Body robust, pleon segments tumid, pleosome with a posterior strongly sinuous dorsal margin. Coxal plates short, margins smooth. Posterior margin of epimeral plate 3 convex, smooth, distal angle with a small tooth. Head with a small pointed rostrum, lateral lobes broadly truncated; eyes very large. Antenna 1 and 2 robust, calceoli prominent. Antenna 1 peduncle article 3 with large distal process, flagellum up to about 30-articulate. Antenna 2 equal to length of I, peduncle articles 4 and 5 about equal, flagellum up to about 30-articulate. Gnathopod 1 and 2 similar, larger in male than female, carpus with narrow posterior lobe, propodus oval, palm strongly oblique and poorly defined, margin with 3 to 5 strong spines. Pereiopods 3 to 7 merus broad; pereiopods 5 to 7 basis expanded, posterior margin feebly serrate. Uropod 2 outer ramus much shorter than inner; uropod 3 rami about equal, broad, margins spinose and setose. Telson elongate, entire, apex rounded.
Up to about 14 mm.
Very variable; whitish or greenish with distinct markings or variegation.
From the intertidal to about 20 metres; usually amongst algae on rocky shores, locally numerous on sheltered or moderately exposed shores.
Circumpolar, arctic-boreal; Arctic Ocean; North Atlantic, American and European coasts; Pacific Ocean. European coasts from northern Norway and Åland to north-west France.