Paragnathia formica

(Hesse, 1864)

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 5 articles. 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. Body length up to 4 mm.

Most records of this species are from sheltered estuaries or inlets, usually around high water mark and in mud cliffs such as occur at the edges of a salt marsh.
Adults usually benthic, in galleries of 2 to 2-5 cm long, typically with a single male near the narrow mouth of the gallery (2-3 mm diameter) and up to 10 or 20 females at the innermost region where the gallery widens to about 5 mm diameter.
Newly released praniza larvae often ectoparasitic on fish and feed on the host blood. Late pranizas were found in the mud cliffs, usually scattered singly throughout the mud and not associated with the adult colonies.

There are sporadic records of the species northwards from Morocco to Scotland with one record in the Mediterranean and none in the Baltic.