RESPONSES OF ESTUARINE CRAB MEGALOPAE TO PRESSURE, SALINITY AND LIGHT- IMPLICATIONS FOR FLOOD TIDE TRANSPORT

The megalopal larval stage of many estuarine brachyuran crabs appears to return to adult habitats by undergoing rhythmic vertical migrations which result in saltatory up-estuary transport on flood tides. Larval ascent into the water column during rising tides may be cued by chang ing hydrologic variables. To test this hypothesis, we investigated the responses of field-caught megalopae of the blue crab Callinectes sapi dus and the fiddler crab Uca spp. to constant rates of pressure and sa linity change under laboratory conditions. For both genera, pressure c hanges resulted in increased movement (barokinesis) and upward migrati on in the test chamber, with C. sapidus megalopae having a lower respo nse threshold (2.8x10(-2) mbar s(-1)) than Uca spp. larvae (5x10(-2) m bar s(-1)). Similarly, larvae ascended in response to increasing salin ity, with C. sapidus larvae being more sensitive. Larvae were negative ly phototactic and failed to respond to pressure increases at light le vels above 1.0x10(15) and 1.0x10(13) photons m(-2) s(-1) for C. sapidu s and Uca spp. megalopae, respectively. Such responses are thought to explain the low abundances of larvae in the water column during daytim e flood tides. Nevertheless, threshold sensitivities to increasing pre ssure for both genera were above levels experienced during flood-tide conditions in the field. Similarly, it is unlikely that increasing sal inity is sufficient to induce ascent in Uca spp. postlarvae. However, rates of salinity increase during mid-flood tide typically reach level s necessary to induce an ascent in C. sapidus megalopae. These results are consistent with the hypothesis that fiddler crab megalopae utiliz e an endogenous activity rhythm for flood-tide transport, while blue c rab megalopae rely upon external cues, especially salinity changes, to time their sojourns in the water column.