feeding on dead oysters. It has never been 

 conclusively demonstrated, however, that 

 Nematopsis causes heavy oyster mortality. 



A trematode worm, Bucephalus cuculus , 

 inhabits the gonads of the oyster, sometimes 

 causing complete castration. The parasite 

 may invade other tissues and organs, even- 

 tually killing the oyster. A spirochaete bac- 

 teriunn, Crististira , occurs in the crystalline 

 style of the oyster without causing any appar- 

 ent damage to its host. 



Small crabs. Pinnotheres ostreum , until 

 recently considered as commensals and not 

 parasites, are occasionally present in the 

 oyster. Adults are found on the oyster gills 

 facing the incoming current. They deprive 

 the oyster of food it gathers and during this 

 operation also injure its gills. Occasionally 

 they feed on particles of oyster gills. The 

 early stages of this crab, however, are truly 

 parasitic. Sometimes many small crabs are 

 found in a single oyster, not only on the gills 

 but in other parts of the body. In general, 

 infested oysters are in poorer condition than 

 healthy ones, but there is no evidence of 

 oyster mortality traceable exclusively to crab 

 infestation. 



Predators 



Young and old oysters are attacked by nnany 

 predators. Various marine snails, commonly 

 called drills or conchs, are probably the most 

 important and most widely distributed. Two 

 species of drills, Urosalpinx cmerea (fig. 12) 

 and Eupleura caudata , are found on most 

 Atlantic coast oyster beds where the salinity 

 of the water is high enough for their existence. 

 Another oyster drill, Thais haemostoma . is 

 considered by some authorities to be the most 

 persistent oyster enemy in the Gulf of Mexico, 

 especially in Alabama. While U. cinerea and 

 E. caudata seldom are over 1-1/2 inches long, 

 the southern drill, T. haemostoma , may reach 

 a length of 3 inches. Another snaillike enemy 

 of the oyster, Murex pomum , or apple Murex , 

 is known to drill oysters in southern waters. 



Drills attack oysters by boring small holes 

 in their shells, then inserting a long proboscis, 

 an organ equipped with a rasplike structure 

 called a radula. With this instrument they 

 scrape out pieces of oyster meat. 



Several species of conchs, snails of con- 

 siderable size, also feed on oysters. Among 

 these, Busycon canaliculatum , or channel 



Figure 12. — Group of common oyster drills, Urosalpinx cinerea , of the Atlantic coast. 



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