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(Adams, 1800)

Description:
Body oval in outline. Five anterior pleon segments all fused; sixth pleon segment fused with the telson as a pleotelson which articulates freely with the pleon. Coxal plates of pereion segments 2 to 7 fused with the segments. Uropods lateral; endopod rigidly fused with the peduncle; exopod moveable. Markedly sexually dimorphic. Pereion segments equal in females, male with segment 6 overlapping 7 and produced into two pairs of backwardly directed processes, one small lateral pair and a much larger pair close to the mid-dorsal line; processes only moderately rugose and without a prominent apical projection. Pleotelson domed, keeled or smooth in female, rugose in males with a central tubercle, which is relatively sessile and bluntly bilobed; respiratory channel sometimes closing ventrally. Mouthparts in ovigerous females showing marked loss of chitin and setae. Pleopods 1 to 3 laminate with apical setae, pleopod 2 of male without appendix masculina; pleopods 4 and 5 with exopod and endopod thick, fleshy and with transverse folds. Uropods biramous in both sexes; males more setose than females, endopod of males usually with a small, smooth apical projection. Young carried in brood chamber formed by 4 pairs of oöstegites on pereiopods 1 to 4; does not roll into a tight ball.
Length up to 7 mm in males and 6 mm in females. Colour variable; males mostly brown, young females green, red or yellow, ovigerous females white or colourless, associated with the considerable loss of tissue which takes place at the last moult.

Habitat:
Juveniles occur among intertidal algae upon which they feed. Adults occur in rock crevices or the empty tests of Balanus perforatus (when present), each male usually with a harem of several females. Mainly below MTL.

Distribution:
In southern and western Britain (including Ireland) and southwards along the Atlantic coasts of France, Spain and Portugal.

Dynamene bidentata