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Common Clownfish (Amphiprion ocellaris) in their magnificent sea anemone (Heteractis magnifica) home.

Symbiosis (plural: "symbioses") is the close, interactive association (living together) of members of two or more species. The term symbiosis was coined by the German botanist Anton de Bary in 1879 from the Greek symbioun (to live together), from the prefix sym (sum, together, or together with) and the word bios (life). In the narrowest sense of the term, as popularly used, symbiosis has been defined as the interactive association of two species in a mutually beneficial relationship. However, biologists define it according to the original, comprehensive concept of de Bary: the living together of members of two or more species in all the various forms, whether the interaction is beneficial, harmful, or neutral to one or both organisms.

In a symbiosis, the larger partner (macrospecies) is known as the “host,” whereas the smaller partner (microspecies) is known as the symbiont (plural: "symbionts") or symbiote (plural "symbiotes"). In the case of cattle and the cattle egret (a small white heron), large ruminants are the host and the cattle egret is the symbiont, feeding on the ticks and other ectoparasites of the former, as well as insects that the cattle disturb as they feed.

In nature, there are no organisms that do not interact at some level with both other organisms and the abiotic environment. While organisms tend to interact for their own survival, one or both of the interacting organisms often will also contribute to the benefit of other third parties—each organism has value for the larger ecosystem in which it is immersed. A typical interrelationship is expressed in the form of a food web showing the relationships of eating and of being eaten. In this relationship, one finds a concept of dual purposes for each being: the individual purpose (eating) and whole purpose (providing value to others). This principle of organisms having both individual and whole purposes is further explicated by the phenomena of symbiosis. (See Interactions:Harmony and dual purposes).

Types of symbioses

A. Classification based on location of symbionts relative to host

One manner of classifying symbioses is according to the physical location of the symbionts. Two types of symbioses are recognized:


In ectosymbiosis, symbionts occur on the body surface of the hosts. The body surface can be an external surface, such as the skin, but also includes internal surfaces, such as the linings of digestive canal and respiratory tract, the ducts of glands, and so forth. Examples include the attachment of a leech to the skin of a vertebrate and the presence of Escherichia coli on the mucosal layer of a person's intestine.


In endosymbiosis, symbionts occur inside the cells or the tissue (intercellular space) of the host. A classical example of endosymbiosis is the presence of nitrogen fixing bacteria (Rhizobium sps.) living in root nodules of leguminous plants. Other examples include reef-building corals containing single-celled algae, and several insect species containing bacterial endosymbionts.
Many instances of endosymbioses are obligate, where neither the endosymbiont nor the host can survive without the other, such as "gutless marine worms" that get nutrition from endosymbiotic bacteria. However, not all endosymbioses are obligate. Furthermore, some endosymbioses can be harmful to both organisms involved.
One theory is that certain organelles of the eukaryotic cell, especially mitochondria and chloroplasts, originated as bacterial endosymbionts. This theory is known as the endosymbiotic theory, as postulated and popularized by Lynn Margulis.

B. Classification based on impact on symbiotic partners

Table 1 shows several types of symbioses based on the impact (positive, negative, or neutral) on the symbiotic partners.

Effect on Species A Effect on Species B Type of interaction
0 0 Neutralism
+ + Mutualism
+ 0 Commensalism
0 + Commensalism
+ - Parasitism or Predation
- + Parasitism or Predation
- 0 Amensalism
0 - Amensalism
Table 1. Some types of relationships listed by the effect they have on each partner. '0' is no effect, '-' is detrimental, and '+' is beneficial.

From the table, we can derive five main categories of symbiotic relationships among organisms. They are:

  • (Type 0,0). Neutralism is a lack of benefit or detriment experienced by either members of the pair of interacting organisms.
  • (Type 0,+). Commensalism occurs when one member of the association benefits while the other is not affected. Type 0,+ includes phoresis, which is the transport of one species by another.
  • (Type +,+). Mutualism occurs when the symbiotic association is advantageous to both members of the pair.
  • (Type -,+). Parasitism (as well as predation, competition, and allelopathic interference) is a situation in which the association is disadvantageous or destructive to one of the organisms and beneficial to the other.
  • (Type -,0). Amensalism is found when the association is disadvantageous to one member while the other is not affected.

