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 (e.g.
Trentepohlia) can usually survive alone, so their involvement is considered facultative (optional) 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 .
Ectosymbiosis 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 (such as
food,
water, and
territory) used by both usually facilitates this type of interaction, although the competition can also be for other resources.
Amensalism secretes a chemical from its roots that harms neighboring plants, an example of
antagonism. 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 (or antibiosis). 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 (black walnut), 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 mites travelling (
phoresy) on a fly (
Pseudolynchia canariensis) 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
plants.
Mutualism ,
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. s 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.
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 (
Elacatinus spp.) 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 (
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.
Parasitism Taenia solium is adapted to
parasitism with hooks and suckers to attach to its
host. In a parasitic relationship, the parasite benefits while the host is harmed. 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 intermittently. Parasitism is a successful and common mode of life. About 40% of all animal species are parasites, and the average mammal species is host to 4 nematodes, 2 cestodes, and 2 trematodes.
Mimicry Mimicry is a form of symbiosis in which a species adopts distinct characteristics of another species to alter its relationship dynamic with the species being mimicked, to its own advantage. Among the many types of mimicry are Batesian and Müllerian, the first involving one-sided exploitation, the second providing mutual benefit.
Batesian mimicry is an exploitative three-party interaction where one species, the mimic, has evolved to mimic another, the model, to
deceive a third, the dupe. In terms of
signalling theory, the mimic and model have evolved to send a signal; the dupe has evolved to receive it from the model. This is to the advantage of the mimic but to the detriment of both the model, whose protective signals are effectively weakened, and the dupe, which is deprived of an edible prey. For example, a wasp is a strongly defended model, which signals with its conspicuous black and yellow coloration that it is an unprofitable prey to predators such as birds which hunt by sight; many hoverflies are Batesian mimics of wasps, and any bird that avoids these hoverflies is a dupe. In contrast,
Müllerian mimicry is mutually beneficial as all participants are both models and mimics. For example, different species of
bumblebee mimic each other, with similar warning coloration in combinations of black, white, red, and yellow, and all of them benefit from the relationship.
Cleaning symbiosis Cleaning symbiosis is an association between individuals of two species, where one (the cleaner) removes and eats parasites and other materials from the surface of the other (the client). It is putatively mutually beneficial, but biologists have long debated whether it is mutual selfishness, or simply exploitative. Cleaning symbiosis is well known among marine fish, where some small species of
cleaner fish – notably
wrasses, but also species in other genera – are specialized to feed almost exclusively by cleaning larger fish and other marine animals. Typically, the host visits a specific 'cleaning station' and adopts a stationary posture while cleaners remove parasites. Cleaner fish play an essential role in the reduction of parasitism on marine animals. Some shark species participate in cleaning symbiosis, where cleaner fish remove ectoparasites from the body of the shark. Sharks display specific behaviours in interactions with cleaner wrasse (
Labroides dimidiatus). For example, the lemon shark remains passive and the wrasse swims to it. It scans the shark's body, inspecting the gills, labial regions, and skin. When it enters the shark's mouth, the shark ceases breathing for the inspection. When the shark begins to close its mouth, the wrasse leaves. ==Role in evolution==