Symbiosis
Symbiosis is any close and long-term biological interaction between two organisms of different species. In 1879, Heinrich Anton de Bary defined symbiosis as "the living together of unlike organisms". The term is sometimes more exclusively used in a restricted, mutualistic sense, where both symbionts contribute to each other's subsistence. This means that they benefit each other in some way.
Symbiosis is diverse and can be classified in multiple ways. It can be obligate, meaning that one or both of the organisms depend on each other for survival, or facultative, meaning that they can subsist independently. When one organism lives on the surface of another, such as head lice on humans, it is called ectosymbiosis; when one partner lives inside the tissues of another, such as Symbiodinium within coral, it is termed endosymbiosis. Where the interaction reduces both parties' fitness, it is called competition; where just one party's fitness is reduced, it is called amensalism. Where one benefits but the other is largely unaffected, this is termed commensalism. Where one benefits at the other's expense, it is called parasitism. Finally, where both parties benefit, the relationship is described as
mutualistic.
Symbiosis has often driven the evolution of species; mutualism has enabled species for example to colonise new environments. Symbiogenesis is thought to have helped to create the eukaryotes as bacteria were incorporated as mitochondria and chloroplasts within cells. Major co-evolutionary relationships include mycorrhiza, the pollination of flowers by insects, the protection of acacia trees by ants, seed dispersal by animals, nitrogen fixation by bacteria in the root nodules of legumes, and the mutualistic partnership of algae and fungi to form lichens.
Definition
The term "symbiosis" is derived from Ancient Greek συμβίωσις symbíōsis: living with, companionship < σύν sýn: together; and βίωσις bíōsis: living.The definition of symbiosis was a matter of debate for 130 years. In 1877, Albert Bernhard Frank used the term symbiosis to describe the mutualistic relationship found in lichens. In 1878, the German mycologist Heinrich Anton de Bary defined it more broadly as "the living together of unlike organisms". Over time, the definition has varied among scientists. Some have argued that it should refer only to persistent mutualisms, while others have proposed that it should include all long-term biological interactions, but exclude brief interactions such as predation.
In 1949, Edward Haskell proposed an integrative approach with a classification of "co-actions", which was later adopted by biologists as "interactions".
Types
Obligate versus facultative
Relationships can be obligate, meaning that one or both of the symbionts entirely depend on each other for survival. For example, in lichens, which consist of fungal and photosynthetic symbionts, the fungal partners cannot live on their own. The algal or cyanobacterial symbionts can usually survive alone, so their involvement is considered facultative rather than obligate. When one of the participants in a symbiotic relationship is capable of photosynthesis, as with lichens, it is called photosymbiosis.Ecto- and endosymbiosis
is a symbiotic relationship where the symbiont lives on the surface of the host, including its digestive tract or exocrine gland ducts. Examples include ectoparasites such as lice, and commensals such as barnacles on baleen whales.Contrastingly, endosymbiosis is a symbiotic relationship in which one symbiont lives within the tissues of the other, either within or between its cells. Examples include diverse microbiomes: rhizobia, nitrogen-fixing bacteria that live in root nodules on legume roots; actinomycetes, nitrogen-fixing bacteria such as Frankia, which live in alder root nodules; single-celled algae inside reef-building corals; and bacterial endosymbionts that provide essential nutrients to about 10%–15% of insects. Endosymbionts gain nutrients from their hosts, sometimes modifying the host's genome in their favour. Endosymbionts adapt to the environment inside their hosts. They often have a much reduced genome, losing protein-coding gene for metabolism and DNA repair. When insects' endosymbiotic bacteria are passed on to the insects' offspring, populations of intracellular bacteria are reduced, as compared to free-living bacteria. The endosymbiotic bacteria are unable to reinstate their wild type phenotype via a recombination process called Muller's ratchet. This, together with reduced population, leads to an accretion of deleterious mutations in the non-essential genes of the intracellular bacteria.
Competition
Competition is an interaction in which one organism's fitness is reduced by another's presence. Competition can also occur between cells within the same organism, which is why older cells are usually eliminated from tissues. This allows the organism to stay as healthy as possible by constantly eliminating old cells and making new ones. Limited supply of at least one resource used by both usually facilitates this type of interaction, although the competition can also be for other resources.Amensalism
Amensalism is a non-mutualistic, asymmetric interaction where one species is harmed or killed by the other, and one is unaffected by the other. There are two types of amensalism: competition and antagonism. Competition is where a larger or stronger organism deprives a smaller or weaker one of a resource. Antagonism 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. An example of antagonism is Juglans nigra, secreting juglone, a substance which destroys many herbaceous plants within its root zone.An example in animals is the relationship 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 a detrimental effect on weevil numbers, as they consume significant quantities of plant matter and incidentally ingest the weevils upon it.
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. It derives from a medieval Latin word meaning sharing food, formed from com- and mensa.Commensal relationships may involve one organism using another for transportation or for housing, or it may also involve one organism using something another created, after its death. Examples of metabiosis are hermit crabs using gastropod shells to protect their bodies, and spiders building their webs on plants.
Mutualism
Mutualism or interspecies reciprocal altruism is a long-term relationship between individuals of different species where both individuals benefit. Mutualistic relationships may be either obligate for both species, obligate for one but facultative for the other, or facultative for both.File:Bryolith.jpg|thumb|upright|Bryoliths document a mutualistic symbiosis between a hermit crab and encrusting bryozoans.
Many herbivores have mutualistic gut flora to help them digest plant matter, which is more difficult to digest than animal prey. This gut flora comprises cellulose-digesting protozoans or bacteria living in the herbivores' intestines. Coral reefs result from mutualism between coral organisms and various algae living inside them. Most land plants and land ecosystems rely on mutualism 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 mutualism is the relationship between the ocellaris clownfish that dwell among the tentacles of Riterri sea anemones. The territorial fish protects the anemone from anemone-eating fish, and in turn, the anemone stinging tentacles protect the clownfish from its predators. A special mucus on the clownfish protects it from the stinging tentacles.
A further example is the goby, a 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 touches the shrimp with its tail to warn it, and both quickly retreat into the burrow. Different species of gobies also clean up ectoparasites in other fish, possibly another kind of mutualism.
An example of obligate mutualism is the relationship between the siboglinid tube worms and bacteria in hydrothermal vents and cold seeps. The worm has no digestive tract and is therefore wholly reliant on its internal symbionts for nutrition. The bacteria oxidize either hydrogen sulfide or methane, which the host supplies to them.
Mutualism improves both organisms' competitive ability, enabling them to outperform members of the same species that lack the symbiont.
A facultative symbiosis is seen in encrusting bryozoans and hermit crabs. The bryozoan colony develops a cirumrotatory growth and offers the crab a helicospiral-tubular extension of its living chamber that initially was situated within a gastropod shell.