Head with a large, anteriorly directed spine dorsally. Maximum length 25 mm in male and 20 mm in female.
Antenna 1 about two-thirds of the length of the body, flagellum of 18-26 articles. Flagellum of antenna 2 with 2 articles; peduncle and flagellum with short setae, widely spaced.
Pereion segment 1 with a similar spine; segment 2 with two spines in the middle and one spine on the posterior margin. Pereion segment 3 with a small triangular tubercle dorsal to the insertion of the pereiopod and two small rounded dorsal tubercles mid-way along the segment. Small rounded dorsal tubercle, or occasionally a spine, at junction of pereion segments 3 and 4; tubercles on segment 4 as on segment 3; segment 5 with a small lateral triangular tubercle one-third length along each side.
Pereiopods 3 and 4 minute, two-segmented, with a long terminal seta. Pereiopods 5, 6 and 7, six-segmented, about equal in length. Gnathopod 2 inserted one-third to one-half length along segment; basis with a downwardly directed spine on the distal anterior edge.
Gills ovoid, on segments 3 and 4 only.
Abdomen with two pairs of non-jointed appendages in male, without appendages in female. Brood pouch of female with an anterior pair of lamellae entirely setose, posterior pair setose only on the posterior half.
Usually sublittoral, often on algae and large hydroids.
All British coasts, probably more common in the south.