Editing Ectosymbiosis

{{short description|Symbiosis in which the symbiont lives on the body surface of the host}}
{{More citations needed|date=May 2017}}
[[File:European Mistletoe Growing On Trees.jpg|thumb|[[European mistletoe]] is an example of an ectosymbiotic parasite that lives on top of trees and removes nutrients and water.]]
'''Ectosymbiosis''' is a form of [[Symbiosis|symbiotic]] behavior in which an organism lives on the body surface of another organism (the [[Host (biology)|host]]), including internal surfaces such as the lining of the [[Gastrointestinal tract|digestive tube]] and the [[duct (anatomy)|duct]]s of [[gland]]s. The ectosymbiotic species, or '''ectosymbiont''', is generally an immobile (or [[Sessility (motility)|sessile]]) organism existing off of [[Biotic material|biotic]] substrate through [[Mutualism (biology)|mutualism]], [[commensalism]], or [[parasitism]].<ref name=":0">{{Cite web|url=http://www.ingentaconnect.com/content/umrsmas/bullmar/1996/00000058/00000002/art00004|title=Epizoic Bryozoans, Horseshoe Crabs, and other Mobile Benthic Substrates|last1=M.|first1=Key Jr., Marcus|last2=B.|first2=Jeffries, William|website=www.ingentaconnect.com|language=en|access-date=2018-11-29|last3=K.|first3=Voris, Harold|last4=M.|first4=Yang, Chang|date=March 1996}}</ref><ref name=":1">{{Cite journal|last1=Williams|first1=Jason D|last2=McDermott|first2=John J|date=July 2004|title=Hermit crab biocoenoses: a worldwide review of the diversity and natural history of hermit crab associates|journal=Journal of Experimental Marine Biology and Ecology|volume=305|issue=1|pages=1–128|doi=10.1016/j.jembe.2004.02.020|issn=0022-0981}}</ref> Ectosymbiosis is found throughout a diverse array of environments and in many different species.

In some species the symbiotic environment provided by both the parasite and host are mutually beneficial. In recent research it has been found that these [[Microflora|micro-flora]] will evolve and diversify rapidly in response to a change in the external environment, in order to stabilize and maintain a beneficial ectosymbiotic environment.<ref name=":2">{{Cite journal|last1=Papot|first1=Claire|last2=Massol|first2=François|last3=Jollivet|first3=Didier|last4=Tasiemski|first4=Aurélie|date=2017-05-03|title=Antagonistic evolution of an antibiotic and its molecular chaperone: how to maintain a vital ectosymbiosis in a highly fluctuating habitat|journal=Scientific Reports|language=En|volume=7|issue=1|page=1454|doi=10.1038/s41598-017-01626-2|issn=2045-2322|pmc=5431198|pmid=28469247|bibcode=2017NatSR...7.1454P}}</ref>

== Evolutionary history ==
Ectosymbiosis has evolved independently many times to fill a wide variety of [[ecological niche]]s, both [[Temperate climate|temperate]] and [[Extreme environment|extreme]].<ref name=":3">{{Cite journal|last1=Noda|first1=Satoko|last2=Ohkuma|first2=Moriya|last3=Yamada|first3=Akinori|last4=Hongoh|first4=Yuichi|last5=Kudo|first5=Toshiaki|date=2003-01-01|title=Phylogenetic Position and In Situ Identification of Ectosymbiotic Spirochetes on Protists in the Termite Gut|journal=Appl. Environ. Microbiol.|language=en|volume=69|issue=1|pages=625–633|doi=10.1128/AEM.69.1.625-633.2003|issn=0099-2240|pmc=152436|pmid=12514050|bibcode=2003ApEnM..69..625N}}</ref> Such temperate regions include the seas off the coast of [[Singapore]] while the extreme regions reach to the depths of [[Antarctica]] and [[hydrothermal vent]]s.<ref name=":0" /><ref name=":1" /><ref name=":4">{{Cite journal|last1=Corbari|first1=L|last2=Zbinden|first2=M|last3=Cambon-Bonavita|first3=M|last4=Gaill|first4=F|last5=Compère|first5=P|date=2008-01-15|title=Bacterial symbionts and mineral deposits in the branchial chamber of the hydrothermal vent shrimp Rimicaris exoculata: relationship to moult cycle|url=http://archimer.ifremer.fr/doc/00000/4626/|journal=Aquatic Biology|language=en|volume=1|pages=225–238|doi=10.3354/ab00024|issn=1864-7782|doi-access=free}}</ref> It likely evolved as a niche specialization, which allowed for greater diversity in ectosymbiotic behavior among species. Additionally, in the case of mutualism, the evolution improved the fitness of both species involved, propagating the success of ectosymbiosis. Ectosymbiosis has independently evolved through [[convergent evolution]] in all [[Domain (biology)|domains of life]].<ref name=":3" /><ref>{{Cite journal|last1=Bauermeister|first1=Jan|last2=Ramette|first2=Alban|last3=Dattagupta|first3=Sharmishtha|date=2012-11-29|title=Repeatedly Evolved Host-Specific Ectosymbioses between Sulfur-Oxidizing Bacteria and Amphipods Living in a Cave Ecosystem|journal=PLOS ONE|language=en|volume=7|issue=11|pages=e50254|doi=10.1371/journal.pone.0050254|issn=1932-6203|pmc=3510229|pmid=23209690|bibcode=2012PLoSO...750254B|doi-access=free}}</ref>
[[File:Sea-urchins-sea-animal-aquarium-604231.jpg|thumb|[[Sea urchin]]s, with their many spines, provide protection for the ectosymbiotic parasites that live on them.]]
Ectosymbiosis allows niches to form that would otherwise be unable to exist without the support of their host. Inherently, this added niche opens up a new branch off of the [[Tree of life (biology)|evolutionary tree]]. The evolutionary success of ectosymbiosis is based on the benefits experienced by the ectosymbiont and the host. Due to the dependence of the parasite on the host and the associated benefits and cost to both the parasite and host, the two will continue to [[Coevolution|coevolve]] as explained by the [[Red Queen hypothesis]].<ref name=":5">{{Citation|last=da Silva|first=Jack|date=2018-08-24|pages=1–7|publisher=John Wiley & Sons, Ltd|language=en|doi=10.1002/9780470015902.a0028127|isbn=9780470015902|title=eLS|chapter=Red Queen Theory|s2cid=240300423 }}</ref> The Red Queen hypothesis states that a host will continually evolve defenses against a parasitic attack, and the parasite species will also adapt to these changes in the [[Host-defense|host defense]], the result being competitive coevolution between the two species.<ref name=":5" />

