Jaera ischiosetosa

Forsman, 1949

Body narrowly oval in females; males smaller than females and broadest across the posterior pereion segments. Lateral margins of both sexes fairly sparsely fringed with spines. Antenna 1 very small with a reduced flagellum; antenna 2 well developed but shorter than the body. Eyes large and situated laterally, particularly in males. Mandible with a strong, truncated molar process. Coxal plates of pereion segments not visible from above. Pereiopods of male 1 to 4 normal, 6 and 7 each with a dense cluster of curved setae on the distal portion of the ischium. Pleon of one small, inconspicuous segment, others all fused with the telson to form a large, shield-shaped pleotelson. Pleotelson posterior border fairly deeply excavate where uropods are inserted. Uropods terminal, small, much shorter than the pleotelson and barely projecting beyond the pleotelson notch in which they are set. In females the first pair of pleopods absent, second forming a plate-like operculum covering the remaining pleopods. Preoperculum of male T-shaped with only slight lateral extensions to each arm of the T.
Males up to 2.7 mm body length; females up to 5.0 mm. Colour grey patterned with black in various colour varieties.

Very tolerant of reduced salinity; beneath stones in strong streams flowing over sheltered shores, normally ranging from LWN to above HWN.

It ranges from the coasts of Brittany, northwards to Russia and extends well into the Baltic where it occurs abundantly under stones on Finnish shores.