Carapace medium,0. 6-0.7 mm long, elongate, subovate, compressed. Ventral and dorsal margins weakly convex or straight, converging anteriorly. Male proprtionally more elongate and with a straighter dorsal margin than the female. Fused zone narrow anteriorly, rather broader ventrally and posteriorly.
Mandible coxa short. Second leg with a chelate knee seta. Third podomere of third leg with a strongly spinose anterior margin. Male copulatory appendage with three distal processes: the anterior one small, inconspicuous, the central one large, lamellar, subtriangular, posteriorly acuminate, and the posterior one lamellar overlapping the central one (P. sarniense 5 ). Living specimens light grey with blue/black markings.
It lives amongst littoral and shallow sublittoral algae in marine or somewhat brackish conditions, being tolerant of salinities as low as 25ä.
Several authors have followed Brady & Norman (1889) in considering P. sarniense to be conspecific with P. hibernicum Brady, but Horne & Whittaker (1985d) showed the two species to be distinct (see also Remarks on P. hibernicum). P. sarniense is more compressed and has a straighter dorsal margin than P. hibernicum, and their male copulatory appendages differ in the shape and proportions of the distal processes. P. sarniense is more elongate than P. trieri and two Mediterranean species, P. rarum G. W. Müller, 1894 and P. rotundatum G. W. Müller, 1894, to which it is closely related.
In the British Isles it is apparently confined to southern and southwestern coasts, and it is also found on the coasts of Brittany and Normandy.