Ectosymbiosis adds to the [[biodiversity]] of the environment, whether on land, in freshwater, in deserts, or in [[Deep-sea vents|deep sea vents]].<ref name=":6">{{Cite journal|last1=Hétérier|first1=Vincent|last2=David|first2=Bruno|last3=Ridder|first3=Chantal De|last4=Rigaud|first4=Thierry|date=2008-07-29|title=Ectosymbiosis is a critical factor in the local benthic biodiversity of the Antarctic deep sea|journal=Marine Ecology Progress Series|language=en|volume=364|pages=67–76|doi=10.3354/meps07487|issn=0171-8630|bibcode=2008MEPS..364...67H|doi-access=free}}</ref> Specifically, ectosymbiosis provides a new niche or environment from which many new species can differentiate and flourish.

This niche specialization between species also leads to stabilization of symbiotic relationships between [[Sessility (motility)|sessile]] and [[Motility|motile]] organisms. The ectosymbiont can increase the [[Fitness (biology)|fitness]] of their host by assisting with [[metabolism]], [[nitrogen fixation]], or cleaning the host organism.<ref name=":2" /><ref>{{Cite journal|last1=Tai|first1=Vera|last2=Carpenter|first2=Kevin J.|last3=Weber|first3=Peter K.|last4=Nalepa|first4=Christine A.|last5=Perlman|first5=Steve J.|last6=Keeling|first6=Patrick J.|date=2016-08-01|title=Genome Evolution and Nitrogen Fixation in Bacterial Ectosymbionts of a Protist Inhabiting Wood-Feeding Cockroaches|journal=Appl. Environ. Microbiol.|language=en|volume=82|issue=15|pages=4682–4695|doi=10.1128/AEM.00611-16|issn=0099-2240|pmc=4984305|pmid=27235430|bibcode=2016ApEnM..82.4682T}}</ref><ref name=":7">{{Cite journal|last1=Skelton|first1=James|last2=Farrell|first2=Kaitlin J.|last3=Creed|first3=Robert P.|last4=Williams|first4=Bronwyn W.|last5=Ames|first5=Catlin|last6=Helms|first6=Brian S.|last7=Stoekel|first7=James|last8=Brown|first8=Bryan L.|date=December 2013|title=Servants, scoundrels, and hitchhikers: current understanding of the complex interactions between crayfish and their ectosymbiotic worms (Branchiobdellida)|journal=Freshwater Science|language=en|volume=32|issue=4|pages=1345–1357|doi=10.1899/12-198.1|s2cid=86614891|issn=2161-9549}}</ref> The diversity of advantages has yet to be fully explored, but by virtue of persisting throughout all of recent evolution, they likely confer an adaptive advantage to many of the species that exist solely due to ectosymbiosis.
[[File:Lemonshark.jpg|thumb|[[Remora]] fish form ectosymbiotic commensal interactions with [[lemon shark]]s in order to scavenge food and travel long distances. ]]

== Types of host and parasite dynamic ==
Although ectosymbiosis is typically an evolutionary stable behavior, the different host and parasite dynamics independently vary in their stability.

