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.
See more at Wikipedia.org...

Noun
1. tiny soft-bodied crab living commensally in the mantles of certain bivalve mollusks
(hypernym) crab
(hyponym) oyster crab, Pinnotheres ostreum
(member-holonym) Pinnotheres, genus Pinnotheres