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Beneficial symbiosis between species

Commensalism is a long term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. while pilot fish feed on the leftovers of their hosts' meals. Numerous birds perch on bodies of large mammal herbivores or feed on the insects turned up by grazing mammals. Commensality, at the Universities of Oxford and Cambridge, refers to professors eating at the same table as students (as they live in the same "college"). Pierre Joseph van Beneden introduced the term "commensalism" in 1876.

The commensal pathway was traveled by animals that fed on refuse around human habitats or by animals that preyed on other animals drawn to human camps. Those animals established a commensal relationship with humans in which the animals benefited but the humans received little benefit or harm. Those animals that were most capable of taking advantage of the resources associated with human camps would have been the 'tamer' individuals: less aggressive, with shorter fight or flight distances. Later, these animals developed closer social or economic bonds with humans and led to a domestic relationship. In this association, the ophiurans benefit directly by being elevated through facilitating their feeding by suspension, while the octocorals do not seem to benefit or be harmed by this relationship. Recent studies in the Gulf of Mexico have suggested that there are actually some benefits to the octocorals, such as receiving a cleaning action by the brittle star as it slowly moves around the coral. In some cases, a close relationship occurs between cohabiting species, with the interaction beginning from their juvenile stages.

Whether the relationship between humans and some types of gut flora is commensal or mutualistic is still unanswered. Some biologists argue that any close interaction between two organisms is unlikely to be completely neutral for either party, and that relationships identified as commensal are likely mutualistic or parasitic in a subtle way that has not been detected. For example, epiphytes are "nutritional pirates" that may intercept substantial amounts of nutrients that would otherwise go to the host plant. Large numbers of epiphytes can also cause tree limbs to break or shade the host plant and reduce its rate of photosynthesis. Similarly, phoretic mites may hinder their host by making flight more difficult, which may affect its aerial hunting ability or cause it to expend extra energy while carrying these passengers.

Like all ecological interactions, commensalisms vary in strength and duration from intimate, long lived symbioses to brief, weak interactions through intermediaries.

Phoresy is one animal attached to another exclusively for transport, mainly arthropods, examples of which are mites on insects (such as beetles, flies or bees), pseudoscorpions on mammals or birds that live in holes in trees.

Metabiosis is a more indirect dependency, in which one organism creates or prepares a suitable environment for a second. Examples include maggots, which develop on and infest corpses, and hermit crabs, which use gastropod shells to protect their bodies.

Facilitation or probiosis describes species interactions that benefit at least one of the participants and cause harm to neither.

Necromeny is one animal associating with another until the latter dies, then the former feeds on the corpse of the latter. Examples include some nematodes and some mites.