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 symbiotic behavior in which an organism lives on the body surface of another organism (the host), including internal surfaces such as the lining of the digestive tube and the ducts of glands. The ectosymbiotic species, or ectosymbiont, 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.
Evolutionary history
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.
thumb|[[Sea urchins, 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 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 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. 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 sessile and motile organisms. The ectosymbiont 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.
thumb|[[Remora fish form ectosymbiotic commensal interactions with lemon sharks 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
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 remoras which attach to sharks to scavenge and travel. An additional ectosymbiotic example of commensalism is the relationship between small sessile organisms and echinoids in the Southern ocean, where the echinoids provide substrate for the small organisms to grow and the echinoids remain unaffected. 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. 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.