COMMON PARASITES OF FISHES U.S. Department of the Interior Fish and Wildlife Service kVUOUi' HULE, ^Jii^^, COMMON PARASITES OF FISHES UNITED STATES DEPARTMENT OF THE INTERIOR Stewart L. Udall, Secretary- Fish and Wildlife Service Clarence F. Pautzke, Commissioner CIRCULAR 144 1962 CONTENTS Page Int reduction 1 FRESH-WATER PARASITES 3 of the body surfaces and gills 3 of the body muscles 6 of the viscera 8 SALT-WATER PARASITES 10 of the body surfaces and gills 10 of the body mu scles 13 of the viscera 14 Suggested references 15 COMMON PARASITES OF FISHES Glenn L. Hoffnnan, Bureau of Sport Fisheries and Wildlife, Leetown, W. Va. and Carl J. Sindermann, Bureau of Comnnercial Fisheries, Boothbay Harbor, Maine Fish taken conamercially, for sport, or raised by fish fanciers are sometimes found to be abnormal due to injury, deformity, disease, or the presence of parasites. This paper is concerned with connnnon parasites of fishes and their recognition. Life cycles and biology are mentioned briefly, and references to more detailed literature are included. Parasites nnay be defined as "animals that live on or in another animal, the host, at the expense of that animal." There are many kinds of parasites which may be found on the surfaces of fish, including the gills, as well as in the flesh and internal organs (fig. 1). Those on the surface are known as external parasites, and those that live inside are called internal parasites. Several types may occur together in a single fish. Some parasites are fairly large (up to several inches) and can be seen easily, but many are microscopic. Scientists who work on parasites describe the organisms, study the life cycles (some have several intermediate hosts), survey the kinds and numbers found in fish, study damage done to fish, and attempt to control them. Re- search is carried on by the U. S. Fish and Wildlife Service and some colleges and universities. Under natural conditions in both fresh and salt water, nnost of the parasites pro- duced are lost to enemies or to the expanse of water before they can infect fish. Ap- parently a light parasitic infection does little harm to the host. Under crowded conditions or inadequate water and oxygen supply, however, fish may become heavily parasitized; in such cases, more damage is done and the fish may even die. Such conditions sometimes occur in hatcheries, causing heavy loss of fish. Much of the damage to fish appears to be mechanical. Parasites may injure tissues and blood vessels by their burrowing, or block blood vessels entirely with their bodies or their eggs. Some actually eat skin, other tissues, mucus, or body fluids. Some parasites are known to release toxic materials in other hosts, but this has never been demonstrated in fish. Probably fish weakened by parasites are easily captured by predators. Fishermen who find parasites when they clean their catch frequently discard the fish. This is an unnecessary waste. Although worms and other parasitic fornns are un- sightly, none of them can possibly harm humans if the fish is thoroughly cooked. Freezing and hot smoking of the fish will also kill nnost parasites, but sonne may remain alive in brine for a month. A few parasites may develop in man if the fish containing them is eaten raw. Best known in this category is the broad fish tapeworm (Diphyllobothrium latum), whose larvae may be found in the muscles and among the viscera of pike and perch, particularly in our north- ern lakes. Most fish parasites will not live in man. The commercial fishing industry suffers great losses each year because of parasit- ized fish which are condemned for human consumption and can be sold only for animal feed at reduced price. In Minnesota, for example, tullibees (ciscos) are infected with tapeworm cysts (Triaenophoras) \vhichdo not infect humans but are unsightly. The annual catchof 2-l/2 to 3 million pounds, if uninfected, would find ready markets for human use, but as much as 1,500,000 pounds have been condemned by governmental agencies. Sinnilar losses occur in marine fisheries. At present there is no known control for nnost parasites except in relatively small bodies of water such as fish hatcheries, small ponds, and aquaria. References to the methods of treatment of fish parasites nnay be found in Hoffman (1959). The major parasite groups discussed in this report are listed and explained below: Protozoa - single-celled animals, usually microscopic. Parasitic groups of pro- tozoa important to fishes include Ciliata, Flagellata, Myxosporidia, and Microsporidia; may be either external or internal parasites. While the in- dividual protozoan is usually micro- scopic, aggregations nnay cause effects recognizable by the naked eye. Monogenetic trematodes - External flatworms, usually small, commonly calledflukes, often found on the gills; are called monogenetic because they complete their life-cycle on one host (mono = single, genetic - origin). Posterior or- gan of attachment (haptor) well de- veloped with chitinous clamps and hooks. Digenetic trematodes - Internal flatworms, often very small, but occasionally large enough to be seen easily; are called digenetic because at least two hosts are needed for their life-cycle (di = two, genetic = origin). Larva (metacercaria) or adult may be found in fish, first internnediate host is snail or clann; possess oral and ventral suckers; eggs present in adults which may be found in alimentary tract, occasionally else- where; metacercariae in various or- gans (skin, muscle, mesenteries, etc.). Cestodes - Tapeworms, are also flatwornns, but are distinct from "flukes" in that the adult worm is usually composed of a head (scolex) and many egg-producing segments. Found as adults or larvae in fish; intermediate stages in "water fleas". Nematodes - Unsegmented roundworms (ac- tually cylindrical) which occur as lar- vae in tissues, and as adults in the alimentary tract, occasionally else- where. Adult female contains eggs, larva does not. Acanthocephala - "thorny-headed" worms, which occur as larvae or adults in fishes. Parasitic copepods - Small highly special- ized crustaceans related to crayfish and crabs, often called "fish lice," usually found on external surfaces; sonnetimes embedded, sometimes loosely attached; shape may be louse- like to wormlike. Leeches - External worms recognizable by their external segmented appear- ance, frequently bright coloration, and a large sucker at the posterior end. Glochidia - Fresh-water clam larvae in cysts on gills or fins of fish. Lampreys - Primitive fishlike vertebrates and the largest of the fish parasites, ranging up to 3 feet in length. They may be recognized by the large cir- cular mouth containing many ro%vs of thornlike teeth. In most instances marine and freshwater fish have different parasites, therefore, they are discussed separately under FRESH- WATER PARASITES and SALT-WATER PARASITES. The following plan of organization is followed \inder each category: (1) Parasites of the body surfaces and gills (2) Parasites of the body muscles (3) Parasites of the viscera A sinnplified drawing (fig. 1) of an opened fish is included to show the various organs. LIVER STOMACH SWIM BLADDER KIDNEV DORSAL FIN ANAL FIN INTESTINE HEART 'ENTRAL FIN PVLORC CAECA Figure 1.- -Drawing of Lepomis cyaneltus, (from Hile, 1960, U. S. Fish and Wildlife Service. Fishery Leaflet 132). FRESH-WATER PARASITES Parasites of the Body Surfaces and Gills 1. Fun^. White cottony growths on wounds or ulcerations are usually fungi of the genera Saprolegnia and Achlya. The filaments can be seen easily under the microscope. The fungus often develops as a secondary infection following wounding or ulceration due to other agents. 2. Protozoa. These are the smallest ani- mals. They consist of a single cell, and nearly all are too small to be seen with- out a microscope. Some protozoa fornn \vhitish cysts containing many spores. Such cysts are large enough to resemble worm larvae superficially. They may be found in many organs including the skin and gills. Often in aquaria and fish hatcheries, a condition develops which is known as "Ich" or "white spot." The fish are seriously ctffected, and many little white spots can be seen - some of them moving slowly over the fish. These are individual protozoa, Ichthyophthirius muUifilis, the largest protozoan to be found on fish. Under the microscope the most striking characteristics that can be seen are the constantly beating hair- like cilia, which completely cover the para- site (fig. 2.). All other protozoa that live on the skin of fish are very small in comparison to Ichthyophthirius. Although some of them are serious fish disease agents, none is harm- ful to man. Myxosporidian cysts containing many spores (fig. 3) are whitish and usually large enough to be seen easily. They are found in many organs including the skin and gills. 3. Internal flukes (digenetic trematodes). These small worms are flat, leaflike forms from 1/250- to l/S-inch long with two suckers. The trematodes most frequently seen are larvae ("grubs") encysted in the skin or flesh. The largest of these larvae is the yello^v grub, Clinostomum marginatum (fig. 4). The cyst is yellowish, about 1/8 inch in diameter, and is usually seen in the gills and at the bases of the fins but nnay Figure 2.-- Ichthyophthirius multifilis, ciliated protozoan, the "Ich" of aquarium and fish hatchery fish. *- * Figure Z,- - Myxobolus spore from white cysts. KiKure JT, — Vrllow grubs in lltwh of yt-Uow pcTch. (Courlray of llif Nf>v York loiuuTvutiiiii I >Fpartiiirut ' Figure 4.