There is one other theoretical category of biological interactions, but, if occurring, it would be rare and short-lived:

  • (Type -,-). Synnecrosis occurs when an interaction is detrimental to both species.

It is important to note that these interactions are not always static. In many cases, two species will interact differently under different conditions. This is particularly true in, but not limited to, cases where species have multiple, drastically different life stages.

Descriptions of types of symbiosis

Type 0,0. Neutralism

Neutralism is the term used in ecology to signify a biological interaction in which the population density of two species appear to have no effect whatsoever on each other. Examples of this type of symbiosis include pelicans and cormorants feeding in the ocean, spiders and mantis preying on insects of the same bush, diverse songbirds feeding and nesting together in a woodland, and a large number of microorganisms acting as residents in different body organs of human beings, such as skin, nose, mouth, and so forth.

A critical examination of each of the examples of neutralism leads to the conclusion that true neutralism is unlikely to exist and would be difficult, or impossible, to prove. It seems that living together—within the complex networks of interactions presented by ecosystems—would likely involve at least some degree of positive or negative impact of one species on the other. If two species do not interact directly, links can be found via other species and shared resources. Since true neutralism is rare or nonexistent, the term is often extended to situations where interactions are merely insignificant or negligible.

Type 0,+. Phoresis, commensalism

In a Type 0,+ symbiosis, the host is said to experience neither benefit nor harm, whereas the symbiont receives benefit, which ranges from simple transport to protection, food, and shelter.

Phoresis (Phoresy)

Phoresis (meaning "to carry") is the simplest form of type 0,+ symbiosis, occurring mainly among species sharing the same ecological niche. Phoresis involves transport of one organism by another. There is no physiological or biochemical dependence between the host and symbiont. Two organisms come in contact by chance, establish a fairly loose mechanical association, and the smaller one (known as the phoront) is carried by the transport host (mechanical vector). For example, food and water-borne disease organisms (viruses, bacteria, protozoans, and multicellular parasites) are transported from their source to the destination by flies and other insects because of direct contacts. Fungi and free living nematodes are carried by dung beetles, with which they share a common niche. The sucker fish Remora is equipped with a sucker disk on the dorsal side of its head. With the help of this disc, the fish attaches itself to the belly of larger fish so as to get benefit of faster movement of the latter. Because the Remora fish also can obtain some food scraps of the larger fish, this association can also be considered as commensalism.


Commensalism is a loose, usually facultative (neither dies if the relationship ends), and interspecific association between a larger "host" and a smaller "commensal," without any metabolic dependency. Only the smaller commensal derives benefits, such as sharing space, substrate, defense, shelter, transport, and/or food, and does so without helping or harming the host. In some cases, the interaction is obligatory for the commensal. Escherichia coli, and a variety other of bacteria, are harbored comfortably in the large intestine of people to feed on nutrients that are outgoing at any rate. There is no obvious harm or benefit to the host. (Although some benefits have been recognized as contributed by certain bacteria that live within the human digestive system, such as aiding immunity, synthesizing certain vitamins, and fermenting complex indigestible carbohydrates(O'Hara and Shanahan 2006)). Entamoeba gingivalis is also a commensal in the human mouth. Commensalism means "eating at the same table," graphically conveying a situation whereby the commensal is able to feed on food that is not being consumed by the host—offering benefit to the commensal without impacting the host.
Examples of metazoan commensals include barnacles on a whale and epiphytes on a tree. Sedentary adult barnacles attach to a whale, or other large moving objects, and get renewed sources of food and oxygen supply without giving causing obvious harm to the carrying host. Similarly, epiphytic plants like orchids and bromeliads grow perched on sturdier trees for better exposure to sunlight and for absorbing moisture and nutrients from the air.
Even though innumerable examples of commensalism have been recognized, a closer inspection suggests that true commensal relationships tends to be rather rare. For example, the pinworm in the large intestine of human beings has been considered as a commensal relationship. In reality, at the time of the pinworms reproduction, it intentionally causes itching in the anal region. Shrimp has been considered to be commensal on the sea cucumber, using it for passive transport. From time to time, the shrimp leaves the host for food. But it also has been pointed out that the commensal may feed on the parasites of the host body surface and also provide protection to the host because of its warning coloration. Thus, commensalism in many cases appears in reality to fall into the categories of either mutualism or parasitism.

