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Though there exist several overlapping definitions for the term 'symbiotic relationship', it is specifically used to refer to 'symbiosis', i.e., the close interaction between different biological species for a specific period. On the basis of what relationship the two organisms involved share, symbiotic relationships are further categorized into three types: mutualism, commensalism, and parasitism.

Symbiotic Relationships: An Overview

While biological interactions like 'predation' and 'competition' have long fascinated researchers in the past, 'symbiosis' has started to get its due of late. In fact, many researchers are of the opinion that symbiosis has been the selective force behind evolution of lifeforms on the planet. The term 'symbiosis', which means 'living with each other', has been derived from the combination of Greek words: 'syn' meaning 'with' and 'biosis 'meaning 'living'.

Even though the concept of 'living with each other' forms the basis of symbiotic relationships, one has to understand that the interaction need not necessarily be beneficial for both the organisms involved. Given below are the details of each of the three types, which will help you get a better understanding of the entire concept.

Mutualism
In this case, the two biologically different organisms interact with each other in such a manner that both get benefited from the interaction. The terms mutualism and symbiosis are often used interchangeably, which is technically incorrect, as mutualism or mutualistic relationship is just a type of symbiotic relationship. Mutualism is further categorized into three different types on the basis of resource-service relationship between the two species involved.
  • Trophic mutualism - Both species involved benefit in the form of resources.
  • Dispersive mutualism - One organism gets resource benefit, while other gets benefited in the form of service.
  • Defensive mutualism - Both organisms involved provide service benefits to each other.
One of the best examples of mutualism is the interaction between angiosperms (flowering plants) and insects, wherein insects derive food from the plants and, in turn, help them reproduce. Yet another example is the relationship between impala and the Red-billed oