Settlement times of blue crab (Callinectes sapidus) megalopae during flood-tide transport

Settlement by blue crab (Callinectes sapidus Rathbun) megalopae on artificial settlement substrates was monitored relative to tidal currents throughout ten nights from July to September 1997 in which the phase relationship between tides and the light:dark cycle differed. Most megalopae were in intermolt, and the total number settling to collectors sampled at hourly intervals was greater than totals on collectors immersed all night. Maximum settlement occurred at slack water before ebb tide (SBE), with a smaller peak at slack water before flood tide (SBF). These results support the hypothesis that during flood-tide transport (FTT) blue crab megalopae remain swimming during flood tide at night in response to water turbulence and settle in response to the decline in turbulence occurring near SBE. Settlement peaks near SBF can be explained by a behavioral response of megalopae to increasing salinity at the beginning of flood tide, which results in an ascent response lasting only a few minutes. Depth maintenance in the water column is not maintained at SBF because of low water turbulence. Since light inhibits swimming and upward movement into the water column, settlement, and, presumably, transport were reduced when SBE occurred near the times of sunrise and sunset. Collectively, these results suggest that the phase relationship between the tide and light:dark cycles affects FTT, the timing of settlement, and behaviors associated with habitat selection.