An Analysis of Cheliped Asymmetry in Three Species of Fiddler Crabs

Abstract

Left-right cheliped asymmetry occurs in a variety of decapod species, Such asymmetries can be present in early development or can arise during sexual differentiation. Male fiddler crabs have a dimorphic enlargement of one of the two chelipeds. This major cheliped has been associated with male reproductive success as a result of female selection or advantage in male-male combat. Because the major cheliped occurs on either the right or the left side, selection pressure could produce populations with right or left cheliped dominance. To ascertain whether populations of dominantly enlarged right- or left-clawed males are present, three fiddler crab species, Uca longisignalis, U. minax and U. pugnax, from four North American marshes, two on the coast of the Atlantic Ocean, one on the coast of the Chesapeake Bay, and one on the coast of the Gulf of Mexico, were sampled. There were no significant differences in the number of males with right- or left-enlarged chelipeds within all species or marshes sampled. Similarly, when male crabs were grouped arbitrarily into two size classes (major chelipeds <20 mm and 2:20>mm), the number of crabs with right or left major cheliped was similar among the size groups. Mean major propodus size for males was also similar for individuals with either right or left major chelipeds within each of the three species and four marshes. Comparison of morphometric parameters in each population indicated a strong correlation between claw size, carapace width, and carapace length, which was not affected by cheliped laterality. It appears that among these three species of Uca, there is no selection for males with cheliped enlargement on either the right or the left side within the geographical range of the species sampled.