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"Symbiology" redirects here. For use of things that represent other things by association, resemblance, or convention, see Symbology.
This article is about the biological phenomenon. For other uses, see Symbiosis (disambiguation). For the Marvel characters, see Symbiote (comics).
In a symbiotic mutualistic relationship, the clownfish feeds on small invertebrates that otherwise have potential to harm the sea anemone, and the fecal matter from the clownfish provides nutrients to the sea anemone. The clownfish is additionally protected from predators by the anemone's stinging cells, to which the clownfish is immune. The clownfish also emits a high pitched sound that deters butterfly fish, which would otherwise eat the anemone.[1]

Symbiosis (from Greek σύν "together" and βίωσις "living")[2] is close and often long-term interaction between two different biological species. In 1877 Albert Bernhard Frank used the word symbiosis (which previously had been used to depict people living together in community) to describe the mutualistic relationship in lichens.[3] In 1879, the German mycologist Heinrich Anton de Bary defined it as "the living together of unlike organisms."[4][5]

The definition of symbiosis has varied among scientists. Some believe symbiosis should only refer to persistent mutualisms, while others believe it should apply to any type of persistent biological interaction (i.e. mutualistic, commensalistic, or parasitic).[6] After 130 years of debate,[7] current biology and ecology textbooks now use the latter "de Bary" definition or an even broader definition (i.e. symbiosis = all species interactions), with the restrictive definition no longer used (i.e. symbiosis = mutualism).[8]

Some symbiotic relationships are obligate, meaning that both symbionts entirely depend on each other for survival. For example, many lichens consist of fungal and photosynthetic symbionts that cannot live on their own.[4][9][10][11] Others are facultative, meaning that they can, but do not have to live with the other organism.

Symbiotic relationships include those associations in which one organism lives on another (ectosymbiosis, such as mistletoe), or where one partner lives inside the other (endosymbiosis, such as lactobacilli and other bacteria in humans or Symbiodinium in corals).[12][13] Symbiosis is also classified by physical attachment of the organisms; symbiosis in which the organisms have bodily union is called conjunctive symbiosis, and symbiosis in which they are not in union is called disjunctive symbiosis.[14]

Physical interaction[edit]

Main article: Physical interaction
See also: Root microbiome
Alder tree root nodule

Endosymbiosis is any symbiotic relationship in which one symbiont lives within the tissues of the other, either within the cells or extracellularly.[13][15] Examples include diverse microbiomes, rhizobia, nitrogen-fixing bacteria that live in root nodules on legume roots; actinomycete nitrogen-fixing bacteria called Frankia, which live in alder tree root nodules; single-celled algae inside reef-building corals; and bacterial endosymbionts that provide essential nutrients to about 10%–15% of insects.

Ectosymbiosis, also referred to as exosymbiosis, is any symbiotic relationship in which the symbiont lives on the body surface of the host, including the inner surface of the digestive tract or the ducts of exocrine glands.[13][16] Examples of this include ectoparasites such as lice, commensal ectosymbionts such as the barnacles that attach themselves to the jaw of baleen whales, and mutualist ectosymbionts such as cleaner fish.


Main article: Mutualism (biology)
Hermit crab, Calcinus laevimanus, with sea anemone.

Mutualism or interspecies reciprocal altruism is a relationship between individuals of different species where both individuals benefit.[17] In general, only lifelong interactions involving close physical and biochemical contact can properly be considered symbiotic. Mutualistic relationships may be either obligate for both species, obligate for one but facultative for the other, or facultative for both. Many biologists restrict the definition of symbiosis to close mutualist relationships.

Bryoliths document a mutualistic symbiosis between a hermit crab and encrusting bryozoans; Banc d'Arguin, Mauritania

A large percentage of herbivores have mutualistic gut flora that help them digest plant matter, which is more difficult to digest than animal prey.[12] This gut flora is made up of cellulose-digesting protozoans or bacteria living in the herbivores' intestines.[18] Coral reefs are the result of mutualisms between coral organisms and various types of algae that live inside them.[19] Most land plants and land ecosystems rely on mutualisms between the plants, which fix carbon from the air, and mycorrhyzal fungi, which help in extracting water and minerals from the ground.[20]

An example of mutual symbiosis is the relationship between the ocellaris clownfish that dwell among the tentacles of Ritteri sea anemones. The territorial fish protects the anemone from anemone-eating fish, and in turn the stinging tentacles of the anemone protect the clownfish from its predators. A special mucus on the clownfish protects it from the stinging tentacles.[21]

