Editing Ectosymbiosis (section)

== 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. ]]