Photoresponses of crab megalopae in offshore and estuarine waters: Implications for transport

Abstract

This study tested the hypothesis that fiddler crab ( Uca spp.) and blue crab [ Callinectes sapidus (Rathbun)] megalopae have separate behaviors in offshore and estuarine waters that are evoked by chemical cues. Past 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 column during the day in estuaries but are abundant during rising tides at night. Since photoresponses contribute to the day distributions of megalopae, we hypothesize that photobehavior changes dramatically between offshore and estuarine areas and used this as an assay for the effects of water-borne chemical cues. Photoresponses were tested in a light field that mimics the underwater angular light distribution and were quantified by measuring the proportion of megalopae swimming in the upper 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 during the day which is important for onshore transport by wind-generated surface currents. When megalopae were collected in an estuary, both species had greater swimming in offshore water than in estuarine water at all light intensities and in darkness. Thus, the absence of megalopae in the water column during the day in estuaries results from light suppression of swimming. Since the water only differed in chemical composition, the changes in photoresponses were induced by chemical cues. Tests with aged estuarine water indicated the chemical cues were relatively stable or initially present at high concentrations. These results support the hypothesis and suggest that megalopae have separate behaviors in coastal/shelf areas and in estuaries, which are induced by chemical cues in offshore and estuarine waters.