Carapace medium, 0.65-0.8 mm long, subovate to subcuneiform, weakly inflated. Greatest height behind mid-length, posterior extremity well above mid-height. Male larger and proportionally higher than female. Fused zone narrow.
Mandible coxa short. Second leg with a slender knee seta. Fourth podomere of third leg with a strongly spinose anterior margin. Male copulatory appendage with a large, rounded basal capsule and a relatively small distal process consisting of two parts, the anterior one narrow, curved, distally bifid with a lamellar rim, and the posterior one lamellar, distally acuminate (P. variabile 12 ). Living specimens variable in colour, ranging from black through violet, blue, brown and red to pale grey.
A very common species on littoral and sublittoral algae in marine and estuarine conditions; in Britain we have found large populations in salinities as low as 20 ä.
P. variabile is the commonest and best-known British Paradoxostoma species, and although Baird's types are lost there is, with few exceptions, general agreement amongst previous authors as to its identity. As Elofson (1941) suggested, most records of P. variabile from Arctic waters should probably be referred to P. arcticum Elofson, 1941. The two species are similar, but the carapace of P. arcticum is proportionally lower and has a straighter ventral margin than that of P. variabile, and their male copulatory appendages differ in details of shape and proportion. Nevertheless, P. variabile does occur on the northernmost coasts of Norway (Sars, 1928) and in the White Sea (Rudjakov, 1962). As far as we can ascertain it does not live in the Mediterranean, and records from the Black Sea (e.g. Schornikov, 1969) must be treated with suspicion.
P. variabile is set apart from other British members of the genus by several features of carapace and appendages, not the least of which is its distinctive sexual dimorphism. The general rule in Paradoxostoma is for the male to be smaller than the female, but in P. variabile the reverse is true. In this and other respects it resembles a number of Japanese species (see Horne & Whittaker, 1985d) but there appear to be no other European or Mediterranean species with close affinities with this group.
British Isles, Norway, White Sea, Sweden, Baltic Sea, and as far south as Brittany, N France.