symbiosis

symbiosis (sĭmbēō`sĭs), the habitual living together of organisms of different species. The term is usually restricted to a dependent relationship that is beneficial to both participants (also called mutualism) but may be extended to include parasitism, in which the parasite depends upon and is injurious to its host; commensalism, an independent and mutually beneficial relationship; and helotism, a master-slave relationship found among social animals (e.g., the ant and the aphid). True symbiosis is illustrated by the relationship of herbivorous animals (e.g., cockroaches, termites, cows, and rabbits) to the cellulose-digesting protozoans or bacteria that live in their intestines; neither organism could survive without the other. Other symbiotic relationships include the interdependence of the alga and the fungus that form a lichen and the relationship between leguminous plants and the nitrogen-fixing bacteria, which is important in agriculture (see nitrogen cycle). Two obvious examples of a plant-to-animal relationship are yucca and yucca moth, fig and fig wasp; in both cases the insect fertilizes the plant, and the plant supplies food for the larvae of the insect.

---

symbiosis

Any of several living arrangements between members of two different species, including commensalism, mutualism, and parasitism. The species involved are called symbionts. In commensalism, one species (the commensal) obtains nutrients, shelter, support, or locomotion from the host species, which is substantially unaffected (e.g., remoras obtain locomotion and food from sharks). In mutualism, both species benefit. Many mutualistic relationships are obligative; neither species can live without the other (e.g., protozoans in the gut of termites digest the wood ingested by the termites).

---

symbiosis

a close and usually obligatory association of two organisms of different species that live together, often to their mutual benefit