Symbiosis

A close, long-term biological relationship between two different species, which biologists classify by whether each partner benefits, is unaffected or is harmed: mutualism benefits both, commensalism benefits one while leaving the other unaffected, and parasitism benefits one at the other's expense. The reef-building corals of the Great Barrier Reef depend on an obligate mutualism with microscopic algae called zooxanthellae that live inside coral tissue, supplying the coral with the products of photosynthesis in exchange for shelter and access to sunlight; the algae also give the coral its color, so when heat stress causes a coral to expel its algae, a process called bleaching, the coral turns white and, if the algae do not return in time, starves.