A further example is the goby fish, which sometimes lives together with a shrimp. The shrimp digs and cleans up a burrow in the sand in which both the shrimp and the goby fish live. The shrimp is almost blind, leaving it vulnerable to predators when outside its burrow. In case of danger the goby fish touches the shrimp with its tail to warn it. When that happens both the shrimp and goby fish quickly retreat into the burrow.[22] Different species of gobies (Elacatinus spp.) also exhibit mutualistic behavior through cleaning up ectoparasites in other fish.[23]

Another non-obligate symbiosis is known from encrusting bryozoans and hermit crabs that live in a close relationship. The bryozoan colony (Acanthodesia commensale) develops a cirumrotatory growth and offers the crab (Pseudopagurus granulimanus) a helicospiral-tubular extension of its living chamber that initially was situated within a gastropod shell.[24]

One of the most spectacular examples of obligate mutualism is between the siboglinid tube worms and symbiotic bacteria that live at hydrothermal vents and cold seeps. The worm has no digestive tract and is wholly reliant on its internal symbionts for nutrition. The bacteria oxidize either hydrogen sulfide or methane, which the host supplies to them. These worms were discovered in the late 1980s at the hydrothermal vents near the Galapagos Islands and have since been found at deep-sea hydrothermal vents and cold seeps in all of the world's oceans.[25]

There are also many types of tropical and sub-tropical ants that have evolved very complex relationships with certain tree species.[26]

Mutualism and endosymbiosis[edit]

During mutualistic symbioses, the host cell lacks some of the nutrients, which are provided by the endosymbiont. As a result, the host favors endosymbiont's growth processes within itself by producing some specialized cells. These cells affect the genetic composition of the host in order to regulate the increasing population of the endosymbionts and ensuring that these genetic changes are passed onto the offspring via vertical transmission (heredity).[27]

Adaptation of the endosymbiont to the host's lifestyle leads to many changes in the endosymbiont–the foremost being drastic reduction in its genome size. This is due to many genes being lost during the process of metabolism, and DNA repair and recombination. While important genes participating in the DNA to RNA transcription, protein translation and DNA/RNA replication are retained. That is, a decrease in genome size is due to loss of protein coding genes and not due to lessening of inter-genic regions or open reading frame (ORF) size. Thus, species that are naturally evolving and contain reduced sizes of genes can be accounted for an increased number of noticeable differences between them, thereby leading to changes in their evolutionary rates. As the endosymbiotic bacteria related with these insects are passed on to the offspring strictly via vertical genetic transmission, intracellular bacteria goes through many hurdles during the process, resulting in the decrease in effective population sizes when compared to the free living bacteria. This incapability of the endosymbiotic bacteria to reinstate its wild type phenotype via a recombination process is called as Muller's ratchet phenomenon. Muller's ratchet phenomenon together with less effective population sizes has led to an accretion of deleterious mutations in the non-essential genes of the intracellular bacteria.[28] This could have been due to lack of selection mechanisms prevailing in the rich environment of the host.[29][30]


Phoretic mites on a fly (Pseudolynchia canariensis).
Main article: Commensalism

Commensalism describes a relationship between two living organisms where one benefits and the other is not significantly harmed or helped. It is derived from the English word commensal used of human social interaction. The word derives from the medieval Latin word, formed from com- and mensa, meaning "sharing a table".[17][31]

Commensal relationships may involve one organism using another for transportation (phoresy) or for housing (inquilinism), or it may also involve one organism using something another created, after its death (metabiosis). Examples of metabiosis are hermit crabs using gastropod shells to protect their bodies and spiders building their webs on plants.


Flea bites on a human is an example of parasitism.
Main article: Parasitism

A parasitic relationship is one in which one member of the association benefits while the other is harmed.[32] This is also known as antagonistic or antipathetic symbiosis.[14] Parasitic symbioses take many forms, from endoparasites that live within the host's body to ectoparasites that live on its surface. In addition, parasites may be necrotrophic, which is to say they kill their host, or biotrophic, meaning they rely on their host's surviving. Biotrophic parasitism is an extremely successful mode of life. Depending on the definition used, as many as half of all animals have at least one parasitic phase in their life cycles, and it is also frequent in plants and fungi. Moreover, almost all free-living animals are host to one or more parasite taxa. An example of a biotrophic relationship would be a tick feeding on the blood of its host.


