Commensalism Definition, Examples, and Relationships

Benefit Without Harm: Commensalism Explained

Remora fish and shark
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Commensalism Definition

Commensalism is a type of relationship between two living organisms in which one organism benefits from the other without harming it. A commensal species benefits from another species by obtaining locomotion, shelter, food, or support from the host species, which (for the most part) neither benefits nor is harmed. Commensalism ranges from brief interactions between species to life-long symbiosis.

The term was coined in 1876 by Belgian paleontologist and zoologist Pierre-Joseph van Beneden, along with the term 'mutualism.' Beneden initially applied the word to describe the activity of carcass-eating animals that followed predators to eat their waste food. The word commensalism comes from the Latin word commensalis, which means 'sharing a table'. Commensalism is most often discussed in the fields of ecology and biology, although the term extends to other sciences.

Terms Related to Commensalism

Commensalism is often confused with related words:

Mutualism - Mutualism is a relationship in which two organisms benefit from each other.

Amensalism - A relationship in which one organism is harmed while the other is not affected.

Parasitism - A relationship in which one organism benefits and the other is harmed.

There's often debate about whether a particular relationship is an example of commensalism or another type of interaction. For example, some scientists consider the relationship between people and gut bacteria to be an example of commensalism, while others believe it's mutualistic because humans may gain a benefit from the relationship.

Examples of Commensalism

  • Nurse plants are larger plants that offer protection from seedlings from the weather and herbivores, giving them an opportunity to grow.
  • Tree frogs use plants as protection.
  • Golden jackals, once they have been expelled from a pack, will trail a tiger to feed on the remains of its kills.
  • Goby fish live on other sea animals, changing color to blend in with the host, thus gaining protection from predators.
  • Cattle egrets eat the insects stirred up by cattle when they are grazing. The cattle are unaffected, while the birds gain food.
  • The burdock plant produces spiny seeds that cling to the fur of animals or clothing of humans. The plants rely on this method of seed dispersal for reproduction, while the animals are unaffected.

Types of Commensalism (With Examples)

Inquilinism - In inquilinism, one organism uses another for permanent housing. An example is a bird that lives in a tree hole. Sometimes epiphytic plants growing on trees are considered iniquilism, while others might consider this to be a parasitic relationship because the epiphyte might weaken the tree or take nutrients that would otherwise go to the host.

Metabiosis - Metabiosis is a commensalistic relationship in which one organism forms a habitat for another. An example is a hermit crab, which uses a shell from a dead gastropod for protection. Another example would be maggots living on a dead organism.

Phoresy - In phoresy, one animal attaches to another for transport. This type of commensalism is most often seen in arthropods, such as mites living on insects. Other examples include anemone attachment to hermit crab shells, pseudoscorpions living on mammals, and millipedes traveling on birds. Phoresy may be either obligate or facultative.

Microbiota - Microbiota are commensal organisms that form communities within a host organism. An example is the bacterial flora found on human skin. Scientists disagree on whether microbiota is truly a type of commensalism. In the case of skin flora, for example, there is evidence the bacteria confer some protection on the host (which would be mutualism).

Domesticated Animals and Commensalism

Domestic dogs, cats, and other animals appear to have started out with commensal relationships with humans. In the case of the dog, DNA evidence indicates dogs associated themselves with people before humans switched from hunting-gathering to agriculture. It's believed the ancestors of dogs followed hunters to eat remains of carcasses. Over time, the relationship became mutualistic, where humans also benefited from the relationship, gaining defense from other predators and assistance tracking and killing prey. As the relationship changed, so did the characteristics of dogs.

Reference: Larson G (2012). "Rethinking dog domestication by integrating genetics, archeology, and biogeography". Proceedings of the National Academy of Sciences of the United States of America. 109: 8878–83.