The progress of the disease is slow. During the 

 advanced stage shell movements are affected and 

 the oyster has difficulty in closing its valves, thus 

 becoming an easy prey for its enemies. 



Foot disease is found in C. virginica, particularly 

 in oysters inhabiting muddy waters of the southern 

 States, but in my experience it never reaches 

 epizootic proportions. The cysts of an affected 

 oyster (fig. 378) contain a suspension of blood 

 cells, debris, and numerous bacteria which prob- 

 ably represent secondary infection. The disease 

 does not present a serious menace to the oyster 

 fishery of the coastal states. 

 Hexamita 



The flagellate Hexamita inflata was first found 

 in the intestinal tract of 0. edulis (Certes, 1882). 

 It is present in C. inrginica of Prince Edward 

 Island, Canada, and southern Louisiana, and in 

 0. edulis in Dutch waters (Mackin, Korringa, and 

 Hopkins, 1952). Heavy infection with Hexamita 

 causes breakdown of connective tissue cells, gen- 



FiGUR^ 378. — Large cyst filled with blood cells, bacteria, 

 and debris of muscle tissue in C. virginica. 



eral inflammation, the appearance of many tropho- 

 zoites in blood vessels, and necrosis of adjacent 

 tissues. The early stages of the disease appear 

 to be intraceUular, and are usually found in the 

 leucocytes of the blood vessels. The trophozoite 

 is oblong and narrow at the anterior, with si.x 

 anterior and two posterior flagella. Cysts found 

 in advanced stages of the disease are small, about 

 5 (U ill diameter; they contain two or four small 

 nuclei and have no flagellar structure. The com- 

 plete life cycle of the parasite has not been 

 described. The method of infection appears to 

 be by cysts liberated after the disintegration of 

 an infected oyster body. Experimental studies 

 by Stein, Denison, and Mackin (1961), who used 

 diseased 0. lurida, give no evidence that Hexamita 

 is a highly pathogenic parasite because there was 

 no significant difference in the mortality between 

 the experimental and control specimens. 



I^ematopsis 



Cysts of the gregarine Nematopsis are frequently 

 found in the tissues of several European bivalves 

 includmg 0. edulis, Mytilus, Cardium, Dotmx, 

 Tellina, Mactra, Solen, and others (DoUfus, 1922). 

 Observations by Louis Leger (quoted from Dollfus) 

 showed that vegetative stages of the gregarine 

 are often found in the kidneys and that the spores 

 with sporozoites are usually located in the gills. 

 Leger also showed that the intermediary hosts 

 are the crabs Corcinus moneas and Portunus 

 depurator. Nemaiopsis develops in the intestinal 

 canal of the crab and forms cysts which are 

 rejected into water and are transmitted with 

 water currents. There was no evidence that 

 Nematopsis is pathogenic. 



The species A'', ostrearum from 6*. virginica has 

 been described by Prytherch (1940), who found 

 the parasite in the oysters of Virginia, North 

 Carolina, and Louisiana. He expressed the belief 

 that mortality of oysters in Virginia and Louisiana 

 was directly caused by this gregarine. Nema- 

 topsis is widely distributed throughout the waters 

 from the Chesapeake Bay states to Louisiana. 

 Its distribution indicates no correlation with 

 oyster mortalities m that area (Landau and 

 Galtsoff, 1951). 

 Tretnatodes and parasitic copepods 



The trematode, Bucepthalus ha.imeanus Lac. 

 Duth., is occasionally foimd in 0. edulis and C. 

 vir-ginica. According to Tennent (1906), who 

 studied its life history, the worm thrives in oysters 



FACTORS AFFECTING OYSTER POPULATIONS 



419 



