epibiont

thumb|upright=1.5|The Blowhole (biology)|blowholes of a [[gray whale, with barnacle epibionts]]

An epibiont (from the Ancient Greek meaning "living on top of") is an organism that lives on the surface of another living organism, called the basibiont ("living underneath"). The interaction between the two organisms is called epibiosis. An epibiont is, by definition, harmless to its host. In this sense, the interaction between the two organisms can be considered neutralistic or commensalistic; as opposed to being, for example, parasitic, in which case one organism benefits at the expense of the other, or mutualistic, in which both organisms obtain some explicit benefit from their coexistence. These organisms have evolved various adaptations to exploit their hosts for protection, transportation, or access to resources. Examples of common epibionts are bacteria, barnacles, remoras, and algae, many of which live on the surfaces of larger marine organisms such as whales, sharks, sea turtles, and mangrove trees.