Ectosymbiosis

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European mistletoe is an example of an ectosymbiotic parasite that lives on top of trees and removes nutrients and water. European Mistletoe Growing On Trees.jpg
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. [1] [2] Ectosymbiosis is found throughout a diverse array of environments and in many different species.

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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. [3]

Evolutionary history

Ectosymbiosis has evolved independently many times to fill a wide variety of ecological niches, both temperate and extreme. [4] Such temperate regions include the seas off the coast of Singapore while the extreme regions reach to the depths of Antarctica and hydrothermal vents. [1] [2] [5] 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. [4] [6]

Sea urchins, with their many spines, provide protection for the ectosymbiotic parasites that live on them. Sea-urchins-sea-animal-aquarium-604231.jpg
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. [7] 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. [7]

Ectosymbiosis adds to the biodiversity of the environment, whether on land, in freshwater, in deserts, or in deep sea vents. [8] 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. [3] [9] [10] 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.

Remora fish form ectosymbiotic commensal interactions with lemon sharks in order to scavenge food and travel long distances. Lemonshark.jpg
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. [11] 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. [8]

Branchiobdellid annelids are mutualistic parasites. They will attach to a signal crayfish and feed on diatoms, bacteria, and protozoans that accumulate on the exoskeleton. Signal crayfish branchiobdellid crop 1.jpg
Branchiobdellid annelids are mutualistic parasites. They will attach to a signal crayfish and feed on diatoms, bacteria, and protozoans that accumulate on the exoskeleton.

Mutualism

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. [10] Another example is the iron-oxide associated chemoautotrophic bacteria found crusted to the gills of 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. [5] Groups of organisms – greater than a single pair of a host and parasite – can also form mutualistic ectosymbiotic interactions. Bark beetles can work in a dynamic mutualistic fashion with fungi and mites 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. [12] [13] 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. [12] [13]

Mutualistic interactions can be evolutionarily unstable because of the constant battle to maximize one's self-benefits. [14] 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. [14] 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. [14] In this case the parasite takes advantage of the previously mutualistic host and parasite dynamic, gaining greater benefits for itself. [14]

The head louse is an ectosymbiotic parasite that feeds off of the blood of humans by attaching itself to the scalp. Male human head louse.jpg
The head louse is an ectosymbiotic parasite that feeds off of the blood of humans by attaching itself to the scalp.

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 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. [10]

See also

Related Research Articles

<span class="mw-page-title-main">Symbiosis</span> Close, long-term biological interaction between distinct organisms (usually species)

Symbiosis is any type of a close and long-term biological interaction between two biological organisms of different species, termed symbionts, be it mutualistic, commensalistic, or parasitic. In 1879, Heinrich Anton de Bary defined it as "the living together of unlike organisms". The term is sometimes used in the more restricted sense of a mutually beneficial interaction in which both symbionts contribute to each other's support.

<span class="mw-page-title-main">Mutualism (biology)</span> Mutually beneficial interaction between species

Mutualism describes the ecological interaction between two or more species where each species has a net benefit. Mutualism is a common type of ecological interaction, one that can come from a parasitic interaction. Prominent examples include most vascular plants engaged in mutualistic interactions with mycorrhizae, flowering plants being pollinated by animals, vascular plants being dispersed by animals, and corals with zooxanthellae, among many others. Mutualism can be contrasted with interspecific competition, in which each species experiences reduced fitness, and exploitation, or parasitism, in which one species benefits at the expense of the other.

<span class="mw-page-title-main">Commensalism</span> Beneficial symbiosis between species

Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit from each other; amensalism, where one is harmed while the other is unaffected; and parasitism, where one is harmed and the other benefits.

<span class="mw-page-title-main">Coevolution</span> Two or more species influencing each others evolution

In biology, coevolution occurs when two or more species reciprocally affect each other's evolution through the process of natural selection. The term sometimes is used for two traits in the same species affecting each other's evolution, as well as gene-culture coevolution.

<span class="mw-page-title-main">Host (biology)</span> Organism that harbours another organism

In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist guest (symbiont). The guest is typically provided with nourishment and shelter. Examples include animals playing host to parasitic worms, cells harbouring pathogenic (disease-causing) viruses, or a bean plant hosting mutualistic (helpful) nitrogen-fixing bacteria. More specifically in botany, a host plant supplies food resources to micropredators, which have an evolutionarily stable relationship with their hosts similar to ectoparasitism. The host range is the collection of hosts that an organism can use as a partner.

