PHOTORESPONSES OF CRAB MEGALOPAE IN OFFSHORE AND ESTUARINE WATERS - IMPLICATIONS FOR TRANSPORT

This study tested the hypothesis that fiddler crab (Uca spp.) and blue crab [Callinectes sapidus (Rathbun)] megalopae have separate behavior s in offshore and estuarine waters that are evoked by chemical cues. P ast field studies indicate that blue crab megalopae are frequently in the neuston during the day offshore, while fiddler crab megalopae are at middle depths. Both species are rarely collected in the water colum n during the day in estuaries but are abundant during rising tides at night. Since photoresponses contribute to the day distributions of meg alopae, we hypothesize that photobehavior changes dramatically between offshore and estuarine areas and used this as an assay for the effect s of water-borne chemical cues. Photoresponses were tested in a light field that mimics the underwater angular light distribution and were q uantified by measuring the proportion of megalopae swimming in the upp er 1/3 of a water column. For blue crab megalopae collected offshore, swimming was significantly greater in offshore than in estuarine water and was suppressed in estuarine water at high light intensities. Thus , photoresponses contribute to depth maintenance near the surface duri ng the day which is important for onshore transport by wind-generated surface currents. When megalopae were collected in an estuary, both sp ecies had greater swimming in offshore water than in estaurine water a t all light intensities and in darkness. Thus, the absence of megalopa e in the water column during the day in estuaries results from light s uppression of swimming. Since the water only differed in chemical comp osition, the changes in photoresponses were induced by chemical cues. Tests with aged estuarine water indicated the chemical cues were relat ively stable or initially present at high concentrations. These result s support the hypothesis and suggest that megalopae have separate beha viors in coastal/shelf areas and in estuaries, which are induced by ch emical cues in offshore and estuarine waters.