Limnoria are small (up to 3-4 mm) but distinctive wood-boring marine isopods. The three species that have been recorded on British and Irish shores can be distinguished by the patterning of raised areas on the fifth pleonite and on the pleotelson, though these can be hard to discern.
The commonest and most widespread species, L. lignorum (the 'Gribble'), has a raised longitudinal 'carina' on the fifth pleonite and an inverted Y-shaped 'carina' in the middle of the pleotelson.
Distribution and Habitat
L. lignorum is found all around the shores of Britain and Ireland (Jones 1963), though it is least common in the south-west of Britain, where it is partly replaced by L. quadripunctata. It is found in wood pilings that are regularly submerged by the tide and in driftwood (when trapped, for example, in rockpools). The presence of Gribbles in wood is indicated by the characteristic pits and channels created by their boring.
Jones, L. 1963. The geographical and vertical distribution of British Limnoria [Crustacea: Isopoda]. Journal of the Marine Biological Association of the U.K., 43, 589-603.
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.