liM^ 



Figure 11. — Biologists experimenting with artificial collectors for oyster set to find a substitute for old shells, which 

 are not easily available in some areas where oysters are cultivated. Artificial collectors may also be used in oyster 

 hatcheries where mature larvae will be released to set on them after they are placed in small ponds or tanks similar 

 to the one shown in photograph. 



which may be a protozoan, probably belonging 

 to the genus Haplosporidium . 



Another organism, possibly a species of 

 Haplosporidium similar to "MSX", is thought 

 responsible for oyster mortalities along the 

 Maryland-Virginia seaside. Therefore, it is 

 possible that there are two species of Hap- 

 losporidium overlapping ecologically. For 

 convenience's sake, the parasite from the 

 Virginia seaside was given the name "SSO", 

 signifying seaside organism. 



Another protozoan often blamed for oyster 

 mortality is a flagellate protozoanbelonging to 

 the genus Hexamita . In the past it was gen- 

 erally believed that Hexamita was a highly 

 pathogenic organism. Now, however, biologists 

 differ sharply in their opinions on the real 

 role of this organism. One group believes it 

 is a mild parasite; the second group thinks it 

 is a parasite, not strong enough alone to 

 destroy oysters, especially if they are in good 

 physical condition, although it may contribute 

 to mortality of weakened oysters after a long, 

 cold winter or after exposure to adverse 



chemical or physical conditions. Finally, the 

 third group maintains that Hexamita is ahighly 

 dangerous parasite causing extensive mor- 

 tality. 



Still another protozoan parasite of the Amer- 

 ican oyster is Nematopsis ostrearum , a 

 sporozoan protozoan which spends part of its 

 life in crustaceans. Large numbers of Nema- 

 topsis spores are sometimes found in oyster 

 tissues, especially in the gills and mantle. 

 Two species of Nematopsis may affect oysters. 

 The spores of one species, N. ostrearum , are 

 found in almost all oyster tissue except the 

 digestive system. The spores of another 

 species, N. prytherchi , are located exclusively 

 in oyster gills. 



Small crabs, such as Panopeus , which carry 

 one stage (heterogenic) of Nematopsis in their 

 intestinal tracts, are very common on many 

 oyster beds. The stages develop in the crab 

 until gymnospores are formed. Eventually 

 these spores are released by the crab, carried 

 by currents, and some are ingested by oysters, 

 infecting them. Crabs infect themselves by 



19 



