Gammarus mucronatus is a highly motile shallow marine gammarid amphipod endemic to the coastal and estuarine waters of the US Atlantic coast. Individuals are typically light green to greenish-brown, with red to brown spots and subtle banding on antennae and appendages. The rounded head has a pair of large, reniform (kidney-shaped) eyes, typical of the genus. Three posterior body segments (pleosomes 1-3) usually possess sharp, posterioriorly-directed mucronations (points) that give the species it's name. The first pair of antennae are greater or equal to the second in length (Bousfield 1973). As with all gammaridean amphipods, the body is laterally compressed.The species is sexually dimorphic, with adult males being longer-bodied than females, and with the dactyl of the gnathopod (the anterior-most walking appendage) modified into a hook used to grasp females during precopulation and copulation.Bousfield (1973) provides a more detailed species description suitable for professional taxonomic purposes.