Male, female and young all of different form; each with only five pairs of ambulatory pereiopods. Cephalon fused with two pereion segments, limbs of the second fused segment modified as flattened pylopods, which cover the mouthparts ventrally. Adult male pylopod of 2 or 3 articles. Male with the front of the cephalon having a deep central concavity with a fairly acute median forward projection; lateral tooth of the mandible closely applied to the mandible itself. Last pereion segment reduced and without limbs; best seen in males, where it is narrow and equal in width to the pleon segments. Male with large mandibles, which project in front of the cephalon and are forceps-like. Females and late pranizas (young ones) with the pereion segments 3 to 5 fused and inflated, particularly in females, which incubate the eggs internally. Males and females usually ranging from 5 to 7 mm in body length, therefore larger than Gnathia maxillaris , with which it should not be confused.
Sporadic records around the British Isles and reported from the Mediterranean northwards to Greenland.