SYMBIOSIS (from Greek συμβίωσις "living together", from σύν "together" and βίωσις "living ") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic , commensalistic , or parasitic . The organisms may be of the same or of different species . In 1879, Heinrich Anton de Bary defined it as "the living together of unlike organisms".
* 1 Definition * 2 Physical interaction
* 3 Mutualism
* 3.1 Mutualism and endosymbiosis
* 9 Examples * 10 See also * 11 References * 12 Bibliography * 13 External links
Albert Bernhard Frank
After 130 years of debate, current biology and ecology textbooks use the latter "de Bary" definition or an even broader definition where symbiosis means all species interactions, and the restrictive definition where symbiosis means only mutualism is no longer used.
===Obligate versus facultative===
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 . Examples of this include ectoparasites such as lice , commensal ectosymbionts such as the barnacles which attach themselves to the jaw of baleen whales , and mutualist ectosymbionts such as cleaner fish .
Mutualism or interspecies reciprocal altruism is a relationship
between individuals of different species where both individuals
benefit. 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. Bryoliths document a mutualistic symbiosis between a hermit
crab and encrusting bryozoans ; Banc d\'Arguin ,
A large percentage of herbivores have mutualistic gut flora to help them digest plant matter, which is more difficult to digest than animal prey. This gut flora is made up of cellulose-digesting protozoans or bacteria living in the herbivores' intestines. Coral reefs are the result of mutualisms between coral organisms and various types of algae which live inside them. 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.
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.
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. Different species of gobies (
Another non-obligate symbiosis is known from encrusting bryozoans and hermit crabs . 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.
A 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.
There are many types of tropical and sub-tropical ants that have evolved very complex relationships with certain tree species.
MUTUALISM AND ENDOSYMBIOSIS
During mutualistic symbioses, the host cell lacks some of the nutrients which the endosymbiont provides. 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 ensure that these genetic changes are passed onto the offspring via vertical transmission (heredity ).
As the endosymbiont adapts to the host's lifestyle the endosymbiont
changes dramatically. There is a drastic reduction in its genome size,
as many genes are lost during the process of metabolism , and DNA
repair and recombination, while important genes participating in the
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 .
A parasitic relationship is one in which one member of the association benefits while the other is harmed. This is also known as antagonistic or antipathetic symbiosis. 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.
Main article: Biological interaction § Amensalism
Amensalism is the type of relationship that exists where one species
is inhibited or completely obliterated and one is unaffected by the
other. There are two types of amensalism, competition and antibiosis.
Competition is where a larger or stronger organism 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 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 by
the sapling. 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. An example of
Amensalism is an interaction where an organism inflicts harm to another organism without any costs or benefits to the perpetrator. 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.
Synnecrosis is a rare type of symbiosis in which the interaction
between species is detrimental to both organisms involved. 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.
Leafhoppers protected by meat ants
About 80% of vascular plants worldwide form symbiotic relationships with fungi, for example, in arbuscular mycorrhizas .
Some of the following symbioses have been discussed on this page, details on the others may be found on the linked pages.
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