Tlie Relation of Shellfish to Fish in Oneida Lake 23 



EXEMIES OF FEESH-\VATER SIIELLFISIE 



Parasites. Since the snails and clams are an important 

 element in the food of fish, it is of importance to know how 

 they can be protected and increased in number. We have 

 seen that the environment is favorable and that food is abun- 

 dant for them in Oneida Lake. Of unfavorable agencies we 

 know but little. Parasites are known to infest both clams 

 and snails to a marked degree but just how much mortality 

 is produced by this means is not known. Many of these 

 parasites spend but a part of their lives in shellfish while 

 the adult stage of the parasite is passed in birds, fish or other 

 vertebrate animals. The parasites may be worms and infu- 

 sorians. The Sheep Liver-fluke, which causes the death of 

 many sheep, lives for a time as a parasite in the respiratory 

 cavity of a small pond snail (Lymncea^). These parasites 

 are mostly confined to the liver, respiratory cavity, or genital 

 organs of the shellfish. It is probable that upwards of 20 

 per cent of mollusks are affected by these parasites. 



Another agency affecting more or less seriously the life of 

 shellfish is the boring algae which perforate the shell, destroy- 

 ing the protective horny -outer covering or epidermis and 

 permitting the carbon dioxide in the water to dissolve the 

 substance of the shell (carbonate of lime). The effect of 

 such action by algse is usually indicated by a roughened or 

 worn condition of the shell. It is more apparent in clams 

 than in snails, in which the apex of the shell is usually 

 attacked. The constant effort on the part of shellfish to 

 repair the damage caused by this erosion is a drain upon 

 their vitality, and probably causes the ultimate death of 

 many. 



Predatory Enemies. A number of animals prey upon 

 shellfish. These animals thus come into competition with 

 the mollusk-eating fish. This is compensated for, in a meas- 

 ure, by the fact that many fishes live on these animals 

 and thereby receive the benefit of the mollusks, though 

 secondarily. Among the animals feeding on shellfish may 

 be mentioned dragon-fly nymphs, horse-fly larvse, water bugs, 



