at any time of the year. Because they are highly perishable, however, their transportation and storage before re- frigeration was widespread created consid- erable difficulties during warm weather. Fur- thermore, oysters that spawn late in summer become watery and unattractive in appearance and are not as acceptable as during the cold season, when the quantity of stored materials is highest. Nevertheless, there are localities in several States where oysters of fairly good quality may be available during the entire year. The circulatory system of the oyster car- ries body fluids from one part of the animal to another. The heart is located above the adductor muscle in the pericardial chamber. It consists of a single ventricle and a pair of contractile auricles, one on each side. The auricles collect blood largely from the gills and push it to the ventricle, which drives it by rhythmical contractions into the anterior and posterior aortas. The posterior aorta is short and supplies only the adductor muscle and the rectal region. The rest of the body is supplied by the anterior aorta, which divides into a series of smaller blood vessels. These vessels open into the so-called blood sinuses, where the blood flows and washes the different organs coming in contact with it. The used blood, now low in oxygen, collects in the veins and is carried into the gills or the organs of excretion, the kidneys, sometimes called the organs of Bojanus. The kidneys, which purify the blood, are located by the adductor muscle and consist of two convoluted tubes connected internally with the pericardium and externally with the exhalant channber. With the assistance of so-called accessory hearts the blood from the kidneys is pumped into the vessels of the mantle, and eventually this blood, together with blood {rora the gills, is returned to the heart by way of the auricles. The blood cells are colorless and contain neither the red hemoglobin that is found in blood cells of higher animals, nor hemocyanin, a blue-colored substance found in other mol- lusks, such as some snails, squids, and octo- puses. The nervous system of the oyster is com- paratively simple. It consists of two knots, or ganglia, of nervous tissue situated near the mouth, from which two nerves pass to another pair of ganglia lying under the adductor muscle. From these two pairs of ganglia small nerves extend to various parts and organs of the body. Oysters, like other mollusks, do not possess the type of centralized nervous system which is characteristic of vertebrates. GROWTH Oyster growth varies considerably with size, temperature, quantity and quality of food, and seasons of the year. As a rule, growth is more rapid in warm waters, such as the Gulf of Mexico, where a marketable size of 3- 1/2 inches may be reached on some beds in 2 years. In northern waters, such as Long Island Sound, characterized by shorter summers and generally lower water tempera- tures, oysters reach this size in 4 or 5 years. The average size of a Long Island Sound oyster at the end of the first, second, third, fourth, and fifth growing periods is 3/4, 2-1/4, 3, 3-1/2, and 4 inches, respectively. Speci- mens measuring more than 14 inches long have been found on old natural beds. The exact maximum age attained by oysters cannot be stated definitely, but the number of layers composing the shells of some unusually large individuals indicates that they may reach an age of 40 years. REPRODUCTION Gonad Derelopment and Spawning Oyster reproductive organs, or gonads, consist of a mass of tissue made of nriicro- scopic tubules, sex cells, and connective tissue which envelops the stomach, digestive diver- ticula, and the fold of the intestine, and, during the period of ripeness, constitutes a significant portion of the entire oyster body (fig. 2). The gonads become larger and thicker as their eggs and sperm mature. When ripe, the gonadal layer in the region of the stomach of an oyster about 4-1/2 inches long may be as thick as a quarter of an inch. The Eastern oyster exhibits alternative sexuality; i.e., adults function seasonally as separate sexes. In this respect, it is like the Japanese oyster, the Australian oyster, Crassostrea commercialis. and the Portuguese oyster, C. angulata. It differs, however, from another group typified by the European oyster, which has a series of alternating male and female phases throughout its life. During the first spawning season of the Eastern oyster, most young individuals function as males. This condition is known as protandry. Nevertheless, the gonads of young oysters may show all gradations from true males, in which no female cells can be found, to those that develop directly into ovaries containing grow- ing eggs. After the second spawning season the numbers of individuals of each sex are almost equal. The adults function seasonally as separate sexes, but, nevertheless, the sex of an oyster is generally unstable and, therefore, a change of sex from male to female and vice versa sometimes occurs. This change usually takes place in the intervals between the two spawn- ing seasons. In experiments with Long Island Sound oysters, about 9.7 percent reversed their sex. The percentage of reversals was considerably higher among the females (13 percent) than among the males (8 percent). Functional hermaphrodites are found among adult oysters, but, as a rule, they are un- common, constituting less than 1 percent of the adult population. Gonads of Eastern oysters change markedly during the year. Such changes may vary con- siderably from one geographical area of the Atlantic or Gulf coast to another, but, never- theless, they correspond to definite seasons of the year and, in general, nnarkedly affect the physiological behavior of the oysters and the chemical composition of their