Commensalism (from Latin com meaning together and mensa meaning table) is a form of symbiosis, or close association between organisms of two or more species, in which one participant in the relationship benefits, and the relationship is neutral for the other participant. This type of symbiotic relationship is often symbolized as (+, 0). Commensalism is one of three recognized categories of symbiotic relationships. The other two are mutualism, in which both participants benefit, and parasitism, in which one benefits and the other is harmed. Of the three types of symbiotic relationships, commensalism seems to be the least common. In addition, some scientists think it is unlikely that one species is completely unaffected by the relationship. Thus, examples of what at first appear to be commensalism may not prove to truly be commensalism upon more in-depth study. One example of commensalism occurs in the ocean. Certain types of barnacles attach to whales and obtain a free ride as the whale swims. Since barnacles rely on currents to bring them food that they can filter out of the water, movement by the whale greatly benefits the barnacles. Because of the whale's movements, the barnacles are always in a new environment with a new supply of food. The whale, however, does not appear to be helped or harmed by this relationship, and thus it is considered commensalism. There are other examples of commensalism throughout the world.
Orchids, which are plants found in tropical areas, have such a commensal relationship with the trees on which they live. The orchids benefit from the environment provided by these trees, while the trees do not appear to be helped or harmed. Cattle egrets are birds that live near grazing cattle. They seem to benefit from living with the cattle, because the cattle disrupt insects in the grasses, providing the birds with a readily available food source. There seems to be no cost or benefit to the cattle from this relationship. The pilotfish is a marine species that usually swims just in front of sharks. When the shark feeds, the pilotfish benefits by picking up scraps. There does not appear to be any effect, either positive or negative, to the shark. Some scientists even believe that commensal relationships exist between humans and a variety of species of bacteria living within them. For example, bacteria within human digestive systems benefit from the food that is eaten. However, in most cases the human neither benefits nor is harmed from this relationship. Occasionally, however, when these bacteria get into the wrong part of the human body, as is the case when E. coli is ingested, they can do serious harm. In such cases, because one participant is being harmed, the relationship would no longer be considered commensalism. It would be parasitism.
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