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, each termed a symbiont, may be of the same or of different
species. In 1879,
Heinrich Anton de Bary defined it as "the living
together of unlike organisms". The term was subject to a century-long
debate about whether it should specifically denote mutualism, as in
lichens; biologists have now abandoned that restriction.
Symbiosis can be obligatory, which means that one or both of the
symbionts entirely depend on each other for survival, or facultative
(optional) when they can generally live independently.
Symbiosis is also classified by physical attachment; 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. When one organism lives on another such as mistletoe, it
is called ectosymbiosis, or endosymbiosis when one partner lives
inside the tissues of another, as in
Symbiodinium in corals.
1.1 Obligate versus facultative
2 Physical interaction
5.2 Batesian mimicry
7 Cleaning symbiosis
8.2 Co-evolutionary relationships
8.2.3 Acacia ants and acacias
9 See also
13 External links
The definition of symbiosis was a matter of debate for 130 years.
Albert Bernhard Frank
Albert Bernhard Frank used the term symbiosis to describe the
mutualistic relationship in lichens. In 1879, the German mycologist
Heinrich Anton de Bary defined it as "the living together of unlike
organisms." The definition has varied among scientists with some
advocating that it should only refer to persistent mutualisms, while
others thought it should apply to all persistent biological
interactions, in other words mutualisms, commensalism, or parasitism,
but excluding brief interactions such as predation. Current
biology and ecology textbooks use the latter "de Bary" definition, or
an even broader one where symbiosis means all interspecific
interactions; the restrictive definition where symbiosis means only
mutualism is no longer used.
Edward Haskell (1949) proposed an integrative approach,
proposing a classification of "co-actions", later adopted by
biologists as "interactions".
Biological interactions can involve individuals of the same species
(intraspecific interactions) or individuals of different species
(interspecific interactions). These can be further classified by
either the mechanism of the interaction or the strength, duration and
direction of their effects.
The table lists interspecific relationships by their effects on each
Relationships between species
Effect on X
Effect on Y
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 in lichens, such as Trentepohlia, can
generally live independently, and their symbiosis is, therefore,
facultative (optional).
Main article: Physical interaction
See also: Root microbiome
Alder tree root nodule houses endosymbiotic nitrogen-fixing bacteria.
Endosymbiosis is any symbiotic relationship in which one symbiont
lives within the tissues of the other, either within the cells or
extracellularly. 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 root nodules; single-celled algae inside reef-building
corals; and bacterial endosymbionts that provide essential nutrients
to about 10%–15% of insects.
Ectosymbiosis 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.
Male-male interference competition in red deer
Main article: Competition (biology)
Competition can be defined as an interaction between organisms or
species, in which the fitness of one is lowered by the presence of
another. Limited supply of at least one resource (such as food, water,
and territory) used by both usually facilitates this type of
interaction, although the competition may also exist over other
'amenities', such as females for reproduction (in case of male
organisms of the same species).
Main article: Mutualism (biology)
Hermit crab, Calcinus laevimanus, with sea anemone.
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.
Bryoliths document a mutualistic symbiosis between a hermit crab and
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 mutualism 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
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.
When that happens both the shrimp and goby quickly retreat into the
burrow. Different species of gobies (
Elacatinus spp.) also clean
up ectoparasites in other fish, possibly another kind of
A non-obligate symbiosis is seen in 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.
Many types of tropical and sub-tropical ants that have evolved very
complex relationships with certain tree species.
Further information: Endosymbiont
In endosymbiosis, 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
A spectacular example of obligate mutualism is the relationship
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
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
DNA to RNA transcription, protein translation and DNA/RNA replication
are retained. The 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.
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. When endosymbiotic bacteria
related with insects are passed on to the offspring strictly via
vertical genetic transmission, intracellular bacteria go across many
hurdles during the process, resulting in the decrease in effective
population sizes, as compared to the free living bacteria. The
incapability of the endosymbiotic bacteria to reinstate their wild
type phenotype via a recombination process is called Muller's ratchet
Muller's ratchet phenomenon together with less effective
population sizes leads to an accretion of deleterious mutations in the
non-essential genes of the intracellular bacteria. This can be due
to lack of selection mechanisms prevailing in the relatively "rich"
commensal mites travelling (phoresy) on a fly (Pseudolynchia
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. It derives from a medieval Latin word meaning
sharing food, formed from com- (with) and mensa (table).
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
In an exploitative relationship, one partner benefits while the other
is harmed. Among the possible long-term exploitative ecological
relationships are parasitism and Batesian mimicry.
Head (scolex) of tapeworm
Taenia solium is adapted to parasitism with
hooks and suckers to attach to its host.
Main article: Parasitism
In a parasitic relationship, the parasite benefits while the host is
Parasitism takes many forms, from endoparasites that live
within the host's body to ectoparasites and parasitic castrators that
live on its surface and micropredators like mosquitoes that visit
Parasitism is an extremely successful mode of life; 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 animal species are hosts to parasites, often of
more than one species.
Main article: Batesian mimicry
This section needs expansion. You can help by adding to it. (February
Batesian mimicry is n exploitative interaction where one species has
evolved to mimic another, to the advantage of the copying species but
to the detriment of the species being mimicked.
The black walnut secretes a chemical from its roots that harms
neighboring plants, an example of antagonism.
Amensalism is an 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 (or
antibiosis). Competition is where a larger or stronger organism
deprives a smaller or weaker one from 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 (black walnut), secreting juglone, a
substance which destroys many herbaceous plants within its root
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.[citation
needed] 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
Main article: Cleaning symbiosis
This section is empty. You can help by adding to it. (February 2018)
Further information: Co-evolution
Leafhoppers protected by meat ants
Symbiosis is increasingly recognized as an important selective force
behind evolution; many species have a long history of
Main article: Symbiogenesis
Eukaryotes (plants, animals, fungi, and protists) developed by
symbiogenesis from a symbiosis between bacteria and
archaea. Evidence for this includes the fact that
mitochondria and chloroplasts divide independently of the cell, and
the observation that some organelles seem to have their own
The biologist Lynn Margulis, famous for her work on endosymbiosis,
contended that symbiosis is a major driving force behind evolution.
She considered Darwin's notion of evolution, driven by competition, to
be incomplete and claimed 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."
About 80% of vascular plants worldwide form symbiotic relationships
with fungi, in particular in arbuscular mycorrhizas.
Pollination is a mutualism between flowering plants and their animal
Entomophily and Ornithophily
Flowering plants and the animals that pollinate them have co-evolved.
Many plants that are pollinated by insects (in entomophily), bats, or
birds (in ornithophily) have highly specialized flowers modified to
promote pollination by a specific pollinator that is 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, where the plant species can only be pollinated
by one species of insect.
A fig is pollinated by the fig wasp, Blastophaga psenes
Main article: Reproductive coevolution in Ficus
Acacia ants and acacias
Pseudomyrmex ant on bull thorn acacia (
Vachellia cornigera) with
Beltian bodies that provide the ants with protein
Main article: Pseudomyrmex ferruginea
The acacia ant (Pseudomyrmex ferruginea) is an obligate plant ant that
protects at least five species of "Acacia" (Vachellia)[a] from preying
insects and from other plants competing for sunlight, and the tree
provides nourishment and shelter for the ant and its larvae.
List of symbiotic organisms
List of symbiotic relationships
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TED-Education video – Symbiosis: a surprising tale of species
Media related to
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