A symbiotic relationship between individuals of different species in which both individuals benefit from the association.
In this type of symbiosis, both organisms of different species rely on one another for nutrients, protection and other life functions, hence, they are usually found living in close proximity.
It can be thought of as a form of "biological barter" since the species trade resources (for example carbohydrates or inorganic compounds), or services such as gamete or offspring dispersal, or protection from predators.
Also called: mutualism
Compare: amensalism, commensalism, parasitism.
See also: cooperation, symbiosis.