One of the marine pill isopods, Cymodoce truncata can be distinguished from other Sphaeromatid species by the tridentate posterior margin of the pleotelson. On males, this is marked, but in females it is rather less obvious. Adult males can exceeds 11mm, whilst females are smaller in size.
Distribution and Habitat
This species is found on southern and western shores of Britain as far north as Shetland and Orkney, and at least on the southern and eastern coasts of Ireland. It appears to be absent from most of eastern Britain. It can be found (for example by washing) at low tide amongst algae, especially Fucus serratus, under stones and in crevices and holes.
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.