Editing Ectosymbiosis (section)

=== 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.]]