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Rathke, 1843

Description
Body oval, narrowing rather sharply posteriorly. Antenna 1 extending beyond article 2, but not article 3 of the peduncle of antenna 2; the most distal aesthetascs as a pair. Antenna 2 flagellum not reaching the length of the peduncle (except possibly in large males which have tufts of setae on the flagellum); usually less than 1/5 of the body length. Cephalon subquadrate, eyes dorsolateral, prominent. Maxillipedal palp of four articles. Coxal plates fairly narrow reaching the posterior border only in segments 4 to 7. Pereiopods all more or less alike. Pleon of two complete segments, and one partial suture in the pleotelson. Pleotelson narrowing sharply at first, with rather concave lateral sides; posterior border with a long, acute, median spine and very obtusely rounded lateral shoulders; not keeled. Uropods uniramous.
Colour mostly uniformly brown, red or green, depending on the nature of the weed inhabited; occasionally with longitudinal white markings.

Size
Males recognisable around 5 mm, ranging to about 20 mm in body length; females 6-13 mm.

Depth range/habitat
Between tidemarks, may occur free swimming.

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

World distribution
A northern species ranging from the White Sea to S Britain and Normandy.

Idotea granulosa