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Any of several living arrangements between members of two different species, including commensalism, mutualism, and parasitism. The species involved are called symbionts. In commensalism, one species (the commensal) obtains nutrients, shelter, support, or locomotion from the host species, which is substantially unaffected (e.g., remoras obtain locomotion and food from sharks). In mutualism, both species benefit. Many mutualistic relationships are obligative; neither species can live without the other (e.g., protozoans in the gut of termites digest the wood ingested by the termites).
This entry comes from Encyclopædia Britannica Concise. For the full entry on symbiosis, visit Britannica.com.