Commensalism

Commensalism is a specific type of biological interaction known as symbiosis. In this relationship, one species, called the commensal, receives a benefit. Meanwhile, the other species, known as the host, is neither helped nor harmed by the interaction. This differs from mutualism, where both species benefit, and parasitism, where one species benefits while the host is harmed. Commensalism can range from brief encounters to long-term, intimate associations.

The commensal organism can gain several different advantages from its host. Some species obtain nutrients, such as food scraps or waste. Others find shelter or physical support to stay in a safe place. Some even use a host for locomotion, which is the ability to move from one place to another. In many cases, the commensal is much smaller than its host. The host remains unmodified, but the commensal may show great structural adaptations to suit its lifestyle.

Scientists categorize these interactions into different types based on how they work. One type is phoresy, where an animal uses another exclusively for transport. This is common among arthropods, such as mites riding on flies or beetles.

Another type is inquilinism, where one organism lives within the home of another. In the deep sea, brittle stars exhibit commensalism by gripping onto octocorals. This lifts the stars off the sea floor and into currents, which helps them feed through suspension. While this helps the brittle star, the octocoral is generally unaffected.

The term "commensalism" was introduced in 1876 by the Belgian zoologist Pierre-Joseph van Beneden. The word comes from the Medieval Latin "commensalis," which means "sharing a table." This reflects how some animals live near human habitats to eat refuse. This process can lead to domestication, a coevolutionary process where animals adapt to a new niche. Pigeons are a classic example, as they transitioned from rock doves to domestic birds by living alongside humans.

Human history shows how commensalism can change entire species. The dog was the first animal to be domesticated, occurring in Eurasia before the end of the Pleistocene. Archaeological evidence, like the Bonn–Oberkassel dog from approximately 14,000 years ago, shows dogs existed before agriculture. It is hypothesized that less aggressive wolves followed human hunter-gatherer camps to eat carcasses. Over time, these wolves may have developed physical changes, such as neotenization, where they retain juvenile skull features into adulthood. This selection for reduced aggression helped them move from commensals to partners.

Commensalism also occurs in the microscopic world of bacteria and fungi. For example, the bacterium Staphylococcus aureus is a metabiotic commensal for many people. It lives on the skin, in the nose, or in the ear canals of roughly 20% to 30% of the human population. Most of the time, it lives peacefully as part of the natural flora. In the soil and water, microorganisms use a process called nitrification to survive. First, bacteria like Nitrosomonas oxidize ammonium into nitrite. Then, Nitrobacter uses that nitrite as a source of energy to grow. This shows how the waste of one microbe becomes the food for another.

Despite its name, some scientists argue that true commensalism is rare. They suggest that most interactions are actually mutualistic or parasitic in subtle ways. For instance, epiphytes, which are plants that grow on other plants, can sometimes act as "nutritional pirates." They might intercept nutrients or shade the host, potentially reducing the host's rate of photosynthesis. Similarly, phoretic mites might make it harder for an insect to fly. Because of these complexities, biologists continue to study whether any biological relationship is truly neutral for both sides.