A study of particle ingestion by three fiddler crab species foraging on sandy sediments

Abstract

Experiments were conducted to determine the abilities of three species of fiddler crab, Uca pugilator (Bosc), Uca pugnax (Smith), and Uca minax (Le Conte), to remove free particles (5-, 30-, 50-, and 80-μm polystyrene beads and unattached bacteria) and particle-bound bacteria from sandy sediments. The overall ranking of the species with respect to unbound particle ingestion efficiency was Uca pugilator > U. pugnax > U. minax. However, no differences were observed in the abilities of the crabs to extract unbound bacteria or 5-μm plastic beads. Uca pugilator removed twice as many attached bacteria as did U. pugnax; U. minax ingested no bound bacteria. Particle extraction efficiencies correlate well with the number of spoon-tipped hairs found on the second maxillipeds of the investigated crab species, implying that these setal types are an adaptation to foraging on sandy sediments. It is suggested that the microhabitat distributions of U. pugnax and U. minax are restricted to muddy sediments in part because these crabs do not possess mouth parts adapted for foraging in sandy sediments. Several lines of evidence are adduced to show that mouth part adaptations are probably not responsible for the exclusion of U. pugilator from microhabitats occupied by U. pugnax and U. minax.