Carapace rounded to polygonal and smooth. Propodus of third maxilliped projecting beyond dactylus. Chelipeds equal in size and smooth, upper margin of merus, inner of carpus and lower of propodus setose; dactylus of second to fifth pereiopods short and curved, only slightly exceeding half propodal length at the most. Female carapace soft, translucent, rounded or polygonal, postero-lateral margins often diverging strongly. Male carapace hard, opaque (in swimming stage) and rounded, frontal region strongly projecting; cheliped propodus swollen; second to fifth pereiopods stouter than female's and, in swimming stage, with long setae.
Carapace length up to female 10 (13) mm; male (6.0 mm).
Depth range from intertidal region to about 55 metres, and in the following bivalves: Acanthocardia echinata (Linnaeus), Arctica islandica (Linnaeus), Cerastoderma edule (Linnaeus), Donax vittatus (da Costa), Glycymeris glycymeris (Linnaeus), Laevicardium laevigatum (Linnaeus), L. norvegicum (Spengler), Lutratia lutraria (Linnaeus), Mactra corallina (Linnaeus), Modiolus modiolus (Linnaeus), Mytilus edulis Linnaeus, M. galloprovincialis Lamarck, M. incurvatus Philippi, ? Ostrea edulis Linnaeus, Parvicardium exiguum (Gmelin), Spisula solida (Linnaeus), Venerupis decussata (Linnaeus), V. pullastra (Montagu), V. rhomboides (Pennant); and in the ascidian Ascidia mentula O. F. Muller (see Clark, 1909; Holthuis & Heerebout, 1976; Ryland & Nelson-Smith, 1975; Schmitt et al., 1973; Sinel, 1907); also from unspecified region in North Sea in Spisula elliptica (Brown) and S. subtruncata (da Costa) (see Christiansen, 1969). Ovigerous crabs June-August, larvae August-October (Lebour, 1928; Williamson, 1956).
Female carapace translucent beneath which is visible the yellowish organs and usually the red-coloured gonads. Male carapace yellowish grey with brown patches.
Remarks. The taxonomy of North East Atlantic species of Pinnotheres needs reappraising (see Ingle, 1980).
Aberdeen, Firth of Forth, Northumberland, Yorkshire, Wash, Thames, East Channel, Wight, Portland, Channel Isles, Plymouth, N. Cornwall, Bristol Channel, Cardigan Bay, Liverpool Bay, Solway, Isle of Man, Belfast, Clyde & Argyll, Donegal Bay, Mayo, Galway Bay, Cork.