SELECTIVE TIDAL-STREAM TRANSPORT OF THE BLUE CRAB CALLINECTES SAPID US: AN OVERVIEW

摘要

This overview combines our recent studies with existing information to develop more complete conceptual models of selective tidal-stream transport (STST) of ovigerous female and post-larvae of the blue crab Callinectes sapidus. During the first phase of the spawning migration, non-ovigerous females migrate seaward from brackish water to the mouths of estuaries following insemination. After oviposition, females with mature embryos undertake the second phase of the spawning migration, in which they undergo ebb-tide transport for movement seaward to release larvae and then migrate back into estuaries using flood-tide transport. Following larval development offshore, post-larvae or megalopae undergo flood-tide transport for up-estuary movement in which they ascend into the water column during flood tides at night and are on or near the bottom at all other times. This behavioral pattern is not due to a circatidal rhythm in activity since megalopae have a circadian rhythm. The timing of this endogenous rhythm is paradoxical because megalopae are active during the day phase and inactive at night. Neither exposure to a cycle in salinity change that simulates the natural tidal cycle nor step decreases in salinity alter this circadian rhythm. The behavior underlying flood-tide transport consists of behavioral responses to a sequence of cues. Megalopae ascend into the water column in response to the relative rate of increase in salinity during flood tide. Water turbulence due to flood-tide currents induces sustained swimming, and the decline in turbulence during slack water at end of flood tide induces settlement out of the water column. Environmental cues during ebb tide do not induce STST. Since light inhibits swimming, flood-tide transport does not occur during the day and is reduced when the time of slack water after flood tide occurs after sunrise. Future studies are needed to determine the behavioral basis of STST of females, and especially the reversal from ebb-tide to flood-tide transport.