Body oval, narrowing rather sharply posteriorly. Antenna 1 extending beyond article 2, but not article 3 of the peduncle of antenna 2; the most distal aesthetascs as a pair. Antenna 2 flagellum not reaching the length of the peduncle (except possibly in large males which have tufts of setae on the flagellum); usually less than 1/5 of the body length. Cephalon subquadrate, eyes dorsolateral, prominent. Maxillipedal palp of four articles. Coxal plates fairly narrow reaching the posterior border only in segments 4 to 7. Pereiopods all more or less alike. Pleon of two complete segments, and one partial suture in the pleotelson. Pleotelson narrowing sharply at first, with rather concave lateral sides; posterior border with a long, acute, median spine and very obtusely rounded lateral shoulders; not keeled. Uropods uniramous.
Males recognizable around 5 mm, ranging to about 20 mm in body length; females 6-13 mm. Colour mostly uniformly brown, red or green, depending on the nature of the weed inhabited; occasionally with longitudinal white markings.
Between tidemarks, large specimens predominating on Ascophyllum and Fucus species, and small specimens preferring smaller algae such as Cladophora and Polysiphonia species. Replaced on exposed shores by Idotea pelagica and on sheltered estuarine shores by Idotea chelipes .
A northern species ranging from the White Sea to southern Britain and Normandy; also extending into the Gulfs of Bothnia and Finland.