(Zool.) An animal, not truly parasitic, which lives in, with, or on, another, partaking usually of the same food. Both species may be benefited by the association.
... they are themselves. It is these whose customs we are called upon to consider. In the interior of the branchial chamber of many bivalvular Mollusca, and especially the Mussel, there lives a little crustaceous commensal called the Pea-crab (Pinnoteres pisum). He goes, comes, hunts, and retires at the least alarm within his host's shell. The mussel, as the price of its hospitality, no doubt profits by the prizes which fall to the ... — The Industries of Animals • Frederic Houssay