-- Clinostomum marginatum "yellow grubs" in the flesh of yellow perch. (Courtesy of the New York Conservation De- partment). produces eggs which pass out in the feces into the water. After a suitable period, these hatch into a microscopic free-swim- ming larva (miracidium) which has about 8 hours to find the right kind of snail. If it succeeds, it burrows in and continues to develop. It produces many of the next stage larvae (cercariae) which burrow out of the snail. To survive, cercariae have about a day in which to contact a fish and burrow into the tissue. In the fish, the worms may migrate a short distance before reaching their final site where they secrete a cyst wall about themselves and become the "yellow grubs", completing the cycle. Another type of fluke that is often noticed is the "black grub" (fig. 6), a very small larva enclosed in a black cyst about l/l6 inch in diameter. These may be the larvae of any one of many trematodes but the most common is known as Neascus. The life cycles are similar to that of the "yellow grub." Different fish, birds, and snails may be involved. occur almost anywhere, including intern- ally. If the cyst is removed and opened, the larva can -be seen. The life history (fig. 5) is typical for the group. When the fish is eaten by a heron the larva is digested free from the cyst and continues development in the intestine of the bird. Most trematodes develop to maturity in the intestine of the host, but the "yellow grub" migrates to the esophagus where it matures. The adult 4. Gill flukes (monogenetic trematodes). Most of these are microscopic, but one of the larger ones, Discocotyle salmonis (fig. 7) often occurs on the gills of trout and salmon. It is about 1/4 inch long and attaches by its large rear suckerlike attachment organ to the gill of the fish. Sometimes these flukes become so numerous that the fish are seriously affected. Another very com- mon monogenetic trematode, Gyrodactylus, HERON BECOMES INFECTED BY EATING FISH WORM BECOMES ADULT IN MOTTTH CAVITT EGG PASSES OUT IN FECES OR SALIVA BODY OF CERCARIA PENETRATES FISH, BECOMES YELLOW GRtJB MIRACIDIDM PENETRATES HELISOMA SNAIL MOTHER SPOROCTST - - LARVAL STAGES IH SNAIL CERCARIA EMERCES FROM SNAIL Figure 5.--Life cycle of the "yellow grub," Clinostomum marginatum. Outer circle (solid line) includes the worm stages. Inner circle (broken line) includes the hosts. is usually found in greater numbers on the body than on the gills; most of the others occur on the gills only. 5. Parasitic copepods. In some ponds and hatcheries, the "anchor worm" copepod CLemoeoj flourishes. It protrudes wormlike from the fish, usually at the base of a fin, with its head buried in the fish. The visible portion is cylindrical, whitish, and about 1/2 inch long (fig. 8). There are often two egg sacs extending from the end of the creature. With care, the head can be dis- sected out of the fish, and one can then see that some of the forward appendages have become modified into a very efficient "an- chor." This parasite goes through several microscopic developmental stages usually on the gills of fish other than the one on which it finally matures. Because it can do considerable damage to fish if present in large numbers, it should be eradicated from fish rearing ponds and hatcheries, and parasitized fish should not be used for stocking. Sometimes "fish lice" (Argulus) may be seen crawling over a fish. They are among the largest of external parasites and can be seen easily. They are round to oval when Figure 7.-- DiscocotyU salmonis, an ectoparasitic trematode from the gills of trout (after Price). Figures, — Crossiphiala bulboglossa,i Neascus metacercaria of one of the several "black spot grubs" of fish in the fathead minnow, Pimephales promelas. seen from above and are flattened from top to bottom. Four pairs of swimming legs extend from their sides (fig. 9). The eggs are borne in sacs at the end of the female. The eggs drop off when developed, hatch, and the larvae must eventually find a fish or perish. There are several larval stages which precede final development of the adult. Other f ornns , Ergaailus , Achtheres, and Salmincola, are usually found attached to the gills. 6. Glochidia. These small larvae of some of the large fresh-water clams clamp onto the gills or fins of fish where they remain for 10 to 20 days. Some of the fish tissue grows up and over the glochidiunn (larva) forming a small translucent cyst about 1/8 inch in diameter. If nunnerous, they cause considerable fish. damage to the 7. Bloodsuckers (leeches). Except for lampreys, these are the largest external parasites, sometimes reaching an inch in length. There are suckers at each end of the flattened, segmented body. The front sucker contains the mouth with which the parasite rasps a small hole in the fish skin to obtain its meal of blood (fig. 10). Leeches often attach near the bases of or on the fins. They transmit certain protozoan parasites from fish to fish through their feeding. The kinds of leeches that parasitize fish do not attack humans. Parasites of the Body Muscles 1. Protozoa. Myxosporidian cysts contain- ing many spores (fig. 3) are whitish and Figure 8.-- Lemaea tortua, the "anchor worm" of fish (after Wilson). large enough to be seen easily. They are found in many organs but some species found in the muscle produce a large ugly- looking "boil." These are not infective for man. 2. Flukes. Some of the "grubs" discussed in the previous section may be found in the nnuscle. Figure d.--Argutus irilineaius, the "fish louse" (after Guberlet). 