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Plasticity in reproductive timing by crabs in adjacent tidal regimes


Tidal variation may cause reproductive timing by fiddler crabs Uca pugilator (Bosc) to vary geographically and seasonally. Fiddler crabs in a semidiurnal tidal regime along the Atlantic coast of the USA were previously shown to release larvae synchronously during maximum amplitude evening high tides near new and full moons throughout the reproductive season. In the present study, fiddler crabs in a diurnal tidal regime on the Gulf coast of the USA released larvae during intermediate amplitude flood tides near dawn regardless of lunar phase throughout most of the reproductive season. Late in the summer, peak larval release occurred near maximum amplitude high tides as on the Atlantic coast. In a mixed semidiurnal tidal regime on the Gulf coast, peak larval release by fiddler crabs occurred during intermediate amplitude high tides near dawn, but biweekly periodicity was weak late in the reproductive season. Intraspecific variation in reproductive timing arose because nocturnal maximum high tides did not occur along the Gulf coast during most of the summer, but these tides returned during late summer in the diurnal tidal regime. Reproductive timing changed concomitantly with changing phase relationships of tides with light-dark and lunar cycles. Variation in reproductive timing is phenotypic because crabs that were translocated reciprocally between diurnal and mixed semidiurnal tidal regimes entrained to the timing patterns of local crabs. A hierarchy of rhythms regulating reproductive timing was revealed by the degree to which larval release kept phase with lunar, tidal amplitude, tidal and light-dark cycles. These hierarchies enabled crabs to track phase shifts of cycles in variable tidal environments and may enhance mean reproductive success throughout the range of the species.

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