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Previous studies have shown that newly released zoea larvae of the shore crab Carcinus maenas (L.) exhibit ebb-phased tidal vertical migration rhythms of upward swimming associated with transport offshore in ebb tidal flows. This gives rise to the question as to how later larvae of the species return to the intertidal zone. Repeated surface plankton sampling in coastal waters revealed that megalopae, the last larval stage of the crab, swarmed in surface waters mainly during flood tides and particularly at night. It is likely that flood-phased upward swimming ensures onshore transport of the larvae. Employing an infra-red actography system, endogenous circatidal rhythms of swimming activity were detected in constant laboratory conditions in freshly collected megalopae from inshore waters. However, in such conditions, upward swimming occurred at the times of expected ebb, not flood. It is proposed that inshore flood transport of megalopae is driven primarily by exogenous factors, which result in swarming of pre-moult megalopae towards the upper intertidal zone where settlement of first stage juvenile crabs occurs. It is suggested that the ebb-phased endogenous circatidal rhythm of upward swimming is adaptive in reducing the risk of premature stranding of megalopae, permitting them to oscillate between the intertidal and near shore waters until moulting is imminent and a suitable substratum is encountered. Of megalopae collected in inshore coastal waters, and maintained in the laboratory at water temperatures close to those of the field, nearly 60% moulted in 2 d, 95% in 4 d and 100% in 7 d after collection. Since the average development time for C. maenas megalopae is reported to be 13 d, the data provide evidence that only late megalopae return to coastal areas

marine ecology. progress series

Occurrence in coastal waters and endogenous tidal swimming rhythms of late megalopae of the shore crab Carcinus maenas:implications for onshore recruitment