Phoresis orphoresy is a temporarycommensalistic relationship when an organism (a phoront or phoretic) attaches itself to a host organism solely for travel.[2] It has been seen inticks andmites since the 18th century,[3] and in fossils 320 million years old.[2] It is not restricted toarthropods oranimals;plants with seeds that disperse by attaching themselves to animals are also considered to be phoretic.[4]
Phoresis is rooted in the Greek wordsphoras (bearing) andphor (thief).[3] 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, andontogeny are now considered.[3] Phoresis is used as a strategy fordispersal,[5][6]seasonal migration,[7] transport to new host/habitat,[8] escaping ephemeral habitats,[9][3] and reducinginbreeding depression.[10] In addition to the benefits afforded to individuals and species, its presence can add to theecological diversity and complexity of an ecosystem.[4]
The strict definition of phoresis excludes cases in which the relationship is permanent (e.g. that of abarnacle surviving on awhale), 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).[3] Phoresis is acommensal relationship, and deviations result inmutualistic orparasitic 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 afitness 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).[2] The evolutionary plasticity of phoretic relationships allow them to potentially add to the complexity and diversity of ecosystems.[4]
Cases in which the phorontparasitizes or preys upon the host organism after travel are still considered phoresis, as long as the travel behaviour and the feeding or parasitizing behaviour are separate.[2] Similarly, somepseudoscorpions prey upon the same species that act as their phoretic host. The behaviours are completely separate however, since the pseudoscorpion utilizes anatomical features used specifically forpredation when treating the host as prey, but employs anatomical features used for phoresis when travelling.[11]
Examples may be found in thearthropods associated withsloths.Coprophagous sloth moths, such asBradipodicola hahneli andCryptoses choloepi, are unusual in that they exclusively inhabit the fur of sloths, mammals found incentral andSouth America.[12][13] The sloth provides transport for the moths, the females of whichoviposit in the droppings of sloths, which the larvae feed on, and the newly hatched moths move into the forest canopy in search of a new sloth host.
Larvae of the blister beetle (Meloe franciscanus) need to find the nests of their host, the solitary bee (Habropoda pallida), to continue their life cycle. The larvae gather in colonies, and emit chemicals that mimic thepheromones of the female solitary bee. Larvae attach to the attracted males when they visit the false source of pheromones, and then subsequently to any female the male mates with. The blister beetle larvae then infest and parasitize the female bee’s nest.[8]
Some species of Bromeliad treefrog (Scinax littoreus andScinax perpusillus) carryostracods (Elpidium sp.), which in turn carryciliates (Lagenophrys sp.) from onebromeliad plant to another. The plants act asecological islands to the ostracods, and phoresis allows them to disperse over a wider area than would be available to them otherwise. The term for a phoretic organism riding on another phoretic organism ishyperphoresis.[9][14]
Somemites in the cladeAstigmatina have a stage of their life cycle (thedeutonymph or hypopus) that is modified specifically for phoresis. This stage has reduced mouthparts, a well-sclerotised body that resistsdesiccation, and usually a posteroventral organ for attaching to the host animal (which may be an invertebrate or a vertebrate). Astigmatans often live in patchy and ephemeral habitats such as fungal fruiting bodies, dung, carrion, animal nests, tree sap flows and decaying wood. Phoresis allows these mites to quickly leave a depleted habitat and travel to a new one.[15] A specific example is deutonymphs ofLardoglyphus dispersing on beetles in the genusDermestes to reach new habitats (both phoront and host feed on animal materials).[16]
A specialist mite (Parasitellus fucorum) that parasitizes bumble bees (Bombus spp.) avoidsinbreeding depression in a single hive, and remains genetically independent of any specific host lineage by travelling to a new hive. This is accomplished by travelling on a foraging bee to a flower and detaching, and waiting for and attaching to another bee which may be from another hive, and infesting the new hive. These mites can survive on flowers for up to 24 hours, and have shown a preference for opened flowers, where they would be most likely to find a host.[10]
Dung and carrion are ephemeral habitats that are frequently visited by beetles (dung beetles,burying beetles). Phoreticnematodes (Rhabditoides) andmites (e.g. generaMacrocheles,Poecilochirus,Uroobovella) use the beetles to reach these rich resources, where they themselves reproduce.[17][18][19]
The pseudoscorpionCordylochernes scorpioides is frequently found ridingharlequin beetles (Acrocinus longimanus). Initially, there were a number of alternate hypothesis for why thepseudoscorpions were found on the beetles: by accident, to forage for mites inhabiting the beetle, or as anobligate parasite. Evidence suggested, however, that the pseudoscorpions were using beetles to travel from tree to tree, where they preyed upon other beetle larvae.[5]
If their host dies,lice can opportunistically use phoresis to hitch a ride on a fly, and attempt to find a new host.[20]
The largest mammalian example of phoresis is human beings directly riding onhorses or other animals, or using them to pull vehicles with humans in them.
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