Astacilla species are distinctive marine isopods that are mostly sublittoral but can occasionally turn up in the intertidal zone. They have long antennae with extended 4th and 5th segments, an elongated middle pereon segment, short walking legs on pereon segments 5 to 7, and usually with adapted, setose legs on pereon segments 1 to 4 that are directed towards the mouth for feeding. The pereon segments are typically ornamented with tubercles and/or spines.
Females of A. longicornis grow up to 30 mm in length, males up to 10 mm. The long middle segment is smooth in males but is covered in small tubercles in females, with two conspicuous ones towards the head end.
Distribution and Habitat
Astacilla species are filter feeders that typically cling to seaweed, echinoderms, coelenterates, etc. Found sublittorally around Britain and Ireland, A. longicornis is occasionally recorded intertidally, especially in south-west England and south Wales.
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.