(symbiotic) association of two different types of living organisms that is often. but not always. beneficial to each.

Merriam-Webster Online Dictionary
symbiosis (noun)
the living together in more or less intimate association or close union of two dissimilar organisms (as in parasitism or commensalism) , especially - mutualism
a cooperative relationship (as between two persons or groups) - the symbiosis…between the resident population and the immigrants John Geipel
symbiosis (Wikipedia)
In a cleaning symbiosis, the clownfish feeds on small invertebrates that otherwise have potential to harm the sea anemone, and the fecal matter from the clownfish provides nutrients to the sea anemone. The clownfish is protected from predators by the anemone's stinging cells, to which the clownfish is immune. The clownfish emits a high pitched sound that deters butterfly fish, which would otherwise eat the anemone, making the relationship mutualistic.

Symbiosis (from Greek συμβίωσις, symbíōsis, "living together", from σύν, sýn, "together", and βίωσις, bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasitic. The organisms, each termed a symbiont, must be of different species. In 1879, Heinrich Anton de Bary defined it as "the living together of unlike organisms". The term was subject to a century-long debate about whether it should specifically denote mutualism, as in lichens. Biologists have now abandoned that restriction.[citation needed]

Symbiosis can be obligatory, which means that one or more of the symbionts depend on each other for survival, or facultative (optional), when they can generally live independently.

Symbiosis is also classified by physical attachment. When symbionts form a single body it is called conjunctive symbiosis, while all other arrangements are called disjunctive symbiosis. When one organism lives on the surface of another, such as head lice on humans, it is called ectosymbiosis; when one partner lives inside the tissues of another, such as Symbiodinium within coral, it is termed endosymbiosis.

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