KOCH, A.S.*; GILLEN, C.M.; Kenyon College, Gambier, OH: Acute physiological and behavioral responses to dilute seawater in Uca pugilator.

We investigated the behavior and physiology of sand fiddler crabs, Uca pugilator, during acute exposures to dilute artificial seawater (ASW). We measured the time crabs spent submerged in water or on wet sand when given the choice between sand and 100% ASW or sand and 10% ASW. Observations were made each hour during 6 hours of exposure. Crabs exposed to 10% ASW spent less time in the water throughout the entire trial. At 6 hours, crabs in 100% ASW were in the water about half the time (42.2 +/- 10.2 %), crabs in 10% ASW were in the water only a small portion of the time (4.4 +/- 2.6 %). We measured hemolymph osmolality in completely submerged crabs and in crabs allowed free movement between seawater and sand. After 6 hours, crabs allowed to move freely between 10% ASW and sand showed a smaller decrease in hemolymph osmolality (81.0 +/- 9.3 mOsm/kg) compared to crabs completely submerged in 10% ASW (150.1 +/- 27.6 mOsm/kg). To investigate physiological mechanisms, we estimated ion transport rates in crabs exposed to 20% and 100% ASW, using rubidium (Rb) as a tracer for potassium (K). Rb uptake was measured in a 20-minute assay in 20% ASW with the K replaced with 2 mM Rb. After 3 hours exposure to 100% or 20% ASW, Rb uptake rates were higher in crabs exposed to 100% ASW than in crabs exposed to 20% ASW. At 72 hours of exposure, crabs in 20% ASW had higher Rb uptake rates than crabs in 100% ASW. These results are consistent with acute physiological and behavioral responses during exposure to dilute seawater that differ from the chronic response. Supported by the Kenyon College Summer Science Scholar Program.