Jaera (Jaera) ischiosetosa Forsman, 1949
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 ischiosetosa is one of the 'albifrons' group of species (along with Jaera albifrons, Jaera forsmani and Jaera praehirsuta), all of which have a T-shaped male preoperculum. It can be distinguished from the other species by the presence of a dense cluster of setae on the ischium of the male 6th and 7th pereopods. Males up to 2.7mm, females up to 5.0mm.
Distribution and Habitat
Most characteristic of sheltered shores in areas of reduced salinity (e.g. under stones with freshwater run off), typically between LWN and HWN. Almost certainly much more common and widespread than indicated in the NBN map (e.g. it has been commonly recorded in south-east Scotland).
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.