ENDOGENOUS SWIMMING RHYTHMS IN ESTUARINE CRAB MEGALOPAE - IMPLICATIONS FOR FLOOD-TIDE TRANSPORT

Up-estuary migration of crab larvae to adult habitats is thought to be accomplished by selective tidal transport in which late-stage larvae enter the water column on flood tides and remain on or near the bottom on ebb tides. This study measured endogenous rhythms in swimming by t he last larval stage (megalopa) of blue crabs Callinectes sapidus and fiddler crabs Uca spp. Previous field studies found that megalopae of both species were only abundant in the estuarine water column on noctu rnal rising tides. Megalopae were collected from the Newport River Est uary, North Carolina (34 degrees 41'N; 76 degrees 40'W) during August- September 1992 and swimming activity was recorded for 4.5 to 7 d under constant conditions with a video system. Rhythms exhibited by both ge nera in the laboratory were not identical to those recorded in the fie ld. Uca spp. displayed a circatidal rhythm, with maximum swimming occu rring near the time of high tide in the field. Rhythm amplitude increa sed when crushed oyster shells were present, which suggested that mega lopae bury or cling to the substrate during quiescent periods. In cont rast, C. sapidus had a circadian rhythm in which maximum swimming coin cided with the day phase in the field. In most trials, the activity of blue crab megalopae was unrelated to the expected tidal cycle. It was concluded that a tidal rhythm in swimming was the behavioral basis of flood-tide transport for fiddler crab larvae. The endogenous rhythm i n blue crabs does not participate in transport, which probably results from behavioral responses to environmental cues associated with flood tide.