3. Tapeworms. A tapew^orm that is con- sidered important because it may infect man through eating improperly cooked infected fresh-water fish is the broad fish tape- worm,/;i;)Ay/io6o«^rium Zoium. The white larva (plerocercoid) may be found among the viscera as well as in the flesh, and may be up to an inch in length, but has no other outstanding characteristics. Normally the parasite ■will attain maturity in the intestine of bears, man, dogs, and perhaps other animals if they eat infected fish (fig. 11). It grows into a large worm - up to 30 feet in length - and produces millions of eggs which pass out with the feces of the animal. If the eggs fall or are washed into the lake, a small swimming larva (coracidium) hatches out. This very active creature develops into the next larval stage (pro- cercoid) when eaten by certain species of "water fleas" (copepods). Small fish feed on the infected copepods, and if a pike or perch eats such fish, the larva will continue to develop. Humans beconne infected by eating raw fish containing the larval stages. 4. Roundworms (nematodes). The largest one seen in muscle is Kustrongylides (fig. 12). It is a red worm coiled up in a cyst about 1/4 inch in diameter and is sometimes seen while dressing a fish. It may also be found in the body cavity. The life cycle is not entirely known, but it is probable that a bird is the final host. Figure 10.-- Pis cicola salmositica, a leech or "blood-sucker" (after Meyer). Parasites of the Viscera 1. Protozoa. Internal fish protozoa are usually not seen by the casual observer except for the myxosporidian cysts dis- cussed in previous sections. 2. Flukes. "White grubs" are sometimes found in the visceral organs. These are somewhat similar to the "yellow grubs" discussed previously, and are not harmful to man. Adult flukes of fish occur in the intestine and stomach, but are usually small and not seen. 3. Tapeworms (cestodes). Two forms of these are seen in fish. One, the adult, lives in the intestine of fish. It is flat like a tape, and often several inches long. It is this stage that is seen when the intestine is accidentally cut or torn during cleaning of the fish. The other form is the larva (plerocercoid), which may or may not be the same species as the adult seen in the fish. Larvae are smaller, nonsegmented, and may occur among the internal organs as well as in the flesh of the fish. Cal- careous corpuscles (round concretions of lime) can be seen microscopically in all tapeworm larvae; this is sometimes a very helpful characteristic for identifying a lar- val tapeworm parasite. The adult from fish is never harmful to man, but some of the larvae, if eaten raw, can develop in man. We have previously discussed the best known one in this group, the broad fish tapeworm (p. 7). One of the most noteworthy fish tape- worms is the bass tapeworm, Proteocephalus ambloplitis (fig. 13). The larvae (plerocer- coids) often cause sterility in black bass. The adult lives in the intestine of black bass and produces large numbers of eggs which pass out with the feces of the fish into the water. The small larva in the egg will develop to the next stage larva (pro- cercoid) if eaten by the proper kind of copepod. After development in the copepod, this larva invades small fish which eat the infected copepods. The larva is freed in the intestine of the small fish, burrows through the intestinal wall, and wanders among the internal organs. When large numbers of these larvae (plerocercoids) are present they cause considerable damage to the fish. Small bass, once infected, will retain the larvae for a long period of time. Larger bass accumulate larvae by eating small fish that have recently fed on infected cope- pods. Larvae, if still in the stomach or intestine of the small fish when eaten by the larger fish, will migrate through the intes- tinal wall and into the visceral cavity of II III I IJIIIIIIIII ■■ /" mrn 'Mmnim m TrnrtncnnnixcarnT J//i'UJJ/Trrrn77TTrPTrm-rn-rr [jmrrnTrmTiiiiiiiiiJiijiiiDnnin^^ ADULT WORM IN SWAIX lUTESTINE CF BEAR, MAN, DOG MAN IS INFECTED BT EATING MPROPERXT COOKED INFECTED FISH EGGS ARE PASSED IN FECES AND REACH WATER NORTHERN PIKE, WALLEYE PIKE, PERCH, TROTTT EATS INFECTED COPEPOD OR SMALL FISH .^ PLEROCERCOID LARVA DEVELOPS IN MUSCLE OR VISCERA OF FISH :^\ <■ SMALL FISH EATS IHFECTH) COPEPOD EGGS HATCH IN WATER AND REIEASE CORACIDIDM CORACTrrtIM IS INGESTED BY WATER FIEA (COPEPOD) PROCERCOID LARVA DEVELOPS IN COPEPCJ) Figure ll.--Life cycle of the broad fish tapeworm, Diphyllobothrium latum. Outer circle (soLd line) includes the worm stages. Inner circle (broken line) includes the hosts. the larger fish. The damage done by the larvae moving among the reproductive or- gans may render the fish sterile. The life cycle is completed when a larger bass eats an infected smaller fish in which the larvae have had time to become established in the visceral cavity. The larva is then freed in the intestine of the larger bass and grows into the adult tapeworm. The large larva of another tapeworm, Ligula intestinalis, is often seen in the body cavity of minnows and suckers (fig. 14). Sometimes it becomes so large in small fish that it causes the body wall to burst, releasing the worm. This worm develops into an adult in the intestine of fish-eating birds. 