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(Pallas, 1772)

Body oblong oval. Antenna 1 extending to or just beyond article 3 of the peduncle of antenna 2; antenna 2 flagellum longer than the peduncle and about a quarter of the body length. Maxillipedal palp of four articles. Coxal plates large in adults, extending from the anterior to the posterior borders of the pereion segments 2 or 3 to 7. Pereiopods all more or less alike. Pleon of two complete segments, and one partial suture in the pleotelson. Pleotelson dorsally keeled, with more or less straight sides, tapering to a tridentate posterior border with the median process long and acute (juveniles lack the tridentate posterior border). Uropods uniramous.
Colour sometimes uniformly green or brown but often with white spots or longitudinal lines; female often darker than male.

Males first recognisable at about 10 mm body length, ranging to 30 mm; females 10-18 mm in length.

Depth range/habitat:
Generally offshore, but not infrequently found among attached algae on the shore and often cast up in large numbers among drift weed.

Distribution in the North Sea
All North Sea; also in the Baltic Sea, the Gulf of Bothnia and the Gulf of Finland.

World distribution
Fairly cosmopolitan, and widespread in Europe, from N Norway to the Black

Idotea balthica