Difference between revisions of "Symbiosis" - New World Encyclopedia

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====Parasitism====
 
====Parasitism====
 
: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 the parasite has to break through the different layers of defense mechanism of the host for localizing on the host, establishing in the host as well as for growth and reproduction in the host, the parasite must be equipped with specific mechanisms also. Therefore, the relationship is strongly species specific, i.e., 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 blockade of food, blood, lymph, bile passages, causing tissue damages by rupture, enzymatic digestion, induction of autolysis, or secretion of toxin.
 
: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 the parasite has to break through the different layers of defense mechanism of the host for localizing on the host, establishing in the host as well as for growth and reproduction in the host, the parasite must be equipped with specific mechanisms also. Therefore, the relationship is strongly species specific, i.e., 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 blockade of food, blood, lymph, bile passages, causing tissue damages by rupture, enzymatic digestion, induction of autolysis, or secretion of toxin.
:'''''Types of Parasites''''':- Macroparasites are multicellular parasites visible to naked eyes, e.g., helminth parasites. Mesoparasites are the ones which penetrate the external openings such as the buccal cavity, cloaca, external ear. Microparasites are unicellular and invisible to naked eyes, e.g., protozoan parasites. Ectoparasites, like leech, ticks, lice are macroparasites occurring on the body surface of the host for shorter or longer period of time. Endoparasites, on the other hand, enter the body of the host via different means and remain in specific sites permanently or for longer period of time, e.g., intestinal parasites, blood parasites. Temporary parasites (leech, bed bug) visit its host only for a short period of time. Permanent parasites spent whole or a part of their life cycle in the host. Facultative parasites need association of host, but can also survive without parasitic mode of life. So, they are opportunistic. For example, parasitic nematode, ''Strongyloides stercoralis'', can also be free living.  
+
:'''''Types of Parasites''''': Macroparasites are multicellular parasites that are visible to the naked eye, e.g., helminth parasites. Mesoparasites are the ones that penetrate external openings, such as the buccal cavity, cloaca, external ear. Microparasites are unicellular and invisible to the naked eye, e.g., 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, e.g., intestinal parasites and blood parasites. Temporary parasites (leeches, bed bugs) visit their host only for a short period of time. Permanent parasites spent whole or a part of their life cycle in the host. Facultative parasites need association with host, but can also survive without the parasitic mode of life. So, they are opportunistic. For example, parasitic nematode, ''Strongyloides stercoralis'', can also be free living.  
  
:The majority of parasites are obligatory parasites and are totally dependent on the host for food, shelter, and/or protection, so they cannot survive without host. Accidental parasites are those which happen to infect unusual host, other than normal definite host. Wandering or aberrant parasite instead of arriving at the site of infection in the definitive host reach unusual place as dead end and becoming unable to complete the life cycle. For example, tape worm (''Taenia solium'') reach to brain and remains there unless operated out. 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, for example, ''Entamoeba histolytica'', but the digenetic parasite needs, in addition to primary host, also secondary host to complete the whole life cycle. ''Plasmodium vivax'' (malaria parasite) complete its asexual part of life cycle in man  and the sexual part in female ''Anopheles'' mosquito.
+
:The majority of parasites are obligatory parasites and are totally dependent on the host for food, shelter, and/or protection; they cannot survive without host. Accidental parasites are those which happen to infect unusual hosts, other than the normal definite host. Wandering or aberrant parasite, instead of arriving at the site of infection in the definitive host, reach an unusual place as a dead end and becoming unable to complete the life cycle. For example, a tape worm (''Taenia solium'') migrating to the brain remains there unless removed via an operation.  
:'''''Types of Hosts''''':- Definitive host is usually the main host. For digenetic parasites, it is the host for adult stage and for the completion of sexual part of life cycle. Intermediate or secondary host occurs only in the case of digenetic parasite for the completion of larval stage, asexual reproduction and for transmission to the definitive host. Accidental host is the one which can function as normal host but is infected only occasionally because of some reasons for example due to the lack of exposer or means of transmission. Permissive host is either definitive, intermediate or accidental host which allow parasite to complete its life cycle part or the whole. Non-permissive host, on the other hand, is host organism other than true definitive host but receiving the parasite in the position of definitive host so that the parasite find itself in a dead end. Paratenic host or transport host refer to host organism other than true intermediate host but receiving the parasite in the position of intermediate host so that the parasite are helped to go to the definitive host. For example ''Echinococcus granulosus'' normally passes to the dog through intermediate host like goat, sheep, etc. But the parasite instead of passing to intermediate host, may come to infect human being and remain in causing hydatiditis, as dog has no chance to get it from man. Reservoir host are permissive host alternative to definitive host so that infective stage can be passed from the host to the population of definitive host. Vector is usually the intermediate host playing active role in the transmission of the parasite.
+
: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, for example, ''Entamoeba histolytica'', but the 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 adult stage and for the completion of sexual part of life cycle. Intermediate or secondary host occurs only in the case of digenetic parasite for the completion of larval stage, asexual reproduction, and for transmission to the definitive host. Accidental host may be one that can function as 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 parasite to complete its life cycle in part or the whole. A non-permissive host, on the other hand, is a host organism other than true definitive host, but while receiving the parasite in the position of definitive host, the the parasite finds itself in a dead end. Paratenic host or transport host refer to host organism other than true intermediate host but receiving 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 like goat, sheep, and so forth. 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, so that infective stage can be passed from the host to the population of definitive host. Vector is usually the intermediate host playing 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 has been no record of species extinction because of parasitism. There are also examples where the parasitism help the host in fulfilling its purpose of life: existence and species continuation. Further, some parasitism result in something which has higher value than the both host and parasite together. ''Cordyceps sinesis'' is a fungus and infects in the head a type of underground caterpillar, in such a way that it finally  projects itself out of the head giving the appearance of half fungi and half caterpillar. The whole body is now known as [http://www.visitnepal.com/nepal_information/yarsagumba.php yarsagumba] (gold rush) and is very much valuable in medicine as a vitalizing agent. Further, when we carefully analyze, we find that all the human parasitic diseases are the result of failure of human responsibility.
+
:In parasitism, there is always exploitation and harm. However, such an association has been continuing and there has been no record of species extinction because of parasitism. There are also examples where the parasitism helps the host in fulfilling its purpose of life: existence and species continuation. Further, some parasitism result in something that has higher value than the both host and parasite together. ''Cordyceps sinesis'' is a fungus and infects a type of underground caterpillar in the head in such a way that it finally  projects itself out of the head, giving the appearance of half fungi and half caterpillar. The whole body is now known as yarsagumba ([http://www.visitnepal.com/nepal_information/yarsagumba.php]) (gold rush) and is very valuable in medicine as a vitalizing agent.  
  
 
====Predation====
 
====Predation====
:It is a relationship between two species but not an association and represents the obvious example of eating and being eaten process. For example, a tiger which hunts is called 'predator' and a deer which is being hunted is known as 'prey'. Even though, it is included in - + type of relationship, this relationship contribute for the well being of both predator and prey species. The predators get food and the preys are protected from mass collapse. This is a good example of individual sacrifice for species' existence and development.
+
:Predation is a relationship between two species, but not an association but rather represents the obvious example of eating and being eaten process. For example, a tiger that hunts is called a 'predator' and a deer that is being hunted is known as 'prey'. Even though, it is included in - + type of relationship, this relationship contribute for the well being of both predator and prey species. The predators get food and the preys are protected from mass collapse. This is a good example of individual sacrifice for species' existence and development.
 
====Competition====
 
====Competition====
:Interspecific competition is the relationship between the populations of two species resulting from utilization of same resources of the same place and at the same time. This interaction may not need the direct confrontation, but still seems that each is hampering the another. Because of this, some authors have catagorized competition in - - type. However, this is an interaction to set demarcation between species for coexistence or mutual survival, as every time at least one of the involved sides gets benefit in terms of species development in the dynamic environment.
+
:Interspecific competition is the relationship between populations of two species resulting from utilization of same resources of the same place and at the same time. This interaction may not need be direct confrontation, but still each is hampering the another. Because of this, some authors have catagorized competition in as type -,-. However, this is an interaction to set demarcation between species for coexistence or mutual survival, as every time at least one of the involved sides gets benefit in terms of species development in the dynamic environment.
  
 
====Allolopathic Interference====
 
====Allolopathic Interference====
:This is similar to amensalism and competition. For example, desert plant, because of scarcity of nutrients and water, secrete certain chemicals and inhibits the seed germination of other plants in the vicinity. The interference can be both intraspecific as well as interspecific. Pine tree exhibits allelopathic interference by making the surrounding soil acidic.
+
:This is similar to amensalism (see below) and competition. For example, desert plant, because of scarcity of nutrients and water, secrete certain chemicals and inhibits the seed germination of other plants in the vicinity. The interference can be both intraspecific as well as interspecific. Pine tree exhibits allelopathic interference by making the surrounding soil acidic.
  
 
===Type 0,-. Amensalism===
 
===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.
 
'''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.
  
The bread mold [[Penicillium]] is a common example of this; penicillium secrete [[penicillin]], which is a chemical that kills bacteria. A second example is the [[Walnut|Black walnut]] tree (Juglans nigra) species. Its roots secrete [[juglone]], a chemical that often kills neighboring plants.
+
The bread mold [[Penicillium]] is a common example of this; penicillium secrete [[penicillin]], which is a chemical that kills bacteria. A second example is the [[Walnut|Black walnut]] tree (Juglans nigra) species. Its roots secrete [[juglone]], a chemical that often kills neighboring plants. Plants in certain [[biome|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.
  
Plants in certain [[biome|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 a cow walks on grass to get to a river to drink. The grass is damaged, but the cow is 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.
 
 
 
 
what it is
 
Can be just biological interaction, such as when cow walks on grass on path to river (where not close, intimate interaction) or symbiosis, where is close, intimate interaction.
 
 
 
In this interspecific interaction, one species of microorganism like ''Penicillium'', by the secretion of antibiotics or antimicrobial metabolites inhibits the growth and development of other microorganisms, like bacteria, without being self affected. Because the first species avoids competitors of its resources in this way, this interaction is a - + type.
 
 
 
In reality this seems to be -,+ type in most cases when look closely
 
  
 
==Conclusion==
 
==Conclusion==
There is no such categorization of symbioses in nature as described above. In fact, it is a continuum of different degree of intimacy and cooperation between the involved species. Therefore, demarcation line between the categories are artificial and many examples surpass the boundary, e.g., remosa and shark.
+
However, mutualism, parasitism, and commensalism are often not discrete categories of interactions and should rather be perceived as a continuum of interaction ranging from parasitism to mutualism. In fact, the direction of a symbiotic interaction 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. Therefore, demarcation line between the categories are artificial and many examples surpass the boundary, e.g., remosa and shark.
 
 
However, mutualism, parasitism, and commensalism are often not discrete categories of interactions and should rather be perceived as a continuum of interaction ranging from parasitism to mutualism. In fact, the direction of a symbiotic interaction 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.
 
 
 
Conclusion: Before discussing each of the categories in detail, it is necessary to point out the conclusion that we can draw from the table above. There is no symbioses known where there is harm to both partners, or one is harmed and other no effect. This proves that the interactions among species are intentionally aimed at fulfilling dual purposes as mentioned above. Even amensalism
 
  
As it has been explained in the introduction, different species interact not only for their own benefit, but also to contribute to others. This is obvious in phoresis, commensalism and in mutualism. Mutual sharing reduces interspecific competition, avoids competitive exclusion as suggested Gause principle, promotes efficient use of resources as well as contributes to the dynamicity and diversity of the natural ecosystem, as more niches are created and new interactions are generated.
+
Note that there is no symbioses known where there is harm to both partners, or cases where one is harmed and other is not effective seem rare. This supports the view that interactions among species are aligned with the fulfilling of dual purposes as mentioned above.  As it has been discussed, different species interact not only for their own benefit, but also to contribute to others. This is obvious in phoresis, commensalism, and in mutualism. Mutual sharing reduces interspecific competition, avoids competitive exclusion as suggested in Gause principle, promotes efficient use of resources as well as contributes to the dynamicity and diversity of the natural ecosystem, as more niches are created and new interactions are generated.
  
 
Even the - + type of associations or relationships should be considered as contribution to the existence of others even at one's own expense or species development at the expense of its individual instead of simply saying exploitation. If the exploitation of one by another leads to something like yarsagumba, then we should consider it as dictated by the directive force of nature to maintain dynamicity and diversity and ultimately to serve the purpose of human being.
 
Even the - + type of associations or relationships should be considered as contribution to the existence of others even at one's own expense or species development at the expense of its individual instead of simply saying exploitation. If the exploitation of one by another leads to something like yarsagumba, then we should consider it as dictated by the directive force of nature to maintain dynamicity and diversity and ultimately to serve the purpose of human being.
  
