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Symbiosis (pl. symbioses) is a interaction between two organisms living together in more or less intimate association or even the merging of two dissimilar organisms.

The term host is used for the larger of the two members of a symbiosis. The smaller member is called the symbiont.

Symbiosis may be divided into two distinct categories: ectosymbiosis and endosymbiosis. In ectosymbiosis, the symbiont lives on the body surface of the host, including the inner surface of the digestive tract or the ducts of exocrine glands. In endosymbiosis, the symbiont lives in the intracellular space of the host.

An example of mutual symbiosis is the relationship between anemonefishes[?] of the genus Amphiprion[?] (family, Pomacentridae) that dwell among the tentacles of tropical sea anemones. The territorial fish protects the anemone from anemone-eating fish, and in turn the stinging tentacles of the anemone protects the anemone fish from its predators (a special mucous on the anemone fish protects it from the stinging tentacles).

The biologist Lynn Margulis, famous for the work on endosymbiosis, contends that symbiosis is a major driving force behind evolution. She considers Darwin's notion of evolution, driven by competition is incomplete, and claims evolution is strongly based on co-operation[?], interaction, and mutual dependence[?] among organisms.
According to Margulis and Sagan (1986), Life did not take over the globe by combat, but by networking..

Symbiosis is sometimes also described as being an association in which at least one of the organism benefit from the relation. In this case, the various forms of symbiosis include parasitism, in which the association is disadvantageous or destructive to one of the organisms, mutualism, in which the association is advantageous, or often necessary to one or both and not harmful to either, and commensalism, in which one member of the association benefits while the other is not affected. However, in a true symbiosis, the association is obligatory and benefits both organisms. Symbiosis as defined in this article restrict the term symbiosis to only these mutually beneficial interactions.

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