Behavioral responses of blue crab Callinectes sapidus postlarvae to turbulence:implications for selective tidal stream transport

Abstract

Postlarvae (megalopae) of the blue crab Callinectes sapidus move up estuaries using selective tidal stream transport, in which they are in the water column on flood tides and at or near the bottom at other times. They have no endogenous tidal rhythm in activity, so this behavior must be driven by reactions to exogenous environmental cues. Previous work indicated that rates of increase of salinity cue megalopae to ascend into the water column at the beginning of flood tide. However, this ascent due to salinity changes is a short-term response which cannot explain the duration of transport during flood tide or the descent at the end of flood tide. The present study examined turbulence in flow as a cue to regulate swimming behavior during selective tidal stream transport. Laboratory flow tank experiments show that megalopae react to increases in turbulent kinetic energy (TKE) by ascending in the water column and react to decreases in TKE by descending. Above a threshold TKE, megalopae are stimulated to swim regardless of changes in TKE. Reactions to turbulence were similar whether TKE was generated by variations in flow speed or bottom roughness. A model for the joint regulation of selective tidal stream transport by salinity and turbulence is proposed, in which increases in salinity cue the ascent in the water column at the beginning of flood tide, TKE cues swimming throughout the flood tide, and decreases in TKE cue the descent in the water column at the end of flood tide.