Eurydice are small, fast-swimming predators and scavengers with oval bodies and elongated pleotelsons. The peduncle of the antenna has four segments, whilst the second segment of the antennule is at a right angle to the first. The body is off-white with various darker markings, and the eyes are prominent.
E. affinis differs from the much more common E. pulchra in being somewhat smaller on average (males up to 5 mm, females up to 6 mm) and in the dark markings being restricted to the dorsal surface. The most significant characteristics is that the coxal plates of pereon somites 6 and 7 are not extended into points, and this feature should be examined closely to confirm identification.
In addition to the two other Eurydice species that are encountered intertidally, fieldworkers should be aware of the three other species that occur in (offshore) British waters, E. inermis, E. truncata and E. grimaldii (see Hansen 1905 and Jones & Naylor 1967 for details), the first two of which can come close to the shore at high tide.
Hansen, H. J. 1905. Revision of the European marine forms of Cirolaninae, a subfamily of Crustacea Isopoda. Journal of the Linnean Society, Zoology 29: 337-373.
Jones, D. A. & E. Naylor. 1967. The distribution of Eurydice [Crustacea: Isopoda] in British Waters, including E. affinis new to Britain. Journal of The Marine Biological Association of the United Kingdom 47, 372-82.
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.