Life-History and Biology of the Oyster Crab, Pinnotheres Ostreum Say

It is shown that Pinnotheres ostreum invades its host, Crassostrea virginica in the first crab stage and not in the hard-shelled stage as hitherto believed. 2. The finding of the first crab stage both in plankton samples and inside oysters marks the first find in nature of this stage for any pinnotherid crab. 3. A preliminary description is given of the following pre-hard stages, which were the last unknown stages in the developmental cycle of P. ostreum. Both sexes of these stages were found only in the oyster and are never free-living. A full life-history of a pinnotherid crab is now known for the first time. 4. Invasion of the oyster in Delaware Bay takes place in late summer at a time when most of the oyster spat have grown sufficiently large to harbor one or more crabs. 5. More crabs invade spat than yearlings and older oysters, but the survival rate is higher for crabs invading the latter groups of oysters. 6. The growth rate of the crab from the invasive to the mature stage is shown to be positively correlated with the growth rate of the host. 7. Development of the crab is not retarded in slow-growing oysters to the same extent as the rate of growth. This results in a considerable size variation of female crabs just moulted into the mature stage, viz. from 4.4 to about 9.0 mm. in carapace width. 8. The hard stage, hitherto believed to be the invasive stage, is shown to be a specialized stage which primarily serves the purpose of uniting the two sexes for copulatory purposes. The males leave their hosts in this stage to search for females in other oysters, but this free-swimming period is only a phase in the otherwise parasitic life of the crab. 9. It is shown that males do not develop beyond the hard stage. They disappear shortly after copulation with one or more females, which usually takes place in late June and in July. In contrast to the females they there fore only become one year old or less. 10. Females become ovigerous in their first summer but do not reach maximum size before their second summer. At least some of them become three years old. 11. Ovigerous crabs are found in Delaware Bay from early June to middle October with a distinct maximum occurring between late July and late August. The older crabs deposit eggs before the one-year-old crabs. The eggs are carried from three to five weeks, and the length of the larval period is three to four weeks. 12. The possible influence of P. ostreum on the growth and reproduction of the host is discussed. It is believed that the crab exerts no (discernible) influence in its first year but that it probably does in many cases in its second (and third) year.