In case of human parasites, we can see that it is the result of coupling of human beings failure to fulfill their responsibility and organisms inherent tendency to colonize new niches. Person practicing good hygienic life style does not get intestinal parasites. People with sexual discipline need not worry about HIV infection.
+
In case of human parasites, some see this as the result of coupling of human beings failure to fulfill their responsibility with organisms inherent tendency to colonize new niches. A person practicing a good hygienic life style normally does not get intestinal parasites, just as those practicing sexual discipline need not worry about sexual transmission of HIV.
  
 
The biologist [[Lynn Margulis]], famous for the 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 [[co-operation]], [[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 don't.
 
The biologist [[Lynn Margulis]], famous for the 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 [[co-operation]], [[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 don't.
 
  
 
==Further references==
 
==Further references==

Revision as of 22:38, 29 March 2006


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

Symbiosis (pl. 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, which is popularly used, symbiosis has been defined as interactive association of two species in a mutually beneficial relationship. However, biologists define it according to the original, comprehensive concept of de Barry, which is 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 (macro-species) is known as 'host' whereas the smaller partner (micro-species) is known as the symbiont (plural: symbionts) or symbiote (plural: symbiotes). In the case of cattle and the cattle egret, large ruminants are the host and the cattle egret, a small white heron, is the symbiont as it feeds on the ticks and other ectoparasites of the former, as well as insects the cattle disturb as they feed. When a microscopic symbiont lives inside the cells of a host, it is referred to as an endosymbiont.

In nature, one does not find organisms that do not relate with other organisms and the abiotic environment. It is not only that they interact for their own survival, but also they contribute to others. Each organism has value for the ecosystem in which it is living. One such inter-relationship is expressed in the form of a food web, in other words, the relationship of eating and being eaten. In this relationship, one finds a concept of dual purposes of each being: the individual purpose (eating) and whole purpose (providing value to others). The phenomena further explicates the inter-relationship of organisms. (See Interactions:Harmony and dual purposes).

Types of symbioses

A. Based on the occurrence of symbionts on the host

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

1. Ectosymbiosis

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 avertebrate and the presence of Escherichia coli on the mucosal layer of a person's intestine.

2. Endosymbiosis

It endosymbiosis, symbionts occur inside the cells or the tissue (intercellular space) of the host. A classical example of endosymbiosis is 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 other examples of endosymbiosis exist.

Many instances of endosymbiosis are obligate, where neither the endosymbiont nor the host can survive without the other, such as gutless marine worms that get nutrition from their endosymbiotic bacteria. However, not all endosymbioses are obligate. Also, some endosymbioses can be harmful to either of the organisms involved.

It is generally agreed that certain organelles of the eukaryotic cell, especially mitochondria and chloroplasts, originated as bacterial endosymbionts. This theory is known as the endosymbiotic theory, confirmed and popularized by Lynn Margulis.

B. Based on impact on symbiotic partners

Symbioses can be categorized into following types:

Effect on Species A Effect on Species B Type of interaction
0 0 Neutralism
+ + Mutualism
+ 0 Commensalism
0 + Commensalism
+ - Predation or Parasitism
- + Predation or Parasitism
- 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.

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.

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

  • (type 0,0) Neutralism is a lack of interaction. Since all species sharing an environment interact in some way, a complete lack of interaction is rarely seen in nature. However, the term can also signify a relationship in which each species derives neither benefit nor detriment to any measurable degree.
  • (type 0,+) Commensalism (including phoresis), in which one member of the association benefits while the other is not affected
  • (type +,+) Mutualism, in which the association is advantageous to both.
  • (type -,+) Parasitism (Predation, Competition, Allelopathic interference), in which the associaiton is disadvantageous or destructive to one of hte organisms and beneficial to the other.
  • (type -,0) Amensalism, in which the association is disadvantageous to one member while the other is not affected.

There are one other theoretical categories of biological interactions, but, if occuring, it would be rare and short-lived:

  • (type -,-) Synnecrosis, which would be detrimental to both species.

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, numerous songs birds feeding and nesting together in a woodland, and large number of microorganisms acting as residents in different body organs of human beings, such as skin, nose, mouth, and so forth.

