In ecology, commensalism is a class of relationships between two organisms where one organism benefits from the other without affecting it. This is in contrast with mutualism, in which both organisms benefit from each other, amensalism, where one is harmed while the other is unaffected, and parasitism, where one benefits while the other is harmed.
The word "commensalism" is derived from the word "commensal", meaning "eating at the same table" in human social interaction, which in turn comes through French from the Medieval Latin commensalis, meaning "sharing a table", from the prefix com-, meaning "together", and mensa, meaning "table" or "meal". Originally, the term was used to describe the use of waste food by second animals, like the carcass eaters that follow hunting animals, but wait until they have finished their meal.
Commensalism, in biology, is a relation between individuals of two species in which one species obtains food or other benefits from the other without either harming or benefiting the latter. The commensal (the species that benefits from the association) may obtain nutrients, shelter, support, or locomotion from the host species, which is substantially unaffected. The commensal relation is often between a larger host and a smaller commensal; the host organism is unmodified, whereas the commensal species may show great structural adaptation consonant with its habits, as in the remoras that ride attached to sharks and other fishes. Both remoras and pilot fishes feed on the leftovers of their hosts’ meals. A commensal relation based on shelter is seen in clown fishes (Amphiprion percula), which live unharmed among the stinging tentacles of sea anemones, where they are protected from predators. Numerous birds feed on the insects turned up by grazing mammals, while other birds obtain soil organisms stirred up by the plow. Various biting lice, fleas, and louse flies are commensals in that they feed harmlessly on the feathers of birds and on sloughed-off flakes of skin from mammals
Examples of commensal relationships
Commensalism is harder to demonstrate or explain than parasitism and mutualism, as it is easier to show a single instance in which the host is affected than it is to disprove that possibility. One example is a whale and barnacles. Another is the titan triggerfish (Balistoides viridescens) which creates feeding opportunities for smaller fish by moving large rocks too big for them to shift themselves. Yet another example is the remora, which eats leftover food from a whale and "hitches a ride". Another example of a commensal relationship is the relationship between Cacique birds and the wasp Polybia rejecta.However, the best example for this is of a vulture and a lion. Once the lion has finished its meal, the vulture swoops down and finishes off the carcass. The lion is not affected by this while the vulture gets to eat.
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 beetles, and millipedes on birds. Phoresy can be either obligate or facultative (induced by environmental conditions).
Metabiosis is a more indirect dependency, in which one organism creates or prepares a suitable environment for a second. Examples include maggots, which feast and develop on corpses, and hermit crabs, which use gastropod shells to protect their bodies.
- Symbiosis – long-term interactions between different biological species, which can be mutualistic, commensal or parasitic
- Mutualism – where both organisms experience mutual benefit in the relationship
- Parasitism – where one organism benefits at the expense of another organism.
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