The female is a very round crab and her carapace is colourless and soft as she relies upon the bivalve for protection. The male is flatter and his carapace is harder and is a brownish colour. He is smaller than the female.

The female scavenges the food of the bivalve and can cause quite a bit of damage as she does so. The male scavenges in the seaweed around the bivalve beds.

The female crab is a parasite inside bivalves, especially mussels where she steals the mussels filtered lunch and damages the gill surface of the mussel. The male lives in seaweed around mussel beds and is likely attracted to the female by a special scent she releases called a pheromone. When he smells this he will stroke the bivalves mantle edge to get it to open up further so he can get in.

Photo Credit: Rod Morris. Photos can be purchased from https://www.rodmorris.co.nz/