=== Commensalism ===
{{main|Commensalism}}

Commensalism is a form of symbiosis where one species is benefiting from the interactions between species and the other is neither helped nor harmed from the interaction. Ectosymbiotic commensalistic behavior is found frequently in organisms that attach themselves to larger species in order to move long distances or scavenge food easily; this is documented in [[remora]]s which attach to [[shark]]s to scavenge and travel.<ref>Britz, R. & G. D. Johnson. 2012. Ontogeny and homology of the skeletal elements that form the sucking disc of remoras (Teleostei, Echeneoidei, Echeneidae). Journal of Morphology, 273 (12) 1353-1366, DOI: 10.1002/jmor.20063</ref> An additional ectosymbiotic example of commensalism is the relationship between small sessile organisms and [[Sea urchin|echinoids]] in the [[Southern Ocean|Southern ocean]], where the echinoids provide substrate for the small organisms to grow and the echinoids remain unaffected.<ref name=":6" />
[[File:Signal crayfish branchiobdellid crop 1.jpg|thumb|''[[Branchiobdellida|Branchiobdellid]]'' annelids are mutualistic parasites. They will attach to a [[signal crayfish]] and feed on [[diatom]]s, [[bacteria]], and [[protozoa]]ns that accumulate on the exoskeleton.]]

=== Mutualism ===
{{main|Mutualism (biology)}}

Mutualism is a form of ectosymbiosis where both the host and parasitic species benefit from the interaction. There are many examples of mutualistic ectosymbiosis that occur in nature. One such relationship is between ''[[Branchiobdellida]]'' and [[crayfish]] in which the ''Branchiobdellida'' acts as a bacterial gut cleaner for the crayfish species.<ref name=":7" /> Another example is the iron-oxide associated chemoautotrophic bacteria found crusted to the gills of ''[[Alvinocarididae|Rimicaris exoculata]]'' shrimp that provide the shrimp with vital organic material for their survival while simultaneously supporting the bacteria with different organic material that the bacterial cannot produce itself.<ref name=":4" /> Groups of organisms – greater than a single pair of a host and parasite – can also form mutualistic ectosymbiotic interactions. [[Bark beetle]]s can work in a dynamic mutualistic fashion with [[Fungus|fungi]] and [[mite]]s attached to their exoskeletons, both of which feed off of trees to provide vital energy to the beetles while the beetles provide necessary organic material to the fungi and mites to survive.<ref name=":9">{{Cite journal|last1=Klepzig|first1=Kier D.|last2=Moser|first2=J.C.|last3=Lombardero|first3=F.J.|last4=Hofstetter|first4=R.W.|last5=Ayres|first5=M.P.|date=2001|title=Symbiosis and competition: complex interactions among beetles, fungi, and mites|url=http://www.fs.usda.gov/treesearch/pubs/2400|journal=Symbiosis |volume=30 |pages=83–96|language=en}}</ref><ref name=":10">{{Cite journal|last1=Six|first1=D. L.|last2=Bentz|first2=B. J.|date=July 2007|title=Temperature determines symbiont abundance in a multipartite bark beetle-fungus ectosymbiosis|journal=Microbial Ecology|volume=54|issue=1|pages=112–118|doi=10.1007/s00248-006-9178-x|issn=0095-3628|pmid=17264992|s2cid=970799}}</ref> In this case, the relationship between the fungi and mites is functional because while both do the same job, they are optimally functional at different temperatures.<ref name=":9" /><ref name=":10" />

Mutualistic interactions can be [[Evolutionarily stable strategy|evolutionarily unstable]] because of the constant battle to maximize one's self-benefits.<ref name=":8">{{Cite journal|last1=Holland|first1=J. Nathaniel|last2=DeAngelis|first2=Donald L.|last3=Schultz|first3=Stewart T.|date=2004-09-07|title=Evolutionary stability of mutualism: interspecific population regulation as an evolutionarily stable strategy|journal=Proceedings of the Royal Society of London B: Biological Sciences|language=en|volume=271|issue=1550|pages=1807–1814|doi=10.1098/rspb.2004.2789|issn=0962-8452|pmc=1691799|pmid=15315896}}</ref> This is due to the limited benefits offered to both the parasite and the host, with the possible outcome for at least one of the species to die out if the other species begins to take advantage of the other.<ref name=":8" /> In the case that the mutualistic behavior persists for enough generations, the dynamic can evolve into parasitism, which is a more stable dynamic due to the increased benefit to the parasite that propagates the behavior.<ref name=":8" /> In this case the parasite takes advantage of the previously mutualistic host and parasite dynamic, gaining greater benefits for itself.<ref name=":8" />
[[File:Male human head louse.jpg|thumb|The [[head louse]] is an ectosymbiotic parasite that feeds off of the blood of humans by attaching itself to the scalp.]]

=== Parasitism ===
{{main|Parasitism}}

Parasitism is a form of symbiosis in which one species benefits from the interactions between species while the other organism is actively harmed. This is the most common form of ectosymbiotic interactions. One of the many examples of ectosymbiotic parasites includes head [[Louse|lice]] in humans, which feed on blood by attaching to a human's scalp. Additionally, mature ''[[Branchiobdellida]]'' bacteria act as a nutrient thief in the gut of [[crayfish]] species to exist. In these cases, the head lice and the ''Branchiobdellida'' are both parasites interacting with host species.<ref name=":7" />

== See also ==
* [[Epibiosis]]
* [[Epixenosomes]]

==References==
{{Reflist}}

[[Category:Symbiosis]]