ENDOGENOUS SWIMMING RHYTHMS OF BLUE-CRAB, CALLINECTES-SAPIDUS, MEGALOPAE - EFFECTS OF OFFSHORE AND ESTUARINE CUES

Larvae of the blue crab Callinectes sapidus Rathbun develop on the con tinental shelf. The postlarval stage (megalopa) occurs near the surfac e and is transported shoreward by wind-driven surface currents. It the n uses selective tidal stream transport for migration up an estuary. E ndogenous swimming rhythms were measured under constant dark condition s in the laboratory in megalopae collected from the Newport River Estu ary (North Carolina), the Delaware Bay, and offshore from the Newport River Estuary. Megalopae from all areas had a similar circadian activi ty rhythm, in which they swam during the time of the day phase in the field and were inactive at night. This rhythm pr-edicts the presence o f a reverse, diel, vertical-migration pattern offshore which would con tribute 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 i ncrease 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 rhy thm also continued in the presence of the eelgrass Zostera marina, whi ch is a site of settlement and metamorphosis in the field. Thus, megal opae 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 en tering an estuary, megalopae were transported back to offshore areas.