This is a distinctive species due to its habitat, very large size (up to 30mm in length), large eyes comprising more than 40 ommatidia, and its antennal flagellum composed of numerous bead-like segments numerous bead-like segments. In the field immatures may be confused with uncommon Halophiloscia couchii (which has three flagellal segments).
Recent research using molecular markers (rather than traditional morphology) strongly suggests that Ligia is not closely allied to other terrestrial woodlice (Oniscidea) but more closely related to marine isopods of suborders Valvifera and Sphaeromatidea. See BMIG Newsletter 40 (2020), p5.
Distribution and Habitat
It is common around the entire coastline of Britain and Ireland, wherever a ‘hard’ shoreline occurs, such as the base of rocky cliffs, on boulder beaches, or within crevices on harbour walls and jetties. It is remarkably fast and agile and difficult to capture.
This summary is based on the detailed account in Gregory (2009).