4. Roundworms (nematodes). The red worm, Eustrongylides (fig. 12 and p. 7) may occur among the viscera. Some smaller larval nematodes may also be present. A very long, thin nematode, Philonema, causes serious damage to the reproductive organs of sal- monid fish. Usually the adult nematodes in the intestine are not noticed because of their small size. None of these is harmful to man. Figure 12. -The "red worm," larval nematode, Eustrongylides, from the flesh of fish. 5. Thomy-headed worms (Acanthocephala). These cylindrical worms have rows of hooks on their heads which become embedded in the intestinal wall of the fish. Unless large numbers are present no harm is apparent. SALT-WATER PARASITES Parasites of the Body Surfaces and Gills When a fish is first taken from the water it may be carrying a variety of external parasites or external evidences of disease. The mouth and gill chambers are favored sites for certain parasites because these areas afford protection and are in close proximity to the host's blood supply. 1. Fungi and ■protozoa ulcerations of the skin may be due to underlying protozoan or fungus infection of the flesh (as in young sea herring). Such infections kill tissues and cause the skin to slough, creating the external ulcers. Two groups of the protozoa (Myxosporidia and Microsporidia) may in- vade the muscles, producing ulcers of the skin (fig. 15). Fungus organisms, such as Ichthyosporidium hoferi, rnay also cause ulcers. 2. Trematodes or flukes may be found on the body surfaces of the larger marine fishes such as halibut, sharks, skates, and ocean sunfish. These worms may be leauf- or disc-shaped and characteristically pos- sess a conspicuous attachment organ of hooks and/or suckers. Trematodes are also common on the gill bars and filaments of marine fishes, but are usually quite small and not easily observed, unless the fish is heavily parasitized. 3. Grubs of marine fishes --larval trema- todes that localize beneath the skin or in the fins--are common in inshore waters. So-called "pigment spot" of cunner, herring, mackerel, butterfish, and other fish is caused by encystment of such larvae be- neath the skin. The life cycle of the worm (Cryptocotyle) that is responsible involves suc- cessively a snail, a fish, and a sea gull (fig. 16a, 16b). The adult fluke inhabits the bird's digestive tract and sheds its eggs with the droppings of the host. Snails become infected by eating the worm eggs. After a period of larval development in the tissues of the snail, an infective stage, known as the cercaria, emerges from infected snails and is free -swimming vintil it contacts the fish host, where it penetrates the skin and encysts. The cycle is com- pleted when fish carrying encysted larval worms are eaten by the bird. Other larval trematodes may also cause "pigment spot" of marine fish, but they are not as well understood, except that a fish-eating bird and a snail are usually necessary for com- pletion of the life cycle. Flounders are often invaded by larval flukes which do not cause pigment accumulation. Encysted lar- val worms appear as tiny opaque white patches in the fins and on the light under- surface of the fish. 4. Parasitic copepods --fish lice--may be found on external surfaces of many species of marine fishes. These may be of various forms. Some are temporary and retain their mobility, moving freely from fish to fish, while others, such as Sphyrion on the ocean perch (redfish), are permanent tissue in- vaders (fig. 17). Anchorlike projections of the head of this particular copepod grow into the flesh, often causing an \insightly ulcer. This projection persists as a brown- ish mass in the flesh eifter the parasite dies. Copepods may also be found attached to the gills and gill regions of marine fishes. An 10 C3I^ ADULT T/LPEWORll Bi INTESTINE CF BUCK BASS BECOIIBS ADULT WORM IF BATKN BY BLACK BASS / / REMAIKS AS LAFVA IK ADULT BASS "HIGH MAY ALSO RAT PROCERCOIDS THAT ARE IN THE STOMACH OF FOOD FISH \ V COPEPOD BECOMES PliROCERCOH) LARVA lU VISCERA CF FISH '^^ "RIPE" SEGMENTS CF WORM PASS OUT WITH FECES \ BGGS ARE FREED AND KATEN BY COPEPOD BECOMES PROCERCOID LARVA IN COPEPd) Figure 13,--Life cycle of the bass tapeworm, Proteocephalus ambloplitis. Outer circle (solid line) includes the worm staRcs. Inner circle (brolourtesy of the New York Conservation D^>artment). 11 Figure 15,--Skin ulcers in young herring caused by underlying muscle infection with the myxosporidian. Kudoa dupeidae. METACERCAfllA ENCYST BENEATH SKIN CAUSING PIGMENT SPOT CERCARIAE DEVELOP IN REDIA AND EMERGE FROM SNAIL REDIAE DEVELOP I SNAIL DIGESTIVE GLAND Figure 16a.--Life cycle of CryptocotyU lingua, the worm whose larvae cause '•pigment spot" of herring and other fish, Figure 16b.--Pigment spot of herring caused by CryptocotyU lingua. 