meats. During the cold season, the gonad follicles of oysters of northern waters, such as those of Long Island Sound, are small and contain sex cells only in early stages of development. With the end of hibernation, however, as soon as the temperature of the surrounding water increases, the gonads develop at first slowly and later rapidly, and by the end of June many oysters are already ripe (fig. 3). Oysters begin to spawn as soon as the water temperature is sufficiently high. Gonads of partially spawned oysters are characterized by contraction of the follicles, invasion by phagocytes, and rapid increase of connective tissue cells that contain much glycogen. Re- sorption of the gonads is completed inOctober. After October they enter a brief indifferent stage during which the sexes are almost indistinguishable. It is assumed that sex re- versal, if it occurs, takes place during this period when the sex is least defined. At the end of the indifferent phase, gonad develop- ment begins again but it is soon stopped by the low temperatures of approaching winter. In the spring, gonad development is resumed. ^' %l 4I^- *^ \ ,,^*' . ^oW^J-M^. Figure 3. — Portions of gonads of oysters In different stages of development. A. Gonad in early spring before active development of sex cells begins. B. Gonad of ripe male containing large number of spermatozoa (center). C. Gonad of ripe female containing numerous eggs. Low magnification. D. Gonad of ripe female showing highly magnified eggs. Oysters are highly prolific. In one experi- ment, a fennale 4-1/2 inches long released about 70 million eggs in a single spawning. Larger oysters can develop and discharge more eggs. A female or male may spawn a number of times. Most ripe oysters of both sexes spawned after being stimulated to spawn on 5 consecutive days. There was no significant difference in the average number of eggs released during a season whether the oysters were spawned at 3-, 5-, or 7-day intervals until they were completely spent. Female oysters with larger numbers of eggs at the beginning of the season spawned more frequently than females with fewer numbers. Until several years ago it was unknown whether there is an age in the life of oysters when they produce the best, most viable sexual products. Recently, experiments with oysters ranging inage from 2 to 30 or 40 years (fig. 4) showed conclusively that there is no signifi- cant difference in the quality of spawn devel- oped by individuals of different ages or sizes and, therefore, mature oysters of all age groups may be used safely as spawners. The oysters of the oldest group, some over 9 inches long and 4 inches wide, responded to the spawning stimuli somewhat faster than individuals of the youngest group. There was no significant difference in the percentage of fertile eggs because almost all eggs of all age groups were fertilized. Moreover, the per- centage of eggs developing to the straight- hinge larval stage showed no variations that could be ascribed to size or age of parent oysters. Finally, no consistent difference was found either in the size of the early straight-hinge larvae developed from eggs discharged by oysters of different age groups or in survival and growth rate of these larvae. Of special interest was the observation that the sexes of the oldest oysters were about evenly divided, thus contradicting the old, often-expressed conception that females usu- ally predominate among the oldest and largest oysters. Figure 4. — Eastern oysters of different sizes and ages. The two smallest oysters are about 2 years old and the largest are between 30 and 40 years of age. Regardless of the difference In sizes and ages of the oysters their spawn showed equal viability. 10 The length of the spawning period of oysters depends upon the climate. In New England waters it extends on the average from 2-1/2 to 3 months, while in Florida waters oysters with ripe eggs or spermatozoa can be found most of the year. Therefore, until recently, in New England waters and similar areas experiments on larvae of oysters and most other bivalves were confined exclusively to the short period of natural propagation. However, biologists of the Fish and Wildlife Service, Bureau of Connmercial Fisheries, recently found that, by using proper methods, normal development of gonads and spawning can be induced during late fall, winter, and spring (fig. 5). Conditioning oysters to develop mature spawn during winter is relatively simple. Place the moUusks in warm water and then gradually increase the temperature to the desired level. Toward spring, instead of slowly conditioning the nnollusks by gradually increasing the v/ater temperature, the oysters can be placed directly in water of about 70° F. and kept there until they become ripe, which normally takes from 3 to 5 weeks. At higher tempera- tures they will ripen more rapidly. Upon ripening, the oysters can be induced to spawn in the laboratory by quickly raising the tem- perature to about 770 F. and adding a suspen- sion of eggs or spermatozoa to the water (fig. 6). The spawn obtained in this way is no less viable than that released by oysters in the usual manner during sunnmer. Fertilizable eggs can be obtained by strip- ping ripe females, but since oysters spawn so readily in response to chemical and thermal stinnulation, this method is seldom needed. Moreover, among the eggs obtained by induced spawning there are always considerably fewer abnormal ones than among eggs obtained by stripping. In early fall, when Long Island Sound oysters cannot be conditioned because they have not recovered from summer spawning, ripe spawn can be obtained from individuals placed, in spring, in water sufficiently warm to allow the eggs to develop to maturity but not high enough to permit spawning. This has been done successfully with Long Island Sound Figure 5. — Milford Biological Laboratory, one of the centers where extensive research on oysters and other commercial moUusks is conducted by the Bureau of Commercial Fisheries. 