Endogenous swimming rhythms of blue crab, Callinectes sapidus?, megalopae: effects of offshore and estuarine cues |
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Authors: | R B Forward Jr J Swanson R A Tankersely J M Welch |
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Institution: | (1) Duke University, School of the Environment, Marine Laboratory, Pivers Island, Beaufort, North Carolina 28516-9721, USA, IS |
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Abstract: | Larvae of the blue crab Callinectes sapidus Rathbun develop on the continental shelf. The postlarval stage (megalopa) occurs near the surface and is transported shoreward
by wind-driven surface currents. It then uses selective tidal stream transport for migration up an estuary. Endogenous swimming
rhythms were measured under constant dark conditions in the laboratory in megalopae collected from the Newport River Estuary
(North Carolina), the Delaware Bay, and offshore from the Newport River Estuary. Megalopae from all areas had a similar circadian
activity rhythm, in which they swam during the time of the day phase in the field and were inactive at night. This rhythm
predicts the presence of a reverse, diel, vertical-migration pattern offshore which would contribute to the location of megalopae
near the surface during the day. The rhythm lacks obvious ecological significance in estuaries because it does not contribute
to selective tidal stream transport and would increase vulnerability to visual predators during the day. Attempts to entrain
a circatidal rhythm in swimming by cyclic and step changes in salinity were unsuccessful, as the circadian rhythm persisted.
The rhythm also continued in the presence of the eelgrass Zostera marina, which is a site of settlement and metamorphosis in the field. Thus, megalopae enter estuaries with a solar day rhythm in
activity. This rhythm, however, is not expressed, because light inhibits swimming during the day upon exposure to estuarine
water. Since this light inhibition is removed in offshore waters, the rhythm would be expressed if, after entering an estuary,
megalopae were transported back to offshore areas.
Received: 19 December 1995 / Accepted: 2 August 1996 |
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