A second form of symbiosis is called mutualism, in which both participating organisms gain one or more advantages. The clownfish and sea anemone are an example of mutualism. Clownfishes (of which there are about 25 species) live among tentacles of giant tropical sea anemones. Although these anemones stun and devour other species of fish (using their poison bearing tentacles), clownfish are not harmed. Clownfishes were thought to be commensal on the giant sea anemones, but there now is evidence that the aggressively territorial clownfishes chase away butterfly fishes, who eat anemone tentacles. Thus, a clownfish and its anemone offer one another protection. Indeed, the clownfish cannot live without its anemone, as it is quickly picked off by predators when left unprotected, and is never found in nature away from an anemone. However, anemones are sometimes seen growing naturally without clownfishes.

Mutualism between termites and protistans.

Although many insects feed on wood, most are unable to produce the enzyme (cellulase) necessary for digesting it. These insects thrive on food they are unable to digest because of a mutualistic relationship with microorganisms (protistans, bacteria, or fungi) that live in their digestive tracts. An excellent example of this kind of obligatory mutualism is the relationship between certain flagellated protistans and termites.

In most species of termites, vast numbers of flagellates inhabit the hindguts of workers. These symbionts can make up as much as one third the body weight of a worker termite. They consume wood particles by phagocytosis and hydrolyze them to glucose which is then fermented to produce carbon dioxide, organic acids, and other compounds. The acids are used as food by the termites, and, as a result of the action of the flagellates, two thirds of the food ingested is available to the termites.

Since the lining of the hindgut is shed each time an insect molts, the termites must reinfect themselves with flagellates after each molt. They accomplish this by exchanging anal exudates. These exudates contain flagellates plus chemicals important for the social organization of the colony. Many sociobiologists believe that the need to exchange symbionts may have been an important factor in the evolution of sociality in termites. Also, the fact that some cockroaches have flagellates closely related to those of termites supports the view that termites evolved from cockroach ancestors

Mutualism between hydra and algae.

The green color in Chlorohydra viridissima is due to unicellular green algae which live in the hydra's gastrodermis. The algal cells are called zoochlorellae. The relationship between the two kinds of organisms is thought to be mutualistic, with the algae utilizing the carbon dioxide output of metabolism in the hydra to synthesize organic compounds useful to the hydra; the oxygen requirements of the hydra may also be met to some extent by the secretion of the algal cells as a byproduct of photosynthesis. Chlorohydra viridissima kept in the dark eventually loses its algae and dies, even if food is available, indicating that the hydra is dependent on its algae. The advantage to the algal cells is believed to be simply shelter, but some suggest that Chlorohydra also meets physiologic requirements of the algae.