A critical examinaiton 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, in the complex networks of interactions presented by ecosystems, would likely involve at least some degree of positive or negative impact on the partner. If two species do not interact directly, links can be found through other species and shared resources. Since true neutralism is rare or nonexistent, its usage is often extended to situations where interactions are merely insignificant or negligible. Thus, living together is a purposeful interaction of different degree of intimacy.

Type 0, +. Phoresis, Commensalism

In this symbiosis, the host is said to experience neither benefit nor harm, whereas the symbiont gets benefit ranging from simple transport to protection, food, and shelter.

Phoresis (Phoresy)

Phoresis is the simplest form of 0,+ symbiosis, occurring mainly among species sharing the same ecological niche. Phoresis involves transport of one organism by another. Two organisms come in contact by chance, establish fairly a loose mechanical association, and the smaller one (specially known as phoront) is carried by the transport host (mechanical vector). For example, food and water-borne disease organisms (virues, bacteria, protozoans, and multicellular parasites) are transported from their point of sources to the destination by flies and other insects because of direct contacts. Fungi and free living nematodes are carried by dung beetles as they share the same common niche. Remosa (sucker fish) is equipped with a sucker disk on the dorsal side of its head. With the help of the disc, the fish attaches itself to the belly of larger fish so as to get benefit of faster movement of the latter. Because it can also get some food scraps of the larger fish, this association can also be considered as commensalism.

Commensalism

File:Oncidium-longipes.jpg
An orchid plant perched on a tree
The term "commensalism," coined to mean "eating at the same table," 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. Here, only commensal derives benefit of sharing space, substrate, defense, shelter, transport, and/or food 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. Entamoeba gingivalis is also a commensal in the human mouth.
Examples of metazoan commensals include barnacles on whale, epiphytes on tree, and so forth. Sedentary adult barnacles attach to a whale, or any other moving objects, and get renewed sources of food and oxygen supply without giving any trouble to the carrying host. Similarly, epiphytic plants like orchids and bromeliads grow perched on sturdier trees for better exposer to sunlight and for absorbing moisture and nutrients from air.
Even though innumerable examples of commensalism have been recognized, a closer inspection draws the conclusion that true commensal relationships tends to be rather rare. Pin worm in the large intestine of human being has been considered as a commensal, but at the time of its reproduction, it intentionally causes itching at the anal region. Shrimp is a commensal on sea cucumber for passive transport. From time to time, it leaves the host for food. But it 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 seems to be, in reality, either mutualism or parasitism.

Type +, + 'Mutualism'

Mutualism is the species specific interaction of two organisms for mutual support and help. At times, the term symbiosis is used specifically for this + + type of association, synonymously with mutualism. However, considering the overall interspecific associations, mutualism is the most appropriate term for this type of symbiosis. It is a loose to intimate, and facultative to obligatory interaction, in which both involved partners are called 'mutuals'. When the relationship is intimate and obligatory, each mutual is physiologically (metabolically) dependent on another for survival and reproduction. It means that one derives benefit of another with respect to developmental stimuli, nutrition, digestive enzyme, and maturation factors, so neither can survive and reproduce successfully in absence of another partner. Examples of mutualism can be categorized as follow:

Nutritional Symbioses

Both mutuals contribute to each other either simple organic nutrients, inorganic minerals, or digestive enzymes or digestion process. Some important nutritional symbioses are:
  • In nitrogen fixation, Rhizobium sps. of root and/or shoot nodules of legumes fix atmospheric nitrogen to make it available for plants, whereas the plants provides glucose and other organic nutrients as well as biological environment to the bacteria.
  • Mycorrhiza (fungus-root) has been defined as an apparent underground structure developed as a result of mutualistic association between fungi (mainly basidiomycetes) and plant root of almost all plant species. There are different degrees of complexity in association. Plants benefit of having induced root development and increased absorption of water and minerals. Fungus partner in return gets organic food supply from the plant. So mycorrhiza can be used to grow plants in efficient way, even in poor soil condition
  • Syntrophy is the obligatory association of two microorganisms for mutual production of biochemical substances that the partner needs for its physiological processes. For example, sulfur reducing bacterium, Desulfuromonas acetoxidans need sulfur for oxidizing acetic acid to produce carbon dioxide, hydrogen sulfide, and energy. Its partner, green photosynthetic bacterium (Chlorobium), in the same culture use the provided hydrogen sulfide for its photosynthesis to make its partner a continuous supply of sulfur. Based on the principle of syntrophy, effective microorganism (EM) preparation is in use for enhancing all sort of microbial processes.
  • About 50 percent of the food stuff of ruminants is composed of cellulose, which for digestion the ruminants need the assistance of protozoans (ciliates like Diplodinium sp. and Entodinium sp.) and obligate anaerobic bacteria (Ruminococcus albus, R. flavefaciens, etc.) lodged in 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 the ruminants, termites harbor protozoan (flagellates) and bacterial flora in their gastrointestinal (GI) tract for digestion of cellulose.
File:Lichen02.jpg
Lichen over the rock
  • Lichen offers a common example of nutritional symbiosis. It is the obligate and intimate, but ectosymbiotic, association of a fungus (especially ascomycetes and basidiomycetes) and an alga (green or blue green algae). The thallus (body proper) is mainly made of a close network of fungal mycelium (mycobiont) and is responsible for absorption of water, minerals, and gases and to form 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