Amensalism is the type of relationship that exists where one species is inhibited or completely obliterated and one is unaffected. This type of symbiosis is relatively uncommon in rudimentary reference texts, but is omnipresent in the natural world.[citation needed] There are two types of amensalism, competition and antibiosis. Competition is where a larger or stronger organisms deprives a smaller or weaker one from a resource. Antibiosis occurs when one organism is damaged or killed by another through a chemical secretion. An example of competition is a sapling growing under the shadow of a mature tree. The mature tree can begin to rob the sapling of necessary sunlight and, if the mature tree is very large, it can take up rainwater and deplete soil nutrients. Throughout the process the mature tree is unaffected. Indeed, if the sapling dies, the mature tree gains nutrients from the decaying sapling. Note that these nutrients become available because of the sapling's decomposition, rather than from the living sapling, which would be a case of parasitism.[citation needed] An example of antibiosis is Juglans nigra (black walnut), secreting juglone, a substance which destroys many herbaceous plants within its root zone.[33]

Amensalism is an interaction where an organism inflicts harm to another organism without any costs or benefits received by the other.[34] A clear case of amensalism is where sheep or cattle trample grass. Whilst the presence of the grass causes negligible detrimental effects to the animal's hoof, the grass suffers from being crushed. Amensalism is often used to describe strongly asymmetrical competitive interactions, such as has been observed between the Spanish ibex and weevils of the genus Timarcha which feed upon the same type of shrub. Whilst the presence of the weevil has almost no influence on food availability, the presence of ibex has an enormous detrimental effect on weevil numbers, as they consume significant quantities of plant matter and incidentally ingest the weevils upon it.[35]


Synnecrosis is a rare type of symbiosis in which the interaction between species is detrimental to both organisms involved.[14] It is a short-lived condition, as the interaction eventually causes death. Because of this, evolution selects against synnecrosis and it is uncommon in nature. An example of this is the relationship between some species of bees and victims of the bee sting. Species of bees who die after stinging their prey inflict pain on themselves (albeit to protect the hive) as well as on the victim. This term is rarely used.[36]

Symbiosis and evolution[edit]

Leafhoppers protected by meat ants

While historically, symbiosis has received less attention than other interactions such as predation or competition,[37] it is increasingly recognized as an important selective force behind evolution,[12][38] with many species having a long history of interdependent co-evolution.[39] In fact, the evolution of all eukaryotes (plants, animals, fungi, and protists) is believed under the endosymbiotic theory to have resulted from a symbiosis between various sorts of bacteria.[12][40][41] This theory is supported by certain organelles dividing independently of the cell, and the observation that some organelles seem to have their own nucleic acid.[42]

Vascular plants[edit]

About 80% of vascular plants worldwide form symbiotic relationships with fungi, for example, in arbuscular mycorrhizas.[43]


Main article: Symbiogenesis

The biologist Lynn Margulis, famous for her work on endosymbiosis, contends that symbiosis is a major driving force behind evolution. She considers Darwin's notion of evolution, driven by competition, to be incomplete and claims that evolution is strongly based on co-operation, interaction, and mutual dependence among organisms. According to Margulis and Dorion Sagan, "Life did not take over the globe by combat, but by networking."[44]


Symbiosis played a major role in the co-evolution of flowering plants and the animals that pollinate them. Many plants that are pollinated by insects, bats, or birds have highly specialized flowers modified to promote pollination by a specific pollinator that is also correspondingly adapted. The first flowering plants in the fossil record had relatively simple flowers. Adaptive speciation quickly gave rise to many diverse groups of plants, and, at the same time, corresponding speciation occurred in certain insect groups. Some groups of plants developed nectar and large sticky pollen, while insects evolved more specialized morphologies to access and collect these rich food sources. In some taxa of plants and insects the relationship has become dependent,[45] where the plant species can only be pollinated by one species of insect.[46]

See also[edit]


