Contents volume 53

Abstract

Sand fiddler crabs Uca pugilator (Bosc 1802) live in burrows in the high intertidal to supratidal zones of sandflats and salt marshes. Large groups of crabs regularly leave their burrows to forage on exposed sediments in the low intertidal zone. They are known to forage in response to chemical cues. We hypothesized that environmental factors that vary predictably in field sediment play a role in determining where fiddler crabs forage. To test this hypothesis, we determined the effects of four physical sediment characteristics on chemically mediated feeding behavior in U. pugilator: (1) sediment organic content; (2) the salinity 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 dominated 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% that is fully saturated with water. Complete saturation allows the entire dactyl to penetrate the sediment. Consequently, dactyl chemoreceptors are maximally stimulated. Optimal conditions are found in a band several meters wide immediately above the water. The band moves with the tide. Physical factors (sediment water content), rather than biological factors (food content), are responsible for temporal patchiness of the foraging environment of fiddler crabs.