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Symbiosis From Wikipedia, the free encyclopedia

"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 character, see Symbiote (comics).

In a symbiotic mutualism, the clownfishfeeds 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.

Symbiosis (from Ancient Greek σύν "together" and βίωσις "living")[1]is close and often long-term interaction between different biological species. In 1877, Bennett used the word symbiosis (which previously had been used of people living together in community) to describe the mutualistic relationship in lichens.[2]In 1879, by the GermanmycologistHeinrich Anton de Bary, defined it as "the living together of unlike organisms."[3][4]

The definition of symbiosis is controversial among scientists. Some believe symbiosis should only refer to persistent mutualisms, while others believe it should apply to any types of persistent biological interactions (i.e. mutualistic, commensalistic, or parasitic).[5]

Some symbiotic relationships are obligate, meaning that both symbionts entirely depend on each other for survival. For example, many lichensconsist of fungal and photosynthetic symbionts that cannot live on their own.[3][6][7][8]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 lactobacilliand other bacteria in humans or zooxanthellesin corals).[9][10]

Contents[hide] [edit]Physical interaction

Alder tree root nodule

Endosymbiosisis any symbiotic relationship in which one symbiont lives within the tissues of the other, either in the intracellular space or extracellularly.[10][11]Examples include diverse microbiomes, rhizobia, nitrogen-fixing bacteria that live in root noduleson legumeroots; actinomycetenitrogen-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 digestivetract or the ducts of exocrineglands.[10][12]Examples of this include ectoparasitessuch as lice, commensalectosymbionts such as the barnaclesthat attach themselves to the jaw of baleen whales, and mutualistectosymbionts such as cleaner fish.

[edit]MutualismMain article: Mutualism (biology)

Hermit crab, Calcinus laevimanus, with sea anemone.

Mutualism is any relationship between individuals of different species where both individuals derive a benefit.[13]In general, only lifelong interactions involving close physical and biochemicalcontact 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 biologistsrestrict the definition of symbiosis to close mutualist relationships.

A large percentage of herbivoreshave mutualistic gut faunathat help them digest plant matter, which is more difficult to digest than animal prey.[9]Coral reefs are the result of mutualisms between coral organisms and various types of algae that live inside them.[14]Most land plants and land ecosystems rely on mutualisms between the plants, which fix carbon from the air, and mycorrhyzalfungi, which help in extracting minerals from the ground.[15]

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 stingingtentacles of the anemone protect the clownfish from its predators. A special mucuson the clownfish protects it from the stinging tentacles.[16]

Another 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 above ground. 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.[17]

One of the most spectacular examples of obligate mutualism is between the siboglinidtube wormsand 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-seahydrothermal vents and cold seeps in all of the world's oceans.[18]

There are also many types of tropical and sub-tropical ants that have evolved very complex relationships with certain tree species.[19]

[edit]Mutualism and endosymbiosis

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).[20]

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, proteintranslationand 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.[21]This could have been due to lack of selectionmechanisms prevailing in the rich environment of the host.[22][23]


Phoreticmites 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 commensalused of human social interaction. The word derives from the medieval Latin word, formed from com- and mensa, meaning "sharing a table".[13][24]

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 gastropodshells 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 parasiticrelationship is one in which one member of the association benefits while the other is harmed.[25]Parasitic symbioses take many forms, from endoparasitesthat live within the host's body to ectoparasitesthat 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. An example 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.

[edit]Symbiosis and evolution

Leafhoppersprotected by an army of meat ants

While historically, symbiosis has received less attention than other interactions such as predation or competition,[26]it is increasingly recognized as an important selective force behind evolution,[9][27]with many species having a long history of interdependent co-evolution.[28]In fact, the evolution of alleukaryotes(plants, animals, fungi, and protists) is believed under the endosymbiotic theory to have resulted from a symbiosis between various sorts of bacteria.[9][29][30]

[edit]Vascular plants

About 80% of vascular plants worldwide form symbiotic relationships with fungi, for example, in arbuscular mycorrhizas.[31]


The biologist Lynn Margulis, famous for her work on endosymbiosis, contends that symbiosis is a major driving force behind evolution. She considers Darwin'snotion of evolution, driven by competition, to be incomplete and claims that evolution is strongly based on co-operation, interaction, and mutual dependenceamong organisms. According to Margulis and Dorion Sagan, "Life did not take over the globe by combat, but by networking."[32]


Symbiosis played a major role in the co-evolutionof floweringplants and the animals that pollinatethem. Many plants that are pollinated by insects, bats, orbirds 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 speciationquickly 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,[33]where the plant species can only be pollinated by one species of insect.[34]

  1. ^ σύν, βίωσις. Liddell, Henry George; Scott, Robert; A Greek-English Lexicon at Perseus Project
  2. ^"symbiosis". Oxford English Dictionary. Oxford University Press. 3rd ed. 2001.
  3. ^ abWilkinson 2001
  4. ^Douglas 1994, p. 1
  5. ^Douglas, Angela E. (2010), The symbiotic habit, New Jersey: Princeton University Press, pp. 5-12, ISBN 978-0-691-11341-8
  6. ^Isaac 1992, p. 266
  7. ^Saffo 1993
  8. ^Douglas, Angela E. (2010), The symbiotic habit, New Jersey: Princeton University Press, p. 4, ISBN 978-0-691-11341-8
  9. ^ abcdMoran 2006
  10. ^ abcAhmadjian & Paracer 2000, p. 12
  11. ^Sapp 1994, p. 142
  12. ^Nardon & Charles 2002
  13. ^ abAhmadjian & Paracer 2000, p. 6
  14. ^Toller, Rowan & Knowlton 2001
  15. ^Harrison 2005
  16. ^Lee 2003
  17. ^Facey, Helfman & Collette 1997
  18. ^Cordes 2005
  19. ^Piper, Ross(2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
  20. ^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.
  21. ^ 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.
  22. ^Andersson, S.G.; Kurland, C.G. (1998). "Reductive evolution of resident genomes.". Trends in Microbiology 6: 263-268.
  23. ^Wernegreen, J.J. (2002). "Genome evolution in bacterial endosymbionts of insects.". Nature reviews, Genetics3: 850-861.
  24. ^Nair 2005
  25. ^Ahmadjian & Paracer 2000, p. 7
  26. ^Townsend, Begon & Harper 1996
  27. ^Wernegreen 2004
  28. ^Ahmadjian & Paracer 2000, pp. 3-4
  29. ^Brinkman 2002
  30. ^Golding & Gupta 1995
  31. ^Schüßler, A. et al. (2001), "A new fungal phylum, theGlomeromycota: phylogeny and evolution", Mycol. Res.105 (12): 1416, DOI:10.1017/S0953756201005196.
  32. ^Sagan & Margulis 1986
  33. ^Harrison 2002
  34. ^Danforth & Ascher 1997
[edit]See also[edit]References[edit]External linksWikimedia Commons has media related to: Symbiosis Look up symbiosis in Wiktionary, the free dictionary. [hide]

Inter-species biological interactions in ecology






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