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.
Introduction
Adaptive radiation