thumb|upright=1.5|In a [[cleaning symbiosis, the clownfish feeds on small invertebrates, that otherwise have potential to harm the sea anemone, and the fecal matter from the clownfish provides nutrients to the sea anemone. The clownfish is protected from predators by the anemone's stinging cells, to which the clownfish is immune. The relationship is therefore classified as mutualistic.]]
Symbiosis (: symbioses) is any close and long-term biological interaction between two organisms of different species.
Symbiosis is diverse and can be classified in multiple ways. It can be obligate, meaning that one or both of the organisms depend on each other for survival, or facultative, meaning that they can subsist independently. When one organism lives on the surface of another, such as head lice on humans, it is called ectosymbiosis; when one partner lives inside the tissues of another, such as Symbiodinium within coral, it is termed endosymbiosis. Where the interaction reduces both parties' fitness, it is called competition; where just one party's fitness is reduced, it is called amensalism. Where one benefits but the other is largely unaffected, this is termed commensalism. Where one benefits at the other's expense, it is called parasitism. Finally, where both parties benefit, the relationship is described as
mutualistic.
Symbiosis has often driven the evolution of species; mutualism has enabled species for example to colonise new environments. Symbiogenesis is thought to have helped to create the eukaryotes as bacteria were incorporated as mitochondria and chloroplasts within cells. Major co-evolutionary relationships include mycorrhiza, the pollination of flowers by insects, the protection of acacia trees by ants, seed dispersal by animals, nitrogen fixation by bacteria in the root nodules of legumes, and the mutualistic partnership of algae and fungi to form lichens.
Definition
thumb|upright=1.7|Diagram of the six possible types of symbiotic relationship, from mutual benefit to mutual harm.
The term "symbiosis" is derived from Ancient Greek : living with, companionship < : together; and bíōsis: living.
The definition of symbiosis was a matter of debate for 130 years. In 1877, Albert Bernhard Frank used the term symbiosis to describe the mutualistic relationship found in lichens.