These are the facultative and loose associations of mutuals based on the principle that one's parasites are someone's food. Here, therefore, the larger mutual cooperates with and allows smaller mutual to remove its ectoparasites like leech, ticks, and so forth—even from gills, inside of mouth, and nostrils. Some of the cleaning partners are crocodile and crocodile bird (Egyptian Plover); larger fishes and crustaceans or fishes; shark and remosa fish; buffalo and oxpecker; rhino 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 which are potentially harmful for the animal. To that end, the crocodile openly invites the bird to hunt on his body, even going so far as to open the jaws to allow the bird 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 the transport symbiosis, one mutual takes advantage of the fast and secure movement of another mutual, whereas the latter receive mainly cleaning services. The Remosa 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) of New World Tropics making use of a small insect for transport of its eggs to the vertebrate host, including man. The female parasitizes small flies and mosquitoes and glues its eggs underneath their bodies and let them free to reach to the hosts. Upon contact with the warm skin of host, larva comes out of egg, penetrates the skin and develops 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 symbiosis refer to the mutual benefit exchanged between flower-visiting small birds and insects like butterflies and bees, and the flower bearing plants, with the former getting nectar while pollinating the latter.

Defense Symbioses

An example of defense symbiosis is 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. (A special mucus on the anemone fish protects it from the stinging tentacles.)
shrimp and goby fish
Some goby fish species live in symbiosis 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 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 the survey of biological world, one can be convinced that there are innumerable cases of viable mutualistic associations enabling organisms to explore diverse ecological niches. Hence, the mutualism has enabled organisms even to colonize stressful habitats and survive successfully even at self-insufficiency, thus promoting the efficient and optimal use of biological features as well as natural resources together.

Type -, +. Parasitism, Predation, Predation, Allopatric Interference

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

Parasitism

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 the parasite has to break through the different layers of defense mechanism of the host for localizing on the host, establishing in the host as well as for growth and reproduction in the host, the parasite must be equipped with specific mechanisms also. Therefore, the relationship is strongly species specific, i.e., 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 blockade of food, blood, lymph, bile passages, causing tissue damages by rupture, enzymatic digestion, induction of autolysis, or secretion of toxin.
Types of Parasites: Macroparasites are multicellular parasites that are visible to the naked eye, e.g., helminth parasites. Mesoparasites are the ones that penetrate external openings, such as the buccal cavity, cloaca, external ear. Microparasites are unicellular and invisible to the naked eye, e.g., 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, e.g., intestinal parasites and blood parasites. Temporary parasites (leeches, bed bugs) visit their host only for a short period of time. Permanent parasites spent whole or a part of their life cycle in the host. Facultative parasites need association with host, but can also survive without the parasitic mode of life. So, they are opportunistic. For example, parasitic nematode, Strongyloides stercoralis, can also be free living.
The majority of parasites are obligatory parasites and are totally dependent on the host for food, shelter, and/or protection; they cannot survive without host. Accidental parasites are those which happen to infect unusual hosts, other than the normal definite host. Wandering or aberrant parasite, instead of arriving at the site of infection in the definitive host, reach an unusual place as a dead end and becoming unable to complete the life cycle. For example, a tape worm (Taenia solium) migrating to the brain remains 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, for example, Entamoeba histolytica, but the 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 adult stage and for the completion of sexual part of life cycle. Intermediate or secondary host occurs only in the case of digenetic parasite for the completion of larval stage, asexual reproduction, and for transmission to the definitive host. Accidental host may be one that can function as 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 parasite to complete its life cycle in part or the whole. A non-permissive host, on the other hand, is a host organism other than true definitive host, but while receiving the parasite in the position of definitive host, the the parasite finds itself in a dead end. Paratenic host or transport host refer to host organism other than true intermediate host but receiving 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 like goat, sheep, and so forth. 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, so that infective stage can be passed from the host to the population of definitive host. Vector is usually the intermediate host playing 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 has been no record of species extinction because of parasitism. There are also examples where the parasitism helps the host in fulfilling its purpose of life: existence and species continuation. Further, some parasitism result in something that has higher value than the both host and parasite together. Cordyceps sinesis is a fungus and infects a type of underground caterpillar in the head in such a way that it finally projects itself out of the head, giving the appearance of half fungi and half caterpillar. The whole body is now known as yarsagumba ([1]) (gold rush) and is very valuable in medicine as a vitalizing agent.

Predation

Predation is a relationship between two species, but not an association but rather represents the obvious example of eating and being eaten process. For example, a tiger that hunts is called a 'predator' and a deer that is being hunted is known as 'prey'. Even though, it is included in - + type of relationship, this relationship contribute for the well being of both predator and prey species. The predators get food and the preys are protected from mass collapse. This is a good example of individual sacrifice for species' existence and development.

Competition

Interspecific competition is the relationship between populations of two species resulting from utilization of same resources of the same place and at the same time. This interaction may not need be direct confrontation, but still each is hampering the another. Because of this, some authors have catagorized competition in as type -,-. However, this is an interaction to set demarcation between species for coexistence or mutual survival, as every time at least one of the involved sides gets benefit in terms of species development in the dynamic environment.

Allolopathic Interference

This is similar to amensalism (see below) and competition. For example, desert plant, because of scarcity of nutrients and water, secrete certain chemicals and inhibits the seed germination of other plants in the vicinity. The interference can be both intraspecific as well as interspecific. Pine tree exhibits 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.

The bread mold Penicillium is a common example of this; penicillium secrete penicillin, which is a chemical that kills bacteria. A second example is the Black walnut tree (Juglans nigra) species. 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 a cow walks on grass to get to a river to drink. The grass is damaged, but the cow is 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.

Conclusion

However, mutualism, parasitism, and commensalism are often not discrete categories of interactions and should rather be perceived as a continuum of interaction ranging from parasitism to mutualism. In fact, the direction of a symbiotic interaction 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. Therefore, demarcation line between the categories are artificial and many examples surpass the boundary, e.g., remosa and shark.

Note that there is no symbioses known where there is harm to both partners, or cases where one is harmed and other is not effective seem rare. This supports the view that interactions among species are aligned with the fulfilling of dual purposes as mentioned above. As it has been discussed, different species interact not only for their own benefit, but also to contribute to others. This is obvious in phoresis, commensalism, and in mutualism. Mutual sharing reduces interspecific competition, avoids competitive exclusion as suggested in Gause principle, promotes efficient use of resources as well as contributes to the dynamicity and diversity of the natural ecosystem, as more niches are created and new interactions are generated.

Even the - + type of associations or relationships should be considered as contribution to the existence of others even at one's own expense or species development at the expense of its individual instead of simply saying exploitation. If the exploitation of one by another leads to something like yarsagumba, then we should consider it as dictated by the directive force of nature to maintain dynamicity and diversity and ultimately to serve the purpose of human being.

In case of human parasites, some see this as the result of coupling of human beings failure to fulfill their responsibility with organisms inherent tendency to colonize new niches. A person practicing a good hygienic life style normally does not get intestinal parasites, just as those practicing sexual discipline need not worry about sexual transmission of HIV.

The biologist Lynn Margulis, famous for the 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 co-operation, 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 don't.

Further references

General Information on Parasitology With Particular Reference to Parasitic Helminths

Symbiosis

Symbiotic Relationships


See also

  • List of symbiotic relationships


References
ISBN links support NWE through referral fees

  • Lynn Margulis and Dorion Sagan, Microcosmos: Four Billion Years of Evolution from Our Microbial Ancestors. Summit Books, New York, 1986. ISBN 0520210646
  • Jan Sapp Evolution by Association, Oxford University Press, 1994. ISBN 0195088212


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