Editing Ectosymbiosis (section)

== 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" />