page_title: Ectosymbiosis
section_title: Evolutionary History
hierarchical_section_title: Ectosymbiosis / Evolutionary History
caption_attribution_description: English: Sea urchin as a host for ectosymbiotic parasites
context_page_description: Ectosymbiosis is form of symbiotic behavior in which a parasite lives on the body surface of the host, including internal surfaces such as the lining of the digestive tube and the ducts of glands. The parasitic species is generally an immobile, or sessile, organism existing off of biotic substrate through mutualism, commensalism, or parasitism. 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 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.
context_section_description: Ectosymbiosis has evolved independently many times to fill a wide variety of ecological niches, both temperate and extreme. Such temperate regions include the seas off the coast of Singapore while the extreme regions reach to the depths of Antarctica and hydrothermal vents. 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 domains of life.
  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 evolutionary tree
  The evolutionary success of ectosymbiosis is based on the benefits experienced by the parasite 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 coevolve as explained by the Red Queen hypothesis. 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. The result being competitive coevolution between the two species.
  Ectosymbiosis adds to the biodiversity of the environment, whether on land, in freshwater, in deserts, or in deep sea vents.  Specifically, ectosymbiosis provides a new niche or environment for species to  
  from which many new species can differentiate and flourish.
  This niche specialization between species also leads to stabilization of symbiotic relationships between sessile and motile organisms.  The ectosymbiotes can increase the fitness of their host by assisting with metabolism, nitrogen fixation, or cleaning the host organism. 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.
caption: Sea urchins, with their many spines provide protection for the ectosymbiotic parasites that live on them.
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