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 



