Athelges paguri (Rathke, 1843)
Athelges paguri is a protandrous hermaphrodite parasite of the hermit-crabs Pagurus bernhardus (which occurs intertidally) and Anapagurus laevis (only found sub-littorally). Adults attach to the abdomen (or occasionally occur in the gill chamber) of their host, with females reaching 11 mm and males 4 mm. The females are assymetrical with the small head, which is withdrawn into the front of the pereon, and pereon segments covered by large oostegites. The pleotelson has four distinct segments, each with with a pair of biramus, stalked pleopods. The uropods are reduced to tubercles on the end of the pleon. The smaller male retains a typical isopod shape; its pleotelson does not have distinct somites.
Although identification can be made based on these characteristics and on the identity of the host, three other sub-littoral Athelges species should be borne in mind, A. bilobus Sars, 1898, found on the usually sub-littoral Pagurus cuanensis, A. tenuicaudis Sars, 1898, found on the sub-littoral Anapagurus chiroacanthus, and A. prideauxi Giard & Bonnier, 1890, found on the sub-littoral Pagurus prideaux.
Distribution and Habitat
This species' host, Pagurus bernhardus, is a familiar resident of the intertidal zone all around Britain and Ireland, and it appears that A. paguri may occur wherever its host is found, though it is poorly recorded. There are records of it from the Solent, north-west Wales, Northumberland, the Firth of Clyde, south-east Scotland, and off the north coast of Ireland. In south-east Scotland, the species has been found on the abdomen of P. bernhardus inhabiting Periwinkle and Rough Whelk shells collected in shallow rockpools around mid-tide.
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.