Jaera (Jaera) nordmanni (Rathke, 1837)
Jaera are small, ubiquitous intertidal marine isopods which can be identified to species by the examination of male reproductive structures (specifically the preoperculum) and pereopods. Preopercula can be examined using a binocular dissecting microscope, but examination of pereopods is best done using a compound microscope at low power.
Jaera nordmanni is a distinctive species with a relatively large, rounded, flattened body fringed by numerous setae. It is of a fairly uniform colour, lacking the white markings seen in members of the 'albifrons' group. The pointed, neck-tie shaped male preoperculum is diagnostic. Males up to 4.5mm, females up to 3.5mm. See further the Identifying Jaera page.
Distribution and Habitat
Found around the coasts of Britain and Ireland. It is rather more widepread and common than indicated in the NBN map (e.g. it has been recorded in Northumberland and in both south-east and south-west Scotland). Most typically found under stones in freshwater streams joining the shore, from MTL to above HWS.
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.