In ecology and biology, commensalism is a type of symbiotic relationship between two species in which one benefits without harming the other. Usually, the host species offers shelter, support, food, or locomotion. The organism receiving the benefit is called the commensal. Commensalism ranges in duration from brief interactions to life-long symbiosis.
Belgian paleontologist and zoologist Pierre-Joseph van Beneden coined the terms “commensalism” and “mutualism” in 1876. Beneden defined commensalism to describe the activity of animals that follow predators to scavenge leftovers from carcasses. The word commensalism comes from the Latin word commensalis, which means “sharing a table.”
- A golden jackal that has been expelled from its pack trails a tiger to eat the remnants of its kill.
- Foxes trail caribou and prey upon small mammals that are either attracted to caribou foraging or uncovered by it.
- The emperor shrimp attaches to sea cucumbers to hitch a ride to new feeding grounds without expending energy. The host also provides protection to the shrimp. Meanwhile, the shrimp does not impact the movement or activities of the sea cucumber.
- Nurse plants are larger plants that offer protection to seedlings from herbivores and weather, giving them a better opportunity to grow.
- Cattle egrets feed on insects that are disturbed by grazing livestock. The birds gain food, while the cattle obtain no benefit. (Note: some species of birds are mutualistic rather than commensal, if they remove external parasites from their host.)
- Barnacle larvae attach to shells, whales, and other surfaces. The barnacles feed on plankton and waste from the host. They don’t feed directly on the flesh or blood of the host, so they generally don’t cause harm.
- Tree frogs use plants as protection, while the tree gains no benefit from the frogs.
- Birds follow army ants and feed on insects fleeing from the ants. The birds avoid the ants because they bite, while the ants usually can’t catch the birds. So, the birds benefit, while the ants are unaffected.
- Goby fish live on other fish and change color to blend in with them. The goby gains protection from predators, while the host gets no benefit.
- The burdock plant releases spiny seeds that cling to animal fur and human clothing. The plant (the commensal) gains a method of seed dispersal, while the host animal is largely unaffected.
- Pseudoscorpions gain transportation and protection by hiding on animal fur and insect wings. The pseudoscorpions do not benefit the host, but also do not harm it.
- Remora fish attach to sharks, whales, and manta rays. The remora detaches from the larger animal when it feeds and then eats the excess food. However, if the remora removes parasites from the larger animal, then the relationship is an example of mutualism because both species benefit.
Types of Commensalism
Commensalism is classified according to the benefit to the commensal.
- Inquilinism: Inquilinism is a form of commensalism where one organism uses another for permanent shelter. For example, some epiphytic plants grow on trees to gain access to sunlight and absorb nutrients caught by the bark. Ideally, the tree is unharmed. Sometimes this relationship becomes parasitic when the commensal deprives nutrients from the host or weakens its structure. Another example of inquilinism is a bird living in a hole in a tree.
- Metabiosis: In metabiosis, the host provides a habitat for the commensal. For example, maggots are metabiotic commensals on a decaying animal. Another example is a hermit crab, which makes its home in a shell from a dead gastropod.
- Microbiota: Microbiota are commensal communities on or within a host organism. Examples include gut bacteria and flora found on human skin. In many cases, microbiota are mutualistic more than commensal. For example, gut bacteria get food and shelter from their host, while releasing vitamins essential for nutrition or breaking down molecules the host cannot digest.
- Phoresy: Phoresy is a commensal relationship where one species uses another for transportation. Examples include millipedes traveling on birds, pseudoscorpions living on mammals, mites on insects, and anemones on crabs. Phoresy may be obligate (necessary for survival) or facultative (helpful, but not necessary). Phoretic organisms often obtain food from the host without harming it, often by eating waste produced by the host or leftover from its food.
Commensalism gets confused with related terms:
- Mutualism: Mutualism is a symbiotic relationship in which two species benefit each other.
- Amensalism: Amenalism is a relationship where one species is harmed and the other is unaffected.
- Parasitism: Parasitism is a relationship between two species in which one benefits and the other is harmed.
Domesticated Animals and Commensalism
Commensalism often progresses into mutualism. Initially, the relationship between humans and domesticated animals started out as commensalism. For example, DNA evidence indicates dogs followed human hunters before the advent of agriculture in order to scavenge carcasses. Over time, the relationship between humans and dogs became mutualistic. As this change occurred, the characteristics of dogs diverged from those of wild dogs and wolves.
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- Williams, E. H.; et al. (2003). “Echeneid-sirenian associations, with information on sharksucker diet”. Journal of Fish Biology. 63 (5): 1176–1183. doi:10.1046/j.1095-8649.2003.00236.x
- Wilson, E.O. (1975). “Social Symbiosis”. Sociobiology: The New Synthesis. Harvard University Press. ISBN 978-0-674-00089-6.