Body oblong oval. Antenna 1 extending to or just beyond article 3 of the peduncle of antenna 2; antenna 2 flagellum longer than the peduncle and about a quarter of the body length. Maxillipedal palp of four articles. Coxal plates large in adults, extending from the anterior to the posterior borders of the pereion segments 2 or 3 to 7. Pereiopods all more or less alike. Pleon of two complete segments, and one partial suture in the pleotelson. Pleotelson dorsally keeled, with more or less straight sides, tapering to a tridentate posterior border with the median process long and acute (juveniles lack the tridentate posterior border). Uropods uniramous.
Males first recognizable at about 10 mm body length, ranging to 30 mm; females 10-18 mm in length. Colour sometimes uniformly green or brown but often with white spots or longitudinal lines; female often darker than male.
Generally offshore, but not infrequently found among attached algae on the shore and often cast up in large numbers among drift weed.
Fairly cosmopolitan, and widespread in Europe from northern Norway into the Gulfs of Bothnia and Finland, and south to the Black Sea.