<span class="mw-page-title-main">Biological interaction</span> Effect that organisms have on other organisms

In ecology, a biological interaction is the effect that a pair of organisms living together in a community have on each other. They can be either of the same species, or of different species. These effects may be short-term, or long-term, both often strongly influence the adaptation and evolution of the species involved. Biological interactions range from mutualism, beneficial to both partners, to competition, harmful to both partners. Interactions can be direct when physical contact is established or indirect, through intermediaries such as shared resources, territories, ecological services, metabolic waste, toxins or growth inhibitors. This type of relationship can be shown by net effect based on individual effects on both organisms arising out of relationship.

The Prodoxidae are a family of moths, generally small in size and nondescript in appearance. They include species of moderate pest status, such as the currant shoot borer, and others of considerable ecological and evolutionary interest, such as various species of "yucca moths".

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Cheating is a term used in behavioral ecology and ethology to describe behavior whereby organisms receive a benefit at the cost of other organisms. Cheating is common in many mutualistic and altruistic relationships. A cheater is an individual who does not cooperate but can potentially gain the benefit from others cooperating. Cheaters are also those who selfishly use common resources to maximize their individual fitness at the expense of a group. Natural selection favors cheating, but there are mechanisms to regulate it. The stress gradient hypothesis states that facilitation, cooperation or mutualism should be more common in stressful environments, while cheating, competition or parasitism are common in benign environments.

<span class="mw-page-title-main">Myrmecophily</span> Positive interspecies associations between ants and other organisms

Myrmecophily is the term applied to positive interspecies associations between ants and a variety of other organisms, such as plants, other arthropods, and fungi. Myrmecophily refers to mutualistic associations with ants, though in its more general use, the term may also refer to commensal or even parasitic interactions.

<span class="mw-page-title-main">Community (ecology)</span> Associated populations of species in a given area

In ecology, a community is a group or association of populations of two or more different species occupying the same geographical area at the same time, also known as a biocoenosis, biotic community, biological community, ecological community, or life assemblage. The term community has a variety of uses. In its simplest form it refers to groups of organisms in a specific place or time, for example, "the fish community of Lake Ontario before industrialization".

<span class="mw-page-title-main">Insect ecology</span> The study of how insects interact with the surrounding environment

Insect ecology is the scientific study of how insects, individually or as a community, interact with the surrounding environment or ecosystem.

<span class="mw-page-title-main">Phoresis</span> Temporary commensalism for transport

Phoresis or phoresy is a non-permanent, commensalistic interaction in which one organism attaches itself to another solely for the purpose of travel. Phoresis has been observed directly in ticks and mites since the 18th century, and indirectly in fossils 320 million years old. It is not restricted to arthropods or animals; plants with seeds that disperse by attaching themselves to animals are also considered to be phoretic.

<span class="mw-page-title-main">Cleaning symbiosis</span> Mutually beneficial association between individuals of two species

Cleaning symbiosis is a mutually beneficial association between individuals of two species, where one removes and eats parasites and other materials from the surface of the other. Cleaning symbiosis is well-known among marine fish, where some small species of cleaner fish, notably wrasses but also species in other genera, are specialised to feed almost exclusively by cleaning larger fish and other marine animals. Other cleaning symbioses exist between birds and mammals, and in other groups.

<span class="mw-page-title-main">Evolving digital ecological network</span>

Evolving digital ecological networks are webs of interacting, self-replicating, and evolving computer programs that experience the same major ecological interactions as biological organisms. Despite being computational, these programs evolve quickly in an open-ended way, and starting from only one or two ancestral organisms, the formation of ecological networks can be observed in real-time by tracking interactions between the constantly evolving organism phenotypes. These phenotypes may be defined by combinations of logical computations that digital organisms perform and by expressed behaviors that have evolved. The types and outcomes of interactions between phenotypes are determined by task overlap for logic-defined phenotypes and by responses to encounters in the case of behavioral phenotypes. Biologists use these evolving networks to study active and fundamental topics within evolutionary ecology.

<span class="mw-page-title-main">Marine microbial symbiosis</span>

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