The mudflat fiddler, Uca rapax, is one of approximately 97 species belonging to the family Ocypodidae (Rosenberg 2001). Members of this family are characterized by a thick, squarish body and herding behavior (Ruppert & Fox 1988). Male crabs also bear one greatly enlarged pincer, either right or left, for combat and mating rituals; whereas, the claws of females are roughly equal in size. Fiddler crabs share many common morphological characteristics and behaviors, but identification of species is usually easily achieved through examination of body color and claw structure. The carapace of U. rapax is narrow between the eyes, and light tan (Kaplan 1988) or occasionally greenish blue in color (Crane 1975, Ruppert & Fox 1988). Eyestalks and the tips of the major claw in some specimens are also green to blue (Crane 1975). However, the claw in most individuals is gray to greenish yellow, sometimes with hints of orange and white finger tips. The center of the palm is almost smooth, but still bears small granules. Whitening of the carapace common to displaying males in other Uca species is poorly developed in U. rapax, and females are similar in color to the less brilliant males.