A symbiotic relationship may be categorized as being mutualistic, parasitic, or commensal in nature. A more narrow definition would include only relationships where both organisms benefit (mutualism).
Symbiotic relationships include those associations in which one organism lives on another (ectosymbiosis, such as mistletoe), or where one partner lives inside the other (endosymbiosis, such as lactobacilli and other bacteria in humans or zooxanthelles in corals). Symbiotic relationships are sometimes necessary for the survival of at least one of the organisms involved, and other times beneficial but not essential.
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