Greek συμ-βίωσις - "life together", from συμ- - together + βίος - life.

Symbiosis is a mutually beneficial, but not obligatory coexistence of organisms, from which all participants benefit.

For example, stickleback fish attach themselves to the skin of sharks and other large fish with a suction cup fin, moving around with their help.

Unicellular green, yellow-green and brown algae often act as symbionts of animals. In this case, the algae provide the animal with photosynthetic products, in return for which the animal receives food, shelter and a number of other essential substances. Green algae are symbionts of freshwater protozoa, hydra and some freshwater sponges. Brown algae are often found as symbionts of marine protozoa (some species of foraminifera and radiolarians). Similar algae live in symbiosis with corals, actinids and some species of flatworms.

A well-known example of symbiosis is the coexistence of a hermit crab and an actinia. The hermit lives on the shell of the actinia, which, with its tentacles equipped with shoot cells, provides it with extra protection, and the hermit in turn pulls the actinia from place to place, thus expanding its hunting territory; in addition, the actinia can eat the remains of the hermit's meals.