  1. ^ Miller, Allie. "Intricate Relationship Allows the Other to Flourish: the Sea Anemone and the Clownfish". AskNature. The Biomimicry Institute. Retrieved 15 February 2015. 
  2. ^ σύν, βίωσις. Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at the Perseus Project
  3. ^ "symbiosis". Oxford English Dictionary (3rd ed.). Oxford University Press. September 2005.  (Subscription or UK public library membership required.)
  4. ^ a b Wilkinson 2001
  5. ^ Douglas 1994, p. 1
  6. ^ Douglas 2010, pp. 5–12
  7. ^ Martin, Bradford D.; Schwab, Ernest (2012), "Symbiosis: 'Living together' in chaos", Studies in the History of Biology 4 (4): 7–25. 
  8. ^ Martin, Bradford D.; Schwab, Ernest (2013), "Current usage of symbiosis and associated terminology", International Journal of Biology 5 (1): 32–45., doi:10.5539/ijb.v5n1p32 
  9. ^ Isaac 1992, p. 266
  10. ^ Saffo 1993
  11. ^ Douglas 2010, p. 4
  12. ^ a b c d Moran 2006
  13. ^ a b c Paracer & Ahmadjian 2000, p. 12
  14. ^ a b c "symbiosis." Dorland's Illustrated Medical Dictionary. Philadelphia: Elsevier Health Sciences, 2007. Credo Reference. Web. 17 September 2012
  15. ^ Sapp 1994, p. 142
  16. ^ Nardon & Charles 2002
  17. ^ a b Paracer & Ahmadjian 2000, p. 6
  18. ^ "symbiosis." The Columbia Encyclopedia. New York: Columbia University Press, 2008. Credo Reference. Web. 17 September 2012.
  19. ^ Toller, Rowan & Knowlton 2001
  20. ^ Harrison 2005
  21. ^ Lee 2003
  22. ^ Facey, Helfman & Collette 1997
  23. ^ M.C. Soares, I.M. Côté, S.C. Cardoso & R.Bshary (August 2008). "The cleaning goby mutualism: a system without punishment, partner switching or tactile stimulation". Journal of zoology 276 (3): 306–312. doi:10.1111/j.1469-7998.2008.00489.x. 10.1111/j.1469-7998.2008.00489.x. 
  24. ^ Klicpera, A; PD Taylor; H Westphal (1 Dec 2013). "Bryoliths constructed by bryozoans in symbiotic associations with hermit crabs in a tropical heterozoan carbonate system, Golfe d’Arguin, Mauritania". Mar Biodivers (Springer Berlin Heidelberg) 43 (4): 429–444. doi:10.1007/s12526-013-0173-4. ISSN 1867-1616. 
  25. ^ Cordes 2005
  26. ^ Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
  27. ^ Latorre, A.; Durban, A.; Moya, A.; Pereto, J. (2011). The role of symbiosis in eukaryotic evolution. Origins and evolution of life - An astrobiological perspective. pp. 326–339. 
  28. ^ Moran, N. A. (1996). "Accelerated evolution and Muller's ratchet in endosymbiotic bacteria.". Proceedings of the National Academy of Sciences of the United States of America 93: 2873–2878. doi:10.1073/pnas.93.7.2873. PMC 39726. PMID 8610134. 
  29. ^ Andersson, Siv G.E; Kurland, Charles G (1998). "Reductive evolution of resident genomes". Trends in Microbiology 6 (7): 263–8. doi:10.1016/S0966-842X(98)01312-2. PMID 9717214. 
  30. ^ Wernegreen, J.J. (2002). "Genome evolution in bacterial endosymbionts of insects.". Nature reviews, Genetics 3 (11): 850–861. doi:10.1038/nrg931. PMID 12415315. 
  31. ^ Nair 2005
  32. ^ Paracer & Ahmadjian 2000, p. 7
  33. ^ The Editors of Encyclopædia Britannica. (n.d.). Amensalism (biology). Retrieved September 30, 2014, from
  34. ^ Willey, Joanne M.; Sherwood, Linda M.; Woolverton, Cristopher J. (2013). Prescott's Microbiology (9th ed.). pp. 713–38. ISBN 978-0-07-751066-4. 
  35. ^ Gómez, José M.; González-Megías, Adela (2002). "Asymmetrical interactions between ungulates and phytophagous insects: Being different matters". Ecology 83 (1): 203–11. doi:10.1890/0012-9658(2002)083[0203:AIBUAP]2.0.CO;2. 
  36. ^ Lidicker, William Z. (August 1979). "A Clarification of Interactions in Ecological Systems". BioScience 29 (8): 475–7. doi:10.2307/1307540. JSTOR 1307540. 
  37. ^ Townsend, Begon & Harper 1996
  38. ^ Wernegreen 2004
  39. ^ Paracer & Ahmadjian 2000, pp. 3–4
  40. ^ Brinkman 2002
  41. ^ Golding & Gupta 1995
  42. ^ "Symbiosis." Bloomsbury Guide to Human Thought. London: Bloomsbury Publishing Ltd, 1993. Credo Reference. Web. 17 September 2012.
  43. ^ Schüßler, A.; et al. (2001), "A new fungal phylum, the Glomeromycota: phylogeny and evolution", Mycol. Res. 105 (12): 1416, doi:10.1017/S0953756201005196. 
  44. ^ Sagan & Margulis 1986
  45. ^ Harrison 2002
  46. ^ Danforth & Ascher 1997


External links[edit]