- Anton de Bary's original definition says: a long-term relationship between two different species. To this can be added, mutualism, (benefit for both), commensalism (benefit for one neutral for the other) and parasitism (benefit for one, costly for the other).
- The relationship between two different species of organisms that are interdependent; each gains benefits from the other. A relationship between different species where both of the organisms in question benefit from the presence of the other. An example is the hermit crab whose shell offers a niche for anemones to exist in which the anemone can defend the crab with its stinging capabilies.
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