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 in the form of the pleon and pleotelson; female mouthparts reduced. Pereion segments without posterior projections in either sex. Male pleon posterior border and pleotelson anterior border each with two tubercles. Pleotelson domed and smooth in females, with only two bosses or tubercles; male pleotelson central region with two pronounced tubercles or processes, posterior region with a low boss just anterior to trilobed posterior border. Very large specimens have a greater number of tubercles. The setation and granulation of the pleon and pleotelson of the males are variable, forms with dense granulation and setation having previously been allocated to Cymodoce emarginata . Posterior border of male pleotelson markedly tridentate, median projection above and lateral projections at sides of the ventral respiratory channel. Posterior border of female pleotelson usually less obviously tridentate. Pleopods 4 and 5 with the exopod lamellar, endopod thick, fleshy and with transverse corrugations. Uropod with two rami subequal in length. Female with brood developed in internal pouches.
Adult male body length 11 mm or more; females smaller.
Sometimes recorded among algae, but adults characteristically in crevices and old mollusc borings around LW.
Patchily distributed from the Isle of Wight westwards and northwards to the Shetlands.