ADAPTIVE SIGNIFICANCE OF SEMILUNAR CYCLES OF LARVAL RELEASE IN FIDDLER CRABS (GENUSUCA): TEST OF AN HYPOTHESIS

Abstract

The hypothesis that semilunar timing of larval release by fiddler crabs (genus Uca) results in transport of the final larval stage (megalopa) by spring tide currents to substrates in the upper estuary occupied by adults was tested and rejected. Water temperatures in the North Inlet estuary, South Carolina, increased from approximately 20° C to 28° C and the length of larval life decreased during the May-September breeding season. Nevertheless, ovigerous female U. pugilator, U. pugnax, and U. minax, collected bimonthly and maintained in the laboratory, released larvae ± 1.5 d of the full and new moons throughout most of the breeding season. Megalopae of Uca spp. were most abundant in a small tidal creek in the upper estuary during nocturnal flood tides and near the bottom about 5 d before and after the spring tides in September. Uca spp. and several other estuarine crabs appear to release larvae near the times of the high tides that are followed by the nocturnal ebb tides of greatest amplitude during the semilunar cycle. At North Inlet, such timing results in rapid seaward transport of newly hatched zoeae and subsequent export into coastal waters. Convergence among estuarine brachyurans in the timing of larval release probably reflects a shared adaptive response to selective factors, such as lethal combinations of high temperatures and low salinities, or predation by diurnal planktivors, that cause high larval mortality during the day in the upper estuary.