Type +,+. Mutualism

Mutualism is the species-specific interaction of two organisms for mutual benefit. At times, the term symbiosis is used specifically for this type +,+ association, using the term synonymously with mutualism. However, this type of interaction is more appropriately termed mutualism, given the diverse associations to which biologists apply the term symbiosis.

Mutualism is a type of biological interaction that may range from loose to intimate, and from facultative to obligatory with both involved partners being called "mutuals." When the relationship is intimate and obligatory, each mutual is physiologically (metabolically) dependent on another for survival and reproduction. This means that members in the association derive key benefits from the other with respect to development, stimuli, nutrition, digestive enzymes, and maturation factors, such that neither of the mutuals can survive and reproduce successfully in the absence of the other. Examples of mutualism can be categorized as follow:

Nutritional mutualism

In "nutritional mutualism," both mutuals contribute to each other either simple organic nutrients, inorganic minerals, or digestive enzymes for the digestion process. Some important examples of nutritional mutualism are:
  • In nitrogen fixation, the bacteria Rhizobium sps. in the root and/or shoot nodules of legumes fix atmospheric nitrogen to make it available for plants. Meanwhile, the plants provide glucose and other organic nutrients, as well as a biological environment to the bacteria.
  • Mycorrhiza (fungus-root) has been defined as an apparent underground structure developed as a result of the mutualistic association between fungi (mainly basidiomycetes) and the roots of almost all plant species. There are different degrees of complexity in the association. Plants benefit by having enhanced root development and increased absorption of water and minerals. The fungus, in return, receives an organic food supply from the plants. Mycorrhiza can be used to enhance the growth of plants, even under poor soil conditions.
  • Syntrophy is the obligatory association of two microorganisms for mutual production of biochemical substances that the partners need for physiological processes. For example, sulfur reducing bacteria Desulfuromonas acetoxidans need sulfur for oxidizing acetic acid to produce carbon dioxide, hydrogen sulfide, and energy. Their partner organisms, green photosynthetic bacteria (Chlorobium), use the provided hydrogen sulfide for photosynthesis, producing a continuous supply of sulfur. Based on this principle of syntrophy, effective microorganism (EM) preparation is in use for enhancing all sorts of microbial processes.
  • About 50 percent of the food stuff of ruminants is composed of cellulose, which can be digested by the ruminants only with the assistance of protozoans (ciliates like Diplodinium sp. and Entodinium sp.) and obligate anaerobic bacteria (Ruminococcus albus, R. flavefaciens, etc.) lodged in the rumen (a special compartment) of their stomach. As a result of the anaerobic milieu, the bacterial fermentation produces fatty acids, carbon dioxide, and hydrogen gas that are used by methanogenic bacteria (Methanobacterium ruminantium) present there to produce methane. Thus, ruminants are an important source of methane in the atmosphere. Like ruminants, termites harbor protozoan (flagellates) and bacterial flora in their gastrointestinal (GI) tract for digestion of cellulose.
  • Lichen offers a common example of nutritional symbiosis. Lichens are symbiotic organisms made up by the association of microscopic green algae or cyanobacteria and filamentous fungi. They co-exist in an obligate and intimate, but ectosymbiotic, association. The thallus (body proper) of lichen is mainly made of a close network of fungal mycelium (mycobiont) and is responsible for absorption of water, minerals, and gases, as well as forming a protective tissue structure for algal partner. Algal cells (phycobiont), which constitute about 10 percent of the thallus, are embedded in the compact mass of mycelium and are responsible for photosynthesis, nitrogen fixation, and production of food for both partners.