12 extreme example is Lemaeocera branchialis found on cod and some other species (fig. 18). The copepod is located in the gill chamber, but roots formed by extensions of the body extend into the host, eventually penetrating to the heart region. The life cycle includes lumpfish, flatfish, and pos- sibly others as intermediate hosts. Parasites of the Body Muscles Because the flesh of fish is the part that is usually consumed by man, parasites and diseases Eiffecting the body muscles of fish are of primary concern. Though parasites are killed by proper cooking, the presence of worms or other abnormal conditions in the flesh of food fish is esthetically and phy- chologically disturbing, and many fish thus affected are discarded unnecessarily. 1. Protozoa are significant and sometimes conspicuous parasites. Myxosporidia form either spindle-shaped white nodules up to Figure 17.- -Parasitic copepods (Sphyrion lumpi) embedded in redfish. Dissected copepod is shown at right. Figure \6.-- Lemaeocera branchialis from cod. Note the ■■antlers"--anterior projections of the copepod tliat anchor it in the flesh of the host. 1 /Z inch in length or so-called "pus pockets" in the flesh of small herring, alewives, and menhaden from the east coast. Myxosporidia are also responsible for the conditions known as "jellied swordfish" on the Atlantic coast and "wormy halibut" on the Pacific coast. Both are characterized by progressive destruction and liquefaction of the muscles, producing unsightly areas in the flesh that must be cut out and dis- carded, or else the entire fish may be discarded. Z. Fungus infections may produce muscle abnormalities. A fungus infection of herring on the east coast of North America produces small yellow- white nodules. Advanced in- fections result in extensive degeneration of muscles, and diseased fish are difficult to salt or smoke (fig. 19). Another symptom of this fungus infection is the accumulation of black pigment around spores in the flesh, making the fish less desirable for filleting and pickling. 3. Larval trematodes occur commonly in the flesh of many coastal marine fishes . Con- spicuous in this respect are young Atlantic herring, in which larvae of the fluke Cryptocotyle frequently localize in the mus- cles as well as underneath the skin, causing black pigment accumulation and the forma- tion of a conspicuous "pigment spot." Floun- ders are frequently invaded by another larval trematode which appears as a small opaque white cyst in the flesh although there is no pigment response. 4. Larval nematodes in the flesh of marine fishes best fulfill the popular conception of "worms." They may occur free or encysted in the muscles, and may become very active when released. The so-called "codworm" PoTocaecum has received particular attention, especially on the Canadian east coast. This larval roundworm encysts, sometimes in great numbers, in the flesh of cod, smelt, and other fishes (fig. ZO). Its life cycle is not completely understood, but it involves a succession of fish hosts, with the seal as the final host for the adult worm. Other kinds of larval roundworms may be found in haddock and other commercial marine fish, occasionally in great numbers, but usually only a few in any single fish. 13 Usually they are cooked, eaten and never noticed, but occasionally they may be seen and may result in unnecessary waste of the fillet or the fish. None has been demon- strated to be harmful to humans. Usually a fish-eating bird or mammal serves as host for the adult worms. 5. Larval cestodes may localize in the flesh of marine fishes, although this condition seems less frequent than in fresh water. Butterfish of the U. S. east coast fre- quently have tapeworm larvae in small (less than l/Z5-inch) white to yellow cysts in the body muscles. Occasionally enormous numbers of such cysts may be found in individual fish (fig. 21). Other tapeworms may occur as contorted opaque white ribbons in the flesh of fish, often in suf- ficient numbers to require discarding the fish. Parasites of the Viscera Inhabitants of the visceral mass of fishes are many, and are often apparent when fish are dressed. Adult worms--cestodes, tre- matodes, nematodes, and acanthocephala-- usually occupy the digestive tract, while larval stages of members of these groups may be foiind, usually encysted, in the gut wall, the liver, or the supporting mem- branes. In addition to worm parasites, there may be protozoan and fungus infec- tions. 1. Protozoa may occur in nodules or cysts on and in the viscera of fish. These are actually masses of thousands of spores, much like the nodules or cysts in the flesh. Conspicuous in this respect are such forms as Glugea hertwigi, a nnicrosporidian that pro- duces white cysts in the viscera of eastern smelt (fig. 22). This parasite nnay occa- sionally be sufficiently abundant to interfere with reproduction. It varies in abundance geographically, and may occur in over one- quarter of all fish sampled in particular areas. 