(G.O. Sars, 1872)
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 centrally tridentate and without a concavity; lateral corners acutely pronounced. Cephalon broader than long. Lateral tooth of the mandible diverging markedly from 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. Male body lengths 2.8 to 3.8 mm, females up to 4.3 mm.
Adults typically well hidden in crevices in open marine localities. Recorded occasionally between tidemarks in Laminaria holdfasts.
Plymouth and in the Isle of Man. Also recorded in Norway.