LIFE-HISTORY AND BIOLOGY OF THE OYSTER CRAB, PINNOTHERES OSTREUM SAY

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Abstract
The full life-history of a pinnotherid crab (P. ostreum) is described for the first time. The host, Crassostrea virginica, is invaded in the first crab stage and not in the hard-shelled stage (Stage I by earlier authors) as hitherto believed. The former stage has not been found previously in nature for any pinnotherid crab. A preliminary description of the following pre-hard stages (both sexes found in the oyster), which were the last unknown stages in the developmental cycle of the crab, is given. Invasion of the oyster in Delaware Bay, N. J. takes place in late summer, and while more crabs invade spat than yearlings or older oysters, the survival rate of crabs is higher in the latter two. While the growth rate of the crab is positively correlated with the growth rate of the host, development to the mature stage is not retarded in slow-growing oysters to the same extent as the rate of growth, which results in considerable size variation in females just moulted into the mature stage. The hard stage is a specialized stage primarly serving the purpose of uniting the two sexes for copulatory purposes. Males leave their hosts in this stage to search for females in other oysters, but the resulting free-swimming period is only a short phase in the otherwise parasitic life of the crab. The usual copulatory period is in late June and in July. Males do not develop beyond the hard stage, but disappear shortly after copulation with one or more females. In contrast to the females, they therefore live for only one year or less. Females bear eggs in their first summer but do not reach maximum size before their second summer, and at least some may become three years old. Egg-bearing crabs are found in Delaware Bay from early June to middle October with a distinct maximum occurring between late July and late August. Older crabs deposit eggs earlier in the season than one-year-old crabs. The eggs which number 7-9000 for medium size crabs are carried for 3 to 5 weeks, and the length of the larval period is 3 to 4 weeks. The possible influence of P. ostreum on the growth and reproduction of the host is discussed, and it was found that the crab exerts no discernible influence in the first year but probably does so in many cases in the second year. Certain aspects of the biology of the genus Pinnotheres are also discussed.