2. Fungus infections and resulting involve- ment of the viscera are found in such marine fishes as the Atlantic herring, mackerel, and flounder. A fungus disease Figure 19.--Adult herring infected with fungus. Top--skin has been sliced away to show normal muscle. Lower --shows ad- vanced decay of muscles. Figure 21.--Larval tapeworms in the flesh of bunerfish. Figure 20,- -Smelt with codworms encysted in flesh. Figure 22.--Visceral cysts of the microsporidianC/ugea hertwigi in a smelt. 14 of marine fishes, caused hy Ichthyosporidium hoferi, has received some attention in recent years, especially in North America, where it has periodically assunned epidemic pro- portions in the Atlantic herring. Visceral symptoms often include extensive white nodules on and in the heart, liver, gonads, and mesenteries (fig. 23). 3. Adult trematodes are connmon, but their small size and location within the gut make them inconspicuous. Occasionally hundreds or even thousands of these small adult worms may be found in the digestive tract of a single fish. They are usually less than 1/4 inch long, opaque white, with a brownish patch of eggs near the center or posterior part of the body. 4. Larval nematodes may localize in the vis- cera, particularly in the mesenteries ad- jacent to the digestive tract. They are often found encysted in tightly coiled spirals in the Atlantic and Pacific herrings, red- fish, and many other species, and may become very active when released--moving in a typical whiplike manner. Numbers per fish range from a few to many hundreds. 5. AduU nematodes may also be found in the digestive tract, but as a rule are not con- spicuous . 6. Larval cestodes maybe regionally abun- dant in the viscera of many fish species. Frequently these are larvae of tapeworms which mature in sharks and skates or in fish-eating birds. The larvae encyst in the mesenteries or wall of the digestive tract as white ovoid or club-shaped nodules, fre- quently 1/8 to 1/2 inch in length. 7. Adult cestodes inhabit the digestive tract of fish, and may be recognized by their Figure 23.--Visceral nodules in herring caused by the fungus Ichthyosporidium hoferi. extended white ribbonlike appearance--the ribbon being composed of many egg-pro- ducing segments. Such worms may occa- sionally be found extruded from the vent of the fish after death (this often happens with smelt, for example). Worms are usually few in number in any single host, but may occupy the entire length of the intestine. SUGGESTED REFERENCES General Allison, Leonard N. 1950. Common diseases of fish in Michigan. Michigan Department of Conservation, Miscellaneous Pub- lication No. 5, 27 p. Bang'nam, Ralph V. 1941. Parasites of game fish. The Aquarium Journal, vol. 14, No. 3, p. 29-31. 1948. Parasites of freshwater fishes. State of Ohio Department of Agricul- ture, Bulletin No. 229, 12 p. Cameron, T. W. M. 1945. Fish-carried parasites in Can- ada. Canadian Journal of Compara- tive Medicine, vol. 9, p. 245-254. Davis, Herbert S. 1953. Culture and diseases of game fishes. University of California Press, Berkeley and Los Angeles, 332 p. Haderlie, Eugene Clinton 1953. Parasites of the fresh-water fishes of northern California. Uni- versity of California Publications in Zoology, vol. 57, p. 303-440. Hargis, William J. 1958. Parasites and fisheries prob- lems. Proceedings of the Gulf and Caribbean Fisheries Institute. Elev- enth Annual Session, p. 70-75. Heller, Anita F. 1949. Parasites of cod and other ma- rine fish from the Bay of Chaleur region. Canada Journal of Research, vol. 27, p. 243-264. 15 Hoffman, Glenn L. 1959. Recomnfiended treatnnent for fish parasite diseases. U. S. Fish and Wildlife Service, Fishery Leziflet 486, 4 p. Hugghins, Ernest J. 1959. Parasites of fishes in South Dakota. South Dakota Departnnent Game, Fish and Parks Bulletin No. 484, 73 p. Hunter, George W., Ill 1942. Studies on the parasites of fresh- water fishes of Connecticut. Con- necticut Geological and Natural His- tory Survey Bulletin No. 63, p. 228- 288. Meyer, Marvin C. 1954. The larger animal parasites of the fresh-water fishes of Maine. Maine Department of Inland Fisher- ies and Game, Fishery Research and Management Division Bulletin No. 1, 92 p. Northcote, T. G. 1957. Common diseases and parasites of fresh-water fishes in British Co- lumbia. British Columbia Game Commission, Management Publica- tion No. 6, 25 p. Pratt, Henry S. 1929. Parasites of fresh-waterfishes, comprising some general considera- tions. U. S. Department of Commerce, Bureau of Fisheries Econonnic Cir- cular No. 42, 10 p. Schaperclaus, W. 1954. Fischkrankheiten. 