SENSORY PHYSIOLOGY AND BEHAVIOR OF BLUE-CRAB (CALLINECTES-SAPIDUS) POSTLARVAE DURING HORIZONTAL TRANSPORT

Adult blue crabs (Callinectes sapidus) live in estuaries and release l arvae near the entrances to estuaries. Larvae are then transported off shore to continental shelf areas where they undergo development. Postl arvae, or megalopae, remain near the surface and undergo reverse diel vertical migration. The behaviors underlying this migration pattern ar e responses to light and a solar day rhythm in activity, in which mega lopae are active during the day and inactive at night. Onshore transpo rt probably occurs by wind-generated surface currents. Once in the vic inity of an estuary, megalopae move up the estuary by selective tidal stream transport, in which they swim in the water column on rising tid es at night and are on or near the bottom at all other times. Light in hibits swimming during the day. The ascent into the water column on no cturnal rising tides does not result from a biological rhythm in activ ity, but rather is cued by the rate of increase in salinity during ris ing tides. Megalopae have separate behavioural responses in coastal/sh elf areas and in estuaries, which are induced by chemical cues in offs hore and estuarine waters.