Sphaeroma serratum

(Fabricius, 1787)

Body oval in outline, readily rolling into a ball. Antenna 1 with 9 to 15 aesthetascs. Maxillipede palp without lobes, but with plumose setae on articles 2 to 4. 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. Pereiopod 1 ischium with up to about 60 plumose setae in two rows; propodus with a distal row of 15 to 20 setae. Pleotelson with a smooth dorsal surface and not extending beyond uropods. Uropods lateral, with the expod and endopod subequal in length, endopod rounded at the apex. Uropods with 4 to 7 well defined serrations on the outer edge of the exopod. No sexual dimorphism.
Sexes separable between 5 and 7 mm body length, females ranging to 10 mm and males to 11.5 mm.

In rock crevices or under stones at about mean tide level, usually on marine shores or at the mouth of estuaries, but occasionally near freshwater seepages on such shores.

In the British Isles it occurs on the south and west coasts of England and Wales, as far north as Anglesey, and on south and west coasts in Ireland and it ranges southwards to Morocco and the Azores, including the Mediterranean and Black Sea.