A symbiote is an organism that lives in close association with another organism of a different species, known as its host. In many cases, the symbiont benefits from the relationship while the host is either unaffected or benefits less than the symbiont. Examples of well-known symbiotic relationships include those between clownfish and sea anemones, and between termites and certain gut flora.
The term “symbiosis” was first coined by German naturalist Anton Bary in 1879 to describe the mutualistic relationship between corals and algae living within their tissues. The word “symbiote” comes from the Greek symphis (συμφίς), meaning “growing together”.
In 1885, Albert Bernhard Frank used the term to describe relationships between plant root systems and mycorrhizal fungi; he also used it to refer to zooxanthellae, single-celled dinoflagellates that live within reef-building corals. The term has been more recently applied to partnerships between other organisms, such as bacteria and their hosts.
Symbiotic relationships are categorized according to whether both partners benefit (mutualism), one partner benefits while the other is unaffected (commensalism), or one partner benefits while the other is harmed (parasitism). Many symbiotic relationships are mutualistic, but not all; for example, some mycorrhizal fungi only gain nutrients from their hosts without providing any benefit in return.
Symbiosis can occur between members of different species (interspecific) or between members of the same species (intraspecific). Interspecific symbiosis is more common than intraspecific symbiosis; examples include rhizobia–legume associations and nitrogen-fixing bacteria–plant associations. Intraspecific symbiosis occurs when two individuals of the same species share resources or space; examples include social insect colonies, where sterile workers care for breeding queens and larvae, and cleaning shrimp–goby fish pairs.