11 Figure 6. — Biologist Inducing out-of-season spawning of oysters and clams under laboratory conditions. oysters by shipping them, early in May, to Boothbay Harbor, Maine, where the water temperature is considerably cooler than in Long Island Sound. Oysters so kept can be induced to spawn any time between mid- August and late November, thus providing normal sex products which then are unobtain- able from Long Island Sound oysters. By use of the two above-described methods of advancing and delaying gonad development, spawning of the Eastern oyster can be induced any time of the year. As a result of these dis- coveries, much more can nowbe accomplished in the field of spawning and propagating oysters and Sonne other species of commercial mol- lusks. The conditioning methods described above are not equally successful with all groups of the Eastern oyster. This is probably because populations of this species are not genetically alike, but consist of different physiological races. Some experiments in this field strongly support this assumption by demonstrating that, even though all these oysters belong to the sanne species, the temperature requirements for gonad development and spawning of the northern populations are definitely lower than for the southern group. In some of these experiments it was possible in winter to induce spawning of 50 percent of Long Island Sound oysters after only 18 days of conditioning at about 71° F., while 78 days were needed to achieve the same results with New Jersey oysters. Oysters of the more southern groups, kept under conditions identical with those applied to northern oysters, failed, as a rule, to produce 50 percent spawners. Eggs and Larvae Fertilized oyster eggs vary in diameter from about 45 to 62 microns, but the majority measure between 50 and 55 microns. Amicron is one twenty-five thousandth of an inch and is designated by the Greek letter "//". The size of an egg is not influenced by the size of the mother oyster; for example, eggs discharged by females measuring over 9 inches long averaged 50.4//, while eggs from younger and smaller females only 3 to 4 inches long aver- aged 51 // . 12 If kept under favorable conditions, 90 to 95 percent of fertilized eggs will develop to shelled veliger stage, often called the "straight- hinge" stage. The smallest normal straight- hinge larvae nneasure about 68^ long by 55// wide (or deep). At about 72° F. the larvae will attain 75 by 67 /i within 48 hours (fig. 7). At this stage some larvae already begin to take in food composed of microscopic water or- ganisms, principally plants. 9 A 72x61 B 75x67 c 85x80 F 125x133 G 160x170 I 204x210 CRASSOSTREA VIRGINICA XII2 Figure 7. Photomicrographs showing several stages of development of oyster larvae from straight-hinge to metamorphosis. The figures below each larva give its length and width in microns. (A micron (T5haea) virginica. Biological Bulletin, vol. 88, no. 3, p. 269-291. Oyster Industry ANONYMOUS. 1948. Mechanization of oyster cultivation. Part 1 --Recent developments and im- provements in oyster dredges. [U.S.] Fish and Wildlife Service, Commercial Fisheries Review, vol. 10, no. 9i p. 12- 16. LEE, CHARLES F., and F. BRUCE SANFORD. 1963. Oyster industry of Chesapeake Bay, South Atlantic, and Gulf of Mexico. U.S. Fish and Wildlife Service, Commercial Fisheries Review, vol. 25, no. 3, p. 8- 17. MEDCOF, J. C. 1961. Oyster farming in the Maritimes, Fisheries Research Board of Canada, Bulletin No, 131, 158 p, U.S. DEPARTMENT OF HEALTH, EDUCA- TION, and WELFARE, PUBLIC HEALTH SERVICE. 1957. Sanitary control of the shellfish in- dustry. Part II: Harvesting and pro- cessing, p. 1-26. Olympia Oyster, Ostrea lurida GALTSOFF, PAUL S. 1929. Oyster industry of the Pacific coast of the United States. [U.S.] Bureau of Fisheries, Report of the Commissioner of Fisheries for the fiscal year 1929, appendix 13 (Document 1066), p. 367- 400. HOPKINS, A. E. 1937. Experimental observations on spawn- ing, larval development, and setting in the Olympia oyster, Ostrea lurida. [U.S.] Bureau of Fisheries, Bulletin No. 23, vol. 48, p. 439-503. 35 Pacific or Japanese Oyster, Crassostrea gigas CAHN, A. R. 1950. Oyster culture in Japan. U.S. Fish and Wildlife Service, Fishery Leaftlet 383, 80 p. GLUD, JOHN B. 1949. Japanese nnethods of oyster culture. U.S. Fish and Wildlife Service, Conn- nnercial Fisheries Review, vol. 11, no. 8, p. 1-7. KINCAID, TREVOR. 1951. The oyster industry of Willapa Bay, Washington. Calliostonna Company, Seattle, Wash., 45 p. QUAYLE, D. B. 1956. Pacific oyster culture in British Co- lunnbia. Provincial Departnnent of Fish- eries, Victoria, B.C., 33 p. STEELE, E. N. 1964. The immigrant oyster (Ostrea gigas) now known as the Pacific oyster. War- rens Quick Print, Olympia, Wash., 179 p. European oyster, Ostrea edulis KORRINGA, P. 1940. Experiments and observations on swarming, pelagic life, and setting inthe European flat oyster, Ostrea edulis L. Archives Neerlandaises de Zoologie, vol. 5, p. 1-249. LOOSANOFF, V. L. 1955. The European oyster in American waters. Science, vol. 121, no. 3135, p. 119-121. ORTON, J. H. 1937. Oyster biology and oyster-culture. Arnold, London, 211 p. kTs, #1379 36 "BL WHO! Librarv - Serais 5 WHSE 00239 Created in 1849 the Department of the Interior—a depart- ment of conservation — is concerned with the management, conservation, and development of the Nation's water, fish, wildlife, mineral, forest, and park and recreational re- sources. It also has major responsibilities for Indian and Territorial affairs. 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