Commensalism is a type of symbiosis in which one organism benefits from an association while the other organism is neither helped nor harmed.
Explanation
Commensalism describes a relationship between two species where one partner gains an advantage and the other experiences no significant impact. The term originates from the Latin “commensalis,” meaning “sharing a table,” and it captures situations in which organisms share a habitat or resources without direct competition or detriment. This form of interaction is distinct from mutualism, where both partners benefit, and parasitism, where one benefits at the expense of the other. In commensal relationships, the benefitting organism may gain shelter, transportation, or access to food, while the host remains unaffected in terms of fitness or survival.
Many commensal interactions involve microorganisms and their hosts. Skin commensals such as Staphylococcus epidermidis reside on human epidermis and feed on secreted oils and dead cells without harming the host. In the gut, certain bacteria utilize nutrients that are indigestible to the host, and while some contribute to health, others appear neutral. Larger organisms also engage in commensalism; epiphytic plants grow on the surfaces of trees to access sunlight without drawing nutrients from their host, and remora fish attach to sharks or turtles using suction discs to travel and feed on leftover scraps. Although the host generally experiences no measurable effect, subtle influences may occur, and some commensal relationships may shift toward mutualism or parasitism under changing conditions. Recognizing these interactions is important for understanding ecological networks and the evolution of symbiosis.
Examples in Nature
One classic example of commensalism is the relationship between barnacles and whales. Barnacles attach to the skin of whales and filter feed on plankton as the whale moves through the water; the whale neither gains nor loses a significant benefit. Another example is epiphytes such as orchids or mosses growing on tropical tree trunks. By perching on branches, they gain access to light and moisture without extracting nutrients from the tree.
In terrestrial ecosystems, cattle egrets follow grazing mammals like cattle or buffalo. The birds feed on insects disturbed by the large herbivores’ movement but do not affect the mammals. On human skin, Corynebacterium species consume fatty acids from sweat and sebum without influencing the host’s health. In marine environments, remoras hitch rides on sharks, feeding on leftover food and gaining protection; the shark remains unaffected. These cases illustrate the diversity of commensal relationships across habitats and taxa.
Commensalism highlights how organisms can coexist with minimal interaction beyond a one-sided benefit. Such relationships add complexity to ecosystems without imposing costs on the host. Studying commensalism provides insights into ecological balance and the subtle transitions between neutral, beneficial and harmful associations.
Related Terms: Mutualism, Parasitism, Symbiosis, Microbiome, Neutralism