UC Davis

Tidal variation may cause the timing of activities by coastal organisms to vary geographically. This was demonstrated by determining the timing of larval release by the same or closely related species in diurnal and mixed semidiurnal tidal regimes along the northeastern Gulf of Mexico and by contrasting hatching patterns with those previously documented in a semidiurnal tidal regime along the Atlantic coast of the USA. Five species of crabs primarily released larvae during nocturnal maximum amplitude high tides near new and full moons along the Atlantic coast. In contrast, larval release by the same or closely related species on the Gulf coast peaked either when maximum amplitude high tides occurred during the daytime or several days away when submaximal amplitude high tides occurred near dawn. This variation in reproductive timing arose because maximum high tides do not occur at night along the Gulf coast during most of the reproductive season. Reproductive timing also differed for two species pairs along the Gulf coast due to changes in the phasing of the tides and light dark cycle. Species-specific hierarchies of rhythms regulating reproductive timing were revealed by the degree to which larval release kept phase with lunar, tidal amplitude and light-dark cycles. These hierarchies enabled crabs to track phase shifts of cycles in variable tidal environments and may enhance reproductive success across tidal regimes.