Figure 385.— Oyster shells with mud blisters made by P. websteri from the groimds at Assateague Island, Va. 



evidence, however, that infestation by the mud 

 worm constitutes a serious menace to the oyster 

 population. 

 Oyster Crab 



Several species of the large family Pinnotheridae, 

 commonly called oyster or pea crabs, are associated 

 with oysters, mussels, and other bivalves. The 

 adult females have been known since ancient 

 times and were fh-st described by Ai-istotle. The 

 males of the American species. Pinnotheres ostreum 

 Say, are much smaller than the females and are 

 rarely seen. Usually one or two adult crabs per 

 oyster can be found, and the percentage of 

 infestation varies from zero in some N~ew England 

 waters to about 77 percent in New Jersey. The 

 latter figure, quoted from Christensen and 

 McDermott (1958), refers to the "invasion" of the 

 oyster crab on certain grounds of Delaware Bay. 

 The oyster crab is also abundant in Virginia 

 waters, where its life history has been studied by 

 Sandoz and Hopkins (1947). Some oysters con- 

 tain a surprisingly large number of these crabs; 

 the ma.ximum reported in a seed oyster was 262 

 (Stauber, 1945). 



Larvae of the oyster crab are pelagic until late 

 summer. At this time larval development is 

 completed, the fu'st crab stage is reached, and 

 the small crabs invade the mantle cavities of 

 oysters. At this time the carapace width of the 

 young crabs ranges from 0.59 to 0.73 mm. 



The female crab may be found in various parts 

 of the water-conducting system of the oyster, but 

 settles chiefly on the surface of the gills, in the 

 promyal and suprabranchial chambers, and grows 

 with the gi-owth of the host. The males are not 

 permanently attached to their host and may 

 leave to enter other oysters for copulation. 



For many years the oyster crab has been 

 considered an innocuous commensal; however, the 

 female crabs which have settled on the oyster 

 erode its gills and impair their function. More 

 serious lesions may develop and cause leakage of 

 water from the water tubes, which fm-ther reduces 

 the efficiency of the food collecting apparatus and 

 of the gills. Rapid regeneration of the damaged 

 gills probably saves many oysters from death, 

 but interference with the normal gill functions 

 causes a relatively poor condition in many infested 

 oysters. 



FACTORS AFFECTING OYSTER POPULATIONS 



425 



