oyster crab

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A small crab (Zaops ostreum syn. Pinnotheres ostreum) of the western Atlantic Ocean that lives inside the shell of a bivalve mollusk, especially an oyster.

from The Century Dictionary.

• noun One of the little crabs which live with oysters in the shells of the latter; a pea-crab. The kind which lives in the common oyster is a grapsoid crustacean, Pinnotheres ostreum. See Pinnotheres.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun tiny soft-bodied crab living within the mantle cavity of oysters