The sexually dimorphic Atlantic marsh fiddlers can be found to be up to 16 mm long and 26 mm wide. The average male carapace is 15 mm long and 23 mm wide, while the average female carapace is 13 mm long and 18 mm wide. The males are dark olive to almost black in color with a royal blue spot on the center of its carpace. Female U. pugnax are of similar color as the males, but do not have the blue spot. Either one of the male's chelea may grow until it is half of the crabs body weight, while the claws of the female are isomorphic. The chelea are brownish yellow at the base with white fingertips in both sexes. Uca pugnax have slender eyestalks and dark banded walking legs. The dorsal carapace of U. pugnax is flattened rather than convex as in other similar species of crabs.
Range length: 13 to 23 mm.
Average length: 16 mm.
Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry
Sexual Dimorphism: male larger