Gnathia maxillaris

(Montagu, 1804)

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 shallow central concavity with only a slight rounded median forward projection; lateral corners square, cephalon wider than long. Lateral tooth of mandible closely applied to 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 length up to 4.5-5.0 mm.

Probably the most common intertidal gnathiid in open marine localities, adults well hidden in rock crevices and crevice-like situations such as dead barnacles and Laminaria holdfasts.

It is recorded from the southern North Sea, southern England and Wales, Irish Sea and Ireland.