Commensalism (literally, 'at the same table'), the intimate, but never parasitic association of two different kinds of organisms, for the benefit of one, or very often of both. Of such advantageous partnerships there are so many different forms and degrees, that no precise definition of the term can be given. (1) Every one who has looked at shore animals must have observed how often mollusc shells, for instance, are covered with sponges, hydroids, worm-tubes, acorn-shells, and the like. But this is a purely external association, and depends simply on the fact that the shells afford convenient anchorage for the free-swimming embryos. In many cases no great advantage can accrue on either side. The habit is comparable to that of vegetable epiphytes upon trees. This grade might be spoken of as mainly external and unadvantageous association. (2) In other cases, however, the association, though probably accidental, brings its reward. When one sea-mat (Polyzoa) grows entangled with another of greater vigour, or when different kinds of polyps are similarly associated, and that is often, there may be distinct advantage to the weaker form, since without becoming a parasite it enjoys the privileges of a messmate. Or the advantage may take another form, when the associate is carried about by its bearer. Thus, Cirripedes are common upon whales, and have evidently an advantage both in security and continual freshness of feeding-ground over those which adhere to fixed objects. This grade might be distinguished as that of fixed external associates with the advantage on one side. (3) It is, however, evident that if a crab be covered with acorn-shells, or polyps, or sponges, there is no longer a one-sided, but a mutual advantage. It is well for the sedentary growth to be carried about continually to new pastures; but it is well also for the crab to be masked. Covered with a rich growth, either vegetable or animal, the crabs must appear all innocence, and like walking-woods of Birnam, can steal unnoticed upon their victims. A gasteropod may be similarly masked by polyps, and doubtless gives and reaps similar advantages. This grade may be described as that of fixed associates with mutual advantage, to this extent, at least, that the weaker animals are borne about, and the bearers are masked. (4) But a higher stage of fixed association is sometimes exhibited, that, namely, where the partnership is deliberate, where the masked bearer is not passively benefited, but is an active accomplice. Probably the most striking case of such deliberate partnership is that referred to (and illustrated) under the article ANEMONE—viz. the habit which some hermit crabs have of bearing about sea-anemones on the mollusc shell which they inhabit, or even upon their claws. It would appear that in some cases the crustacean deliberately chooses its ally, induces it to fix itself on the shell or claw, and takes care not to leave it behind at the epochs of shell-changing. When deprived of its commensal, the crab is said to be restlessly ill at ease until another of the same species is forthcoming. Off some parts of the British coast, the beautiful sea-anemone (Adamsia palliata) is found enveloping the mollusc home of a hermit crab (Eupagurus prideauxii). The use of the sea-anemone as a mask, and also as equivalent to a stinging organ, is obvious enough, while the hermit crab returns the benefit by carrying about the sea-anemone and giving it a share of the spoil. This grade may therefore be described as deliberate partnership with mutual advantage.
So far only fixed commensals have been spoken of, but organisms may be constantly associated without being attached. Sometimes different organisms, both plant and animal, are found in almost constant association without any obvious connection obtaining between them. In many cases this companionship may be simply due to the fact that similar environmental conditions suit both. Small fishes are sometimes found as free commensals within sea-anemones; the Remora (q.v.) attaches itself temporarily to sharks and other fishes; the little crabs (Pinnotheres, &c.) found living freely inside various bivalves are probably true messmates, and similar habitual partnership is very common among crustaceans; a brittle-star is known to live as a free messmate on a crinoid; many worm-types are found in constant though free associations with other animals; and the same habit is exhibited by some Cœlenterates and Protozoa. Many of the insects which frequent plants are in strict sense commensals, feeding not on their hosts, but on other visitors, &c. In some cases they form an actual bodyguard.
Commensalism must, of course, be distinguished from Parasitism (q.v.), whether external or internal, for in parasitism the one organism more or less directly preys upon the other. Yet it is evident that a commensal may readily degenerate into a parasite. Commensalism must also be distinguished from that most intimate kind of partnership known as Symbiosis (q.v.), and illustrated by the union of algoid and fungoid organisms to form a lichen, or by the occurrence of algæ as constant internal associates of Radiolarians, some Celerenterates, and some worm-types.
As part of the animate environment, commensals have influenced one another in very direct ways. See ENVIRONMENT; P. J. van Beneden's Animal Parasites and Messmates (1876); and Semper's Animal Life (1881).