The males are larger than the females. The adult has a long, ovoid body with variable colouration, either yellow, brown or green, and typically with white spots or a longitudinal line down the centre of the dorsal surface. The females are often darker than the males. The telson (last segment, or appendage of the last segment of the abdomen), is broad and relatively large with three points at the tip, the centre one being the longest. It has straight sides and is dorsally keeled. Length up to 30mm.
It feeds on algae.
Found mainly sub-littoral, sometimes in rockpools and amongst drifting seaweed. Widespread and locally common.
Photographs taken August 2015, in rock pool Meadfoot Beach, Torquay, Devon.