Body oval in outline. Five anterior pleon segments all fused; sixth pleon segment fused with the telson as a pleotelson which articulates freely with the pleon. Coxal plates of pereion segments 2 to 7 fused with the segments. Uropods lateral; endopod rigidly fused with the peduncle; exopod moveable. Markedly sexually dimorphic. Pereion segments equal in females, male with segment 6 overlapping 7 and produced into two pairs of backwardly directed processes, one small lateral pair and a much larger pair close to the mid-dorsal line; processes only moderately rugose and without a prominent apical projection. Pleotelson domed, keeled or smooth in female, rugose in males with a central tubercle, which is relatively sessile and bluntly bilobed; respiratory channel sometimes closing ventrally. Mouthparts in ovigerous females showing marked loss of chitin and setae. Pleopods 1 to 3 laminate with apical setae, pleopod 2 of male without appendix masculina; pleopods 4 and 5 with exopod and endopod thick, fleshy and with transverse folds. Uropods biramous in both sexes; males more setose than females, endopod of males usually with a small, smooth apical projection. Young carried in brood chamber formed by 4 pairs of oöstegites on pereiopods 1 to 4; does not roll into a tight ball.
Length up to 7 mm in males and 6 mm in females. Colour variable; males mostly brown, young females green, red or yellow, ovigerous females white or colourless, associated with the considerable loss of tissue which takes place at the last moult.
Juveniles occur among intertidal algae upon which they feed. Adults occur in rock crevices or the empty tests of Balanus perforatus (when present), each male usually with a harem of several females. Mainly below MTL.
In southern and western Britain (including Ireland) and southwards along the Atlantic coasts of France, Spain and Portugal.