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.
Distribution and Habitat
Fairly common amongst seaweed (Fucus and red seaweeds such as Plocamium cartilagineum) and under rocks in the intertidal zone. It can also be encountered in shallow water over sand at low tide. Found all around the coasts of Britain and Ireland. For further information on the distribution of I. balthica in Ireland, see here.
Naylor, E. & A. Brandt. 2015. Intertidal Marine Isopods. Synopses of the British Fauna (New Series), No. 3. Field Studies Council, for The Linnean Society of London.