The New Zealand pea crab, Pinnotheres novaezelandiae, is a small crab that lives symbiotically in oyster and mussel shells around New Zealand. 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.
Of 10 New Zealand green-lipped mussels taken in Four Fathom Bay, Pelorus Sound, New Zealand, eight hosted pea crabs of various sizes. Their presence does not harm the host, which may be harvested and consumed as usual.