ENVIRONMENTAL-REGULATION OF FORAGING IN THE SAND FIDDLER-CRAB UCA-PUGILATOR (BOSC 1802)

Sand fiddler crabs Uca pugilator (Bosc 1802) live in burrows in the hi gh intertidal to supratidal zones of sandflats and salt marshes. Large groups of crabs regularly leave their burrows to forage on exposed se diments in the low intertidal zone. They are known to forage in respon se to chemical cues. We hypothesized that environmental factors that v ary predictably in field sediment play a role in determining where fid dler crabs forage. To test this hypothesis, we determined the effects of four physical sediment characteristics on chemically mediated feedi ng behavior in U. pugilator: (1) sediment organic content; (2) the sal inity of interstitial water; (3) sediment grain size; and (4) sediment water content. Our results indicate that sediment organic content and sediment water content are the most important factors that determine where fiddler crabs forage. Optimal sediment is a mixture of sizes dom inated by 0.125 and 0.250 mm sands. This condition is constant across the intertidal zone. Salinity, the most variable of the environmental factors, has no effect upon feeding responses. Optimal conditions for feeding are found in sediment with organic content at or above 1.0% th at is fully saturated with water. Complete saturation allows the entir e dactyl to penetrate the sediment. Consequently, dactyl chemoreceptor s are maximally stimulated. Optimal conditions are found in a band sev eral meters wide immediately above the water. The band moves with the tide. Physical factors (sediment water content), rather than biologica l factors (food content), are responsible for temporal patchiness of t he foraging environment of fiddler crabs.