The female is a soft bodied crab lives commensally within the mantle of mussels and sometimes other bivalves. The male crab is rarer, and has a harder shell. It lives free amongst the seaweed.

Found at low tide within the shell of the mussel and sometimes other shellfish such as cockles (females) in seaweed (males).

They scavenge on the food filtered by the gills of the shellfish.

Collins: Gunson