Adaptations for feeding on rock surfaces and sandy sediment by the fiddler crabs (Brachyura: Ocypodidae) Uca tetragonon (Herbst, 1790) and Uca vocans (Linnaeus, 1758)

Two species of fiddler crab, Uca tetragonon (Herbst, 1790) and Uca vocans (Linnaeus, 1758), which belong to the subgenus Gelasimus, dwell on rocky shores and muddy-sandy tidal flats, respectively, in Phuket Is., Thailand. We investigated their feeding ecology in relation to the morphology of their feeding organs: minor food-handling chelipeds and maxillipeds. U. tetragonon fed chiefly on rocks covered by filamentous green algae. U. vocans fed on the emerged sand and in shallow water along the shoreline and in pools. While feeding, both crabs made sand pellets beneath their mouthparts and discarded them, indicating that they divided the matter scooped up with their minor chelipeds into edible and inedible fractions by using the maxillipeds in the water passing through their buccal cavity. The morphology of maxillipeds hardly differed between the two species, which means that both species are flotation-feeders. The morphology of their minor chelipeds, however, differed: the tips of the dactyl and pollex were flat in U. tetragonon and pointed in U. vocans. When the minor cheliped was closed, U. tetragonon had a hemispherical space in the distal one-fourth of the gape, which was closed by the framing keratin layers and a few setae of the dactyl and pollex. On the other hand, U. vocans had an ellipsoidal space in the distal half of the gape. We consider these morphological characters to be adaptations to the different feeding substrates for retaining more food-laden sediment. We discuss the role of the setae on the minor chelipeds on the basis of the morphological differences between populations of U. tetragonon in Phuket Is. and East Africa where the crab inhabits muddy-sandy tidal flats.