The squareback marsh crab, Armases cinereum, is one a several semi-terrestrial crab species occupying the intertidal habitats of the IRL. The carapace is brown to olive and the top margin of the upper finger on the claw bears small tubercles or bumps (Kaplan 1988). The last segment of the 4th walking leg is equipped with black spines on the dorsal and ventral surface. A characteristic tooth or spine behind the eye socket, which is found in several similar species, is absent in A. cinereum (Gosner 1978, Kaplan 1988, Voss 1980). The squareback marsh crab is sexually dimorphic, with male claws growing relatively larger than those of females (Buck et al. 2003).