commensalism

Commensalism is a type of symbiotic interaction in which one species benefits while the other is unaffected. It is one of the classical categories of symbiosis, sitting between mutualism (both benefit) and parasitism (one benefits at the other's expense). In a commensal relationship, the host experiences no obvious harm or benefit, though some authorities note that effects can be subtle or context-dependent.

Common examples include remora fish attaching to sharks to gain transport and access to scraps, while the shark is typically unaffected beyond potential drag; barnacles or other epibionts that anchor to whales, ships, or large plants, using the host as a substrate; and epiphytic plants, such as some orchids or ferns, that grow on trees to obtain light and moisture without taking nutrients from the tree. Also, phoresy—small organisms hitching rides on larger ones to reach new habitats—illustrates commensalism when the carrier is not harmed.

Ecologists emphasize that many supposed commensal relationships are difficult to prove, and the ecological effect on the host may vary with environmental conditions or over time. Some associations classed as commensal may have subtle costs or benefits that are not immediately evident, shifting along a continuum toward mutualism or parasitism.

Commensalism can influence species distributions and community structure by facilitating dispersal, shelter, or feeding opportunities for the benefiting organism without imposing a clear burden on the host.