The pea crab is a small crab in the family Pinnotheridae that lives symbiotically in oysters, clams, mussels and other species of bivalves. The crab is usually about the size of a pea but can grow as large as a small walnut. It is also commonly known as the oyster crab. Females have a large tail flap that is used to cover eggs attached to the bodies when they are breeding, adding to the spherical appearance.
Encyclopaedia Britannica (11th ed., vol. 7) (New York, NY: The Encyclopaedia Britannica Company, 1910)