Idotea balthica (Pallas, 1772)

Idotea are relatively large, conspicuous and common marine isopods with oval or oblong bodies. Although they often have striking colours and patterns, these are not particularly useful for distinguishing between the various species. Rather, identification should be based on characteristics of the antennae and antennules, and the shape of the coxal plates and the pleotelson.

Males of I. balthica grow up to 30 mm in length, whilst females reach 18 mm. The most obvious diagnostic characteristic of the species is the tridentate tip to the pleotelson, though this is absent in juveniles. The antennules extend to the third segment of the peduncle of the antennae or just beyond, whilst the coxal plates extend from the front to the back of pereonites 2 or 3 to 7 in adults. Body colouring and patterning vary considerably and can often be striking.