Pea crab

The pea crab, Pinnotheres pisum, is a small crab that lives symbiotically in oyster and mussel shells. They are about the size of a pea but can be up to the size of a small walnut. It is also commonly called the oyster crab. They have a large tail flap that is used to cover eggs under the body when they are breeding, adding to the spherical appearance.

It is found from southern Scandinavia to western Africa, and throughout the Mediterranean in the mantle cavity of such bivalve molluscs as mussels, and inside ascidians.

Although researchers once thought that pea crabs were not harmful to their hosts, they have since found that their activities do cause some damage to the delicate gills of bivalves; and as such, these crabs are now considered parasites.