3rd ed. Aka- demie- Verlag, Berlin, 708 p. Sindermann, Carl J. 1953. Parasites of fishes of North Central Massachusetts. Massachu- setts Division Fisheries and Game, Fisheries Report for lakes of North Central Massachusetts (1950), p. 4- 28. Sindermann, Carl J., and Aaron Rosenfield 1954. Diseases of fishes of the western North Atlantic. I. Diseases of the sea herring. Maine Department of Sea and Shore Fisheries Research Bulletin No. 18, 23 p. Van Cleave, Harley J., and Justus F. Mueller 1934. Parasites of Oneida Lake fishes . Part III. A biological and ecological survey of the worm parasites. Roose- velt Wildlife Annals, vol. 3,Nos.3& 4, p. 161-334. (Bulletin New York State College of Forestry, vol. 7, No. 1.) Van Duijn, C., Jr. 1956. Diseases of fishes. Water Life, Dorset House, London, 174 p. Viruses, bacteria, fungi U. S. Fish and Wildlife Service, Fish- ery Leaflets 453 - 467, 494, and 497. Protozoa Davis, Herbert S. 1947. Studies on the protozoan para- sites of fresh-water fishes. U. S. Fish and Wildlife Service, Fishery Bulletin 41, vol. 51, p. 1-29. 1953. Culture and diseases of game fishes. University of California Press, Berkeley and Los Angeles, 332 p. Kudo, Richard R. 1954. Protozoology. Charles C. Thonnas Publisher, Springfield, Illi- nois, 966 p. Trematodes Bychowsky, B. E. 1957. Monogenetic trematodes; Their systematics and phylogeny. Moscow - Leningrad (in Russian), 509 p. (Eng- lish translation by Pierre C. Ous- tinoff, edited by William J. Hargis, American Institute of Biological Sciences, Washington, 627 p.) Sproston, Nora G. 1946. A synopsis of the monogenetic trematodes. Transactions Zoological Society of London, vol. 25, No. 4, p. 185-600. 16 Wolfgang, Robert W. 1954. Studies of the trematode Stephana stomum baccatum (Nicoll, 1907). II. Biology, with special reference to the stages affecting the winter flounder. Journal of the Fisheries Research Board of Canada, vol. 11, No. 6, p. 963-987. Yamaguti, Satyu 1953. Systema Helminthum. Part I. Digenetic trematodes of fishes. In- terscience Publishers, Inc., New York, 405 p. Cestodes Miller, Richard B. 1952. A review of the Triaenophorus problem in Canadian lakes. Fisher- ies Research Board of Canada, Bul- letin No. 95. p. 1-42, Wardle, Robert A. 1932a. The Cestoda of Canadianfishes. I. The Pacific Coast region. Contri- butions to Canadian Biology and Fish- eries, vol. 7, p. 220-243. 1932b. The Cestoda of Canadianfishes. II. The Hudson Bay drainage system. Contributions to Canadian Biology and Fisheries, vol. 7, p. 377-403. Wardle, Robert A., and J. A. McLeod 1952. The zoology of tapeworms. Uni- versity of Minnesota Press, Min- neapolis, 780 p. Yamaguti, Satyu 1959. Systema Helminthum, Vol. 2. The cestodes of vertebrates. Inter- science Publishers, Inc., New York, 860 p. Nematodes Yorke, Warrington, and P. A. Maplestone 1926. The nematode parasites of ver- tebrates. J. & A. Churchill, London, 536 p. Parasitic Copepods Harding, J. P. 1950. On some species of Lemaea (Crustacea, Copepods: parasites of fresh-water fish). Bulletin British Museum. (Natural History). Zoology, vol. 1, p. 1-27. Meehean, O. Lloyd 1940. A review of the parasitic Crus- tacea of the genus Arffulus in the col- lections of the United States National Museum. Proceedings U. S. National Museum, vol. 88, No. 3087, p. 459-522. Pennak, Robert W. 1953. Fresh-water invertebrates of the United States, Ronald Press Co,, New York, 769 p. Wilson, Mildred S. 1959. Branchiura and parasitic Cope- poda (In Ward, Henry B., and G. C. Whipple. Fresh-water biology, 2d ed., John Wiley & Sons, New York, p. 862-868), Wilson, Charles B. 1901 - 1942. A series of monographs on the parasitic copepods. Most of these were published in the Pro- ceedings of the U. S, National Mu- seum (complete bibliography may be found in "Index-catalogue of Medical and Veterinary Zoology," Part 17, p. 5542-5544). Leeches Meyer, Marvin C. 1940. A revision of the leeches (Pis- cicolidae) living on fresh-water fishes of North Annerica. Transac- tions American Microscopical Soci- ety, vol, 59, No. 3, p. 354-376. 1946. Further notes on the leeches (Piscicolidae) living on fresh-water fishes of North America, Transac- tions of the American Microscopical Society, vol. 65, No. 3, p. 237-249. Moore, J. Percy 1959. Hirudinea, (In Ward, Henry B., and GeorgeC, Whipple, Fresh-water biology. 2d ed,, John Wiley and Sons, New York, p. 542-557). Pennak, Robert W. 1953. Fresh-water invertebrates of the United States, Ronald Press Co., New York, 769 p. 17 GPO 9 2a 6a I MBL WHOI Ctbrnry Serial SE 00229 The Department of the" Interior, created in 1849, is our Nation's De- partment of Natural Resources, concerned with management, conservation, and development of water, wildlife, fish, mineral, forest, and park and recreational resources. It also has major responsibilities for Indian and Territorial affairs. As America's principal conservation agency, the Department works to assure that nonrenewable resources are developed and used wisely, that park and recreational resources are conserved for the future, and that re- newable resources make their full contribution to the progress, prosperity, and security of the United States, now and in the future. UNITED STATES DEPARTMENT OF THE INTERIOR Stewart L. Udall, Secretary- Frank P. Briggs, Assistant Secretary for Fish and Wildlife FISH AND WILDLIFE SERVICE Clarence F. Pautzke, Commissioner BUREAU OF SPORT FISHERIES AND WILDLIFE Daniel H. Janzen, Director BUREAU OF COMMERCIAL FISHERIES Donald L. McKernan, Director