oyster crab

any of various small, thin-shelled crabs (family Pinnotheridae) that live as commensals in the gill cavities of oysters, clams, etc.

See oyster crab in American Heritage Dictionary 4

noun

A small crab (Pinnotheres ostreum) that lives commensally inside the shell of a bivalve mollusk such as an oyster or a clam.

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