Cleaning symbioses

Cleaning symbioses are facultative and loose associations of mutuals based on the principle that one's parasites are someone else's food. In this case, the larger mutual cooperates with and allows the smaller mutual to remove its ectoparasites, like leeches, ticks, and so forth—even from gills, inside of mouth, and nostrils. Some of the cleaning partners include larger fish and crustaceans or fishes; shark and remora fish; buffalo and oxpecker; rhinocherus and tick bird; and cattle and cattle egret.
A famous land version of symbiosis is the relationship of the Egyptian Plover bird and the crocodile. In this relationship, the bird is well known for preying on parasites that feed on crocodiles and that are potentially harmful for the animal. To that end, the crocodile openly invites the bird to hunt on its body, even going so far as to open the jaws to allow the bird to enter the mouth safely to hunt. For the bird's part, this relationship not only is a ready source of food, but a safe one considering that few predator species would dare strike at the bird at such close proximity to its host.

Transport symbioses

In transport symbiosis, one mutual takes advantage of the fast and secure movement of another mutual, whereas the latter receives mainly cleaning services. The Remora fish and shark, and the shrimp and sea cucumber, have already been described above. Another typical example of this association is the torsalo fly (Dermatobia hominis) making use of a small insect for transport of its eggs to the vertebrate host, including humans. The female parasitizes small flies and mosquitoes and glues its eggs underneath their bodies, then frees them to reach to the host. Upon contact with the warm skin of the host, the larvae come out of the eggs, penetrate the skin and develop underneath the skin of the host. This association can also be considered as a type of parasitism.

Pollination symbioses

Rufous humming bird on a flower.
Pollination symbioses refer to the mutual benefit exchanged between flower-visiting small birds, and insects like humming birds, butterflies and bees, with flower bearing plants, with the former receiving nectar while pollinating the latter.

Defense symbioses

An example of defense symbiosis is exemplified by the relationship between clownfish of the genus Amphiprion (family: Pomacentridae) that dwell among the tentacles of tropical sea anemones. The territorial fish protects the anemone from anemone-eating fish, and in turn the stinging tentacles of the anemone protect the anemone fish from its predators. Special mucus on the clownfish protects it from the stinging tentacles.
Shrimp and goby fish.
Some goby fish species live in symbiosis with a shrimp. The shrimp digs and prepares 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 of imminent danger. When that happens, both the shrimp and goby fish quickly retract into the burrow.

From a survey of the biological world, it is clear that there are innumerable cases of viable mutualistic associations enabling organisms to explore diverse ecological niches. Mutualism even has enabled organisms to colonize stressful habitats and survive successfully even at self-insufficiency. Overall, this promotes the efficient and optimal use of biological features and natural resources.

Type -,+. Parasitism, predation, competition, allelopathic interference

This association involves obvious harm to one partner for the benefit of another. This type of relationship occurs in many forms.


Parasitism may be defined as an interspecific association in which one species, the parasite, lives on or in a second species, the host, for a significant period of its life and exploits the host to obtain nourishment, shelter, and/or protection. This relationship can be loose or mostly intimate, and for the parasite, it is usually obligatory.
As a parasite has to break through the different layers of defense mechanisms of the host in order to localize and thrive on the host, the parasite must be equipped with specialized mechanisms. The relationship, therefore, is strongly species specific, in other words, a particular parasite can infect only a particular host, not any other. The parasite causes harm to the host in many ways, such as by depriving the host of digested food; erecting mechanical blockades of food, blood, lymph, and bile passages; and causing tissue damages by rupture, enzymatic digestion, induction of autolysis, or secretion of toxins.
Types of Parasites
Macroparasites are multicellular parasites that are visible to the naked eye, such as helminth parasites (parasitic worms, such as flukes, tapeworms, and roundworms or nematodes). Mesoparasites are the ones that penetrate external openings, such as the buccal cavity, cloaca, external ear, and so forth. Microparasites are unicellular and invisible to the naked eye, such as protozoan parasites.
Ectoparasites, like leeches, ticks, and lice, are macroparasites occurring on the body surface of the host. Endoparasites, on the other hand, enter the body of the host via different means and remain in specific sites for a long period of time, such as intestinal parasites and blood parasites.
Temporary parasites (leeches, bed bugs) visit their host only for a short period of time. Permanent parasites spend the whole or a part of their life cycle in the host.
Facultative parasites can survive without the parasitic mode of life, but can adapt to it if placed in such a situation; they are opportunistic. For example, the parasitic nematode, Strongyloides stercoralis, can also be free living. The majorities of parasites are obligatory parasites and are totally dependent on the host for food, shelter, and/or protection; they cannot survive without the host. Accidental parasites are those that happen to infect unusual hosts, other than the normal definite host. Wandering or aberrant parasites, instead of arriving at the site of infection in the definitive host, reach an unusual place as a dead end and are unable to complete the life cycle. For example, the tapeworm Taenia solium may migrate to the brain and remain there unless removed via an operation.
When a parasite is being parasitized by another organism, then the latter one is known as hyperpasasite or secondary parasite. It can be used for biological control of the pest and parasites. Monogenic parasites complete the whole life cycle in one host, such as with Entamoeba histolytica. A digenetic parasite needs, in addition to a primary host, also a secondary host to complete the entire life cycle. Plasmodium vivax (malaria parasite) completes its asexual part of life cycle in people and the sexual part in the female Anopheles mosquito.
Types of Hosts
A definitive host is usually the main host. For digenetic parasites, it is the host for the adult stage and for the completion of the sexual part of the life cycle. An intermediate or secondary host is a temporary environment, but one that is essential for the completion of a particular parasite's life cycle. Such a host is found only in the case of digenetic parasites for the completion of larval stage, asexual reproduction, and for transmission to the definitive host. An accidental host may be one that can function as the normal host, but is infected only occasionally for some reason, for example due to the lack of exposure or means of transmission.
A permissive host is either a definitive, intermediate, or accidental host that allows the parasite to complete its life cycle in part or in whole. A non-permissive host, on the other hand, is a host organism other than true definitive host, which receives the parasite but the parasite finds itself in a dead end.
A paratenic host or transport host is a host organism other than true intermediate host that receives the parasite in the position of intermediate host so that the parasite is helped to go to the definitive host. For example Echinococcus granulosus normally passes to a dog through an intermediate host, such as a goat or sheep. But the parasite, instead of passing through the intermediate host, may come to infect a human being and remain, causing hydatiditis, and a dog has no chance to get it from a person. Reservoir hosts are permissive host alternatives to definitive hosts, such that the infective stage can be passed from the host to the population of the definitive host. A vector is usually the intermediate host playing an active role in the transmission of the parasite.
In parasitism, there is always exploitation and harm. However, such an association has been continuing, and there are no known cases of species extinction traced to parasitism. There are examples where the parasitism would appear to help the host in fulfilling a life purpose: existence and species continuation. Furthermore, some parasitism appears to result in something that has higher value than the both host and parasite together. Cordyceps sinesis is a fungus that infects a type of underground caterpillar in the head in such a way that it ultimately projects itself out of the head, giving the appearance of half fungi and half caterpillar. The whole body is now known as yarsagumba (gold rush) and is very valuable in medicine as a vitalizing agent.


Predation is a close relationship between two species, but one that represents the example of eating and being eaten. For example, a tiger that hunts is called a "predator" and a deer that is being hunted is known as the "prey." Even though predation is included as a type -,+ interaction, in the broad view this relationship actually contributes to the well being of both predator and prey species. Predators get food and prey species are protected from the mass collapse that could occur if the prey species proliferates excessively beyond the ecosystem's carrying capacity. This is an example of individual sacrifice for species' existence and development. If symbiosis is viewed as a prolonged, intimate relationship between species, then this does not qualify as symbiosis because it is short-term.


Interspecific competition is the relationship between populations of two species resulting from utilization of the same resources in the same place and at the same time. This interaction need not be direct confrontation, yet each is hampering the other. Because of this, some authors have categorized competition as type -,-. Again, however, this should not be looked at as only a harmful relationship. Rather, this interaction sets a demarcation between species for coexistence or mutual survival, as at least one of the involved sides receives benefit in terms of species development in the dynamic environment. Similar to predation, if symbiosis is considered to be a prolonged, intimate relationship, then competition is not viewed as symbiosis because it is not an intimate association.

Allelopathic interference

The term "allelopathic" refers to effect of one plant species on another. "Allelopathic interference" is a harmful effect of one plant on another. It is similar to amensalism (see below) and competition. For example, desert plants, because of scarcity of nutrients and water, may secrete certain chemicals and inhibit the seed germination of other plants in the vicinity. The interference can be both intraspecific as well as interspecific. Pine trees exhibit allelopathic interference by making the surrounding soil acidic.

Type 0,-. Amensalism

Amensalism is a biological interaction, a type of symbiosis, between two species in which one impedes or restricts the success of the other without being affected, positively or negatively, by the presence of the other. Usually this occurs when one organism exudes a chemical compound as part of its normal metabolism that is detrimental to another organism.

An example is the bread mold Penicillium that produces the penicillin antibiotic, a chemical that kills bacteria. A second example is the black walnut tree (Juglans nigra). Its roots secrete juglone, a chemical that often kills neighboring plants. Plants in certain biomes, such as the chaparral or desert, are very dependent on the effects of amensalism. It stabilizes the community by reducing competition for scarce nutrients in the water.

The term amensalism also refers to biological interactions in which there is not an intimate relationship between species (thus, not a symbiosis), such as when cattle trample grass on the way to a water hole to drink. The grass is damaged, but the cows are unaffected.

Although amensalism is listed as a type 0,- interaction, most examples given for symbiosis are more a type +,- interaction. For example, in the cases of excreting a substance, the first species is able to limit competition for resources.


The categorization of symbiosis types (mutualism, parasitism, commensalism, etc.) is an artificial construct. In reality, biological interactions do not always fit into such discrete categories. Rather, symbioses should be perceived as a continuum of interactions ranging from parasitism to mutualism. Even the direction of a symbiotic relationship can change during the lifetime of the symbionts due to developmental changes, as well as changes in the biotic/abiotic environment in which the interaction occurs. Demarcation lines between categories are not always so clear-cut, and many examples surpass the boundary, such as seen between a remora and its shark host.

What is notable is that there are no known symbioses where there is harm to both partners, and cases of symbioses where one partner is harmed and other is unaffected seem rare. This supports the view that interactions among species are aligned with the principle of dual purposes—living for the individual and for higher purpose, as mentioned above. Species interact not only for their own benefit, but also contribute to others. In the case of phoresis, commensalism, and mutualism, this is obvious. Mutual cooperation reduces interspecific competition, avoids competitive exclusion as suggested by the Gause principle, and promotes efficient use of resources, as well as contributes to the diversity of the natural ecosystem, as more niches are created and new interactions are generated.

However, even type -,+ associations or relationships can be viewed as more than simply exploitation of one species by another, but rather as a member of one species contributing to the existence of others even at one's own expense, or advancing species development at the expense of the individual. If exploitation leads to something helpful to humans,like yarsagumba, then it might be perceived as serving the purpose of the human being.

In the case of human parasites, some tie this phenomenon to failure of human beings themselves. Other organisms naturally have an inherent tendency to colonize new niches. However, a person practicing an hygienic lifestyle, such as exhibiting care about water and food consumed (cooking meat properly, etc.) can limit the possibility of getting intestinal parasites. There also is a larger social responsibility tied to such actions as educating the public how parasites are transmitted, proper treatment fo sewage, and so forth.

Biologist Lynn Margulis, famous for her work on endosymbiosis, contends that symbiosis is a major driving force behind evolution. She considers Darwin's notion of evolution, driven by competition, as incomplete, and claims evolution is strongly based on cooperation, interaction, and mutual dependence among organisms. According to Margulis and Sagan (1986), "Life did not take over the globe by combat, but by networking." As in humans, organisms that cooperate with others of their own or different species often outcompete those that do not.

ISBN links support NWE through referral fees

  • Margulis, L., and D. Sagan. 1986. Microcosmos: Four Billion Years of Evolution from Our Microbial Ancestors. New York: Summit Books.
  • Nair, S. 2005. Bacterial associations: Antagonism to symbiosis. In Marine Microbiology: Facets and opportunities (Editor: Nagappa Ramaiah). Goa, India: National Institute of Oceanography, pp. 115-124.
  • O'Hara, A., and F. Shanahan. 2006. The gut flora as a forgotten organ. EMBO Rep 7(7): 688-93. PMID 16819463.
  • Sapp, J. 1994. Evolution by Association. Oxford University Press.


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