Phoresis

Phoresis or phoresy is a non-permanent, commensalistic interaction in which one organism (a phoront or phoretic) attaches itself to another (the host) solely for the purpose of travel. Phoresis has been observed directly in ticks and mites since the 18th century, and indirectly in fossils 320 million years old. It is not restricted to arthropods or animals; plants with seeds that disperse by attaching themselves to animals are also considered to be phoretic. Phoresis is rooted in the Greek words phoras (bearing) and phor (thief). The term, originally defined in 1896 as a relationship in which the host acts as a vehicle for its passenger, clashed with other terminology being developed at the time, so constraints on the length of time, feeding, and ontogeny are now considered. Phoresis is used as a strategy for dispersal, seasonal migration, transport to new host/habitat, escaping ephemeral habitats, and reducing inbreeding depression. In addition to the benefits afforded to individuals and species, its presence can add to the ecological diversity and complexity of an ecosystem. The strict definition of phoresis excludes cases in which the relationship is permanent (e.g. that of a barnacle surviving on a whale), or those in which the phoront gains any kind of advantage from the host organism (e.g. remoras attaching to sharks for transportation and food). Phoresis is a commensal relationship, and deviations result in mutualistic or parasitic relationships. Phoretic relationships can become parasitic if a cost is inflicted upon the host, such as if the number of mites on a host begins impeding its movement. Parasitic relationships could also be selected from phoretic ones if the phoront gains a fitness advantage from the death of a host (e.g. nutrition). Mutualistic relationships could also develop if the phoront begins to confer a benefit to the host (e.g. predator defense). The evolutionary plasticity of phoretic relationships allow them to potentially add to the complexity and diversity of ecosystems.