Commensal

Commensalism is a type of symbiotic relationship in which one organism benefits from the association while the other organism experiences no obvious benefit or harm. The benefiting partner may gain nutrients, transport, habitat, or access to resources, whereas the host is not meaningfully affected in the short term. Some relationships thought to be commensal can have subtle or context-dependent effects that are difficult to measure, so true commensalism is sometimes hard to prove.

The concept is distinguished from mutualism, in which both partners benefit, and parasitism, in which one benefits at the expense of the other. In practice, many interactions lie on a continuum between these categories; what is commensal in one environment may become mutualistic or parasitic under different conditions. The term derives from the Latin commensalis, meaning “sharing a table,” to reflect the idea of one organism sharing another’s space or resources without harm.

Examples commonly cited in biology include barnacles that attach to whales, remora fish that fasten to larger marine hosts and feed on scraps without noticeably harming the host, and cattle egrets that forage near grazing animals to catch insects disturbed by them. In terrestrial environments, many small arthropods and some epiphytic organisms live on larger hosts without overt effects.

In humans and other animals, the microbiota of the gut and skin is largely regarded as commensal, occupying niches and deriving benefit from host resources. These microbes can contribute to digestion and immune development, but imbalances can lead to disease or opportunistic infection, illustrating that commensal relationships can be context-dependent.