Body elongate, laterally flattened. Coxal plate 1 prolonged, anterior angle rounded, plates 2 to 5 rounded; epimeral plates 1 to 3 rounded. Head with rounded lateral lobes, eyes small, oval. Antenna 1 and 2 about equal in male, in female antenna 1 may be a little longer than antenna 2; antenna 1 peduncle article 3 almost half the length of article 2, flagellum slender, up to about 35-articulate; antenna 2 peduncle robust especially in male, flagellum short, up to about 15-articulate. Gnathopod 1 moderately setose, basis with a moderately large anterodistal lobe, propodus oval, palm oblique, weakly convex, delimited by a small spine, dactylus longer than the palm. Female gnathopod 2 slightly larger than gnathopod 1, carpal lobe slender, palm weakly concave. Male gnathopod 2 much larger than gnathopod 1, basis with a small anterodistal lobe, carpus with a slender posterior lobe, propodus broadly oval, inner surface very strongly setose, palm concave. Pereiopods 5 to 7 basis only slightly expanded, propodus weakly expanded distally. Uropod 3 peduncle inner distal margin with about six small spines, outer ramus with two strong hooks and a small group of long dorsal setae, inner ramus with long apical setae and three or four small spines. Telson rounded, apex convex between small lateral cusps.
Up to about 20 mm.
Variable, red to green with dark markings, often with white spots on the pereion.
A very common intertidal or shallow subtidal amphipod; locally abundant; lives in tubes attached to algae or beneath stones.
North Atlantic, American and European coasts; North Pacific; Arctic Ocean. European coasts from northern Norway to north-west France and the southern Baltic. Very widespread and frequently recorded.