This distinctive species is a parasite of the burrowing mud (or ghost) shrimp, Callianasa subterranea. The larger females (up to 6 or 7 mm) have lateral points on the pereon somites, prominent oostegites with pointed projections, lateral branched pleon processes and pleopods, and long, tubular uropods. The smaller males have simple projecting pleon processes but no distinct pleon segments (unlike the female).
This species inhabits the gill chamber of its host, Callianasa subterranea, which is found around Britain. It is, however, mostly a sublittoral species, but may occasionally be found intertidally. Specimens should be checked for I. thoracica, which has been recorded off the south coast of England and around the Channel Islands, and occasionally elsewhere.
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.
World Register of Marine Species (WoRMS): https://www.marinespecies.org/aphia.php?p=taxdetails&id=118218
Ocean Biodiversity Information System (OBIS) distribution map: https://mapper.obis.org/?taxonid=118218