Hatching Rhythms of Fiddler Crabs and Associated Species at Beaufort, North Carolina

Estuarine crabs usually show hatching periodicities closely synchronized with tidal, lunar, or solar day periodicities. To define their timing, we monitored hatching activity either directly (by observing females during hatching) or indirectly (by measuring larval densities during nocturnal ebbing tides). Larval densities in the Newport River, at a site (Beaufort) near the ocean, revealed two patterns. Intertidal species (Uca spp., Panopeus herbstii) showed highest densities twice monthly, during the spring tides. Subtidal species (Neopanope spp., three pinnotherids, the blue crab Callinectes sapidus) showed larval densities which, on the average, were similar during all ebbing tides. We speculate upon the selection pressures that may have shaped these differences and propose a hypothesis to explain why the patterns are adaptive. Fiddler crabs were used to determine when, in relation to high tide (HT), hatching occurred. Most larval release in the laboratory and under field conditions occurred within 1 h after HT. To determine what environmental cue might synchronize semimonthly hatching rhythms in fiddler crabs, we compared the timing of larval release at two local sites where tidal amplitude pattern was identical but HT time differed each night by an average of 1.92 h. The results suggested that fiddler crabs respond to tidal amplitude and not the time interval between HT and the onset of darkness.