Abstract
The density of spoon-tipped setae (ST setae) along the inner edge of the merus of the left second maxilliped of two Singapore species of fiddler crabs, Uca annulipes and U. vocans, was studied. A larger proportion of the inner merus edge was covered with ST setae in U. annulipes as compared to U. vocans (two-thirds vs. half of the edge, respectively). The number of ST setae did not differ significantly between males and females in both species (ANCOVA, slopes and intercepts, p > 0.05) suggesting there is no sexual dimorphism in mouthpart adaptations for these species. The combined regression equation (male + female data; CW = carapace width) for U. annulipes (n = 91) was #ST setae = 15.6 + 14.7CW with r2 = 0.63, that for U. vocans (n = 100) being #ST setae = 32.4 + 9.91CW with r2 = 0.66. For any given size of crab, significantly more ST setae were present in Uca annulipes (a sandy habitat crab) than in U. vocans (a muddy habitat species). These results provide quantitative evidence that fiddler crabs that inhabit sandy habitats possess more ST setae (an adaptation for processing sandy sediments), than those occurring in muddy habitats.
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