Symbiosis

Symbiosis is a close and long-term biological interaction between two different biological organisms. The organisms, each termed a symbiont, may be of the same or 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.

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

Symbiosis is often classified into three types: mutualism, commensalism, and parasitism, though this classification is incomplete. For example, a more complete classification, proposed by the German mycologist Heinrich Anton de Bary, includes mutualism, commensalism, parasitism, and also competition and neutralism. These latter types are not symbiosis as defined by de Bary, and the term symbiosis has been widely used to denote mutualism, in which both organisms benefit and neither is harmed.

Types of Symbiosis

Mutualism

In mutualism, both organisms benefit. This type is often seen in the gut flora of animals and in the root nodules of legumes, where bacteria fix nitrogen from the atmosphere for the plant, which provides the bacteria with sugars.

Commensalism

In commensalism, one organism benefits while the other is neither helped nor harmed. Examples are barnacles that grow on whales and other marine animals, and orchids growing as epiphytes on trees.

Parasitism

In parasitism, one organism benefits while the other is harmed. Parasites may be ectoparasites, living on the surface of their host, or endoparasites, living inside the host's body.

See also

• Coevolution
• Mutualism (biology)
• Parasitism
• Commensalism

References

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