Head and body smooth, without tubercles or spines; body stout, in female more slender than in male. Maximum length 7 mm in male and 6 mm in female.
Antenna 1 about one-third to one-half of the length of the body, flagellum of eight articles. Antenna 2 with short ventral setae; about as long as the peduncle of antenna 1. Mandible without a palp.
Gnathopod 1 with a short obtuse spine on the distal merus, plus one or two distal setae; carpus triangular. Gnathopod 2 inserted near the middle of the segment; palm of the propodus armed with a strong proximal poison spine and a large sub-median tooth; dactyl slightly sinuous on the inner border. Gnathopod 2 of female without a sub-median tooth.
Gills small, oval, on segments 3 and 4 only.
Pereiopods 3 and 4 represented by single seta only. Pereiopods 5 minute, comprising two segments with a long terminal seta. Pereiopods 6 and 7 with six articles.
Abdomen in male with plaque and a pair of vestigial lobes, female without lobes.
Anterior pair of incubatory lamellae setose.
The species in the North Sea belong to the variety inermis:
antenna 1 without a tubercle; gnathopod 2 with a deep indentation on the palm; propodus of perepods 6 and 7 without spines. On the southern and eastern coasts of Britain variety armatus occurs: with a tubercle on the peduncle of article 3 of antenna 1; without a deep indentation on the palm of gnathopod 2; with a strong proximal spine on the propodus of pereiopods 6 and 7.
Variety inermis is found in the sublittoral, but may occur on the lower shore; on coarse sandy substrata, but often on the aboral surface of the starfish Asterias rubens and Crossaster papposus.
Widely distributed in the North Sea.