oyster crab

American Heritage® Dictionary of the English Language, Fourth Edition

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

Century Dictionary and Cyclopedia

1. n. 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.

GNU Webster's 1913

1. n. (Zoöl.) a small crab (Pinnotheres ostreum) which lives as a commensal in the gill cavity of the oyster.

WordNet 3.0

1. n. tiny soft-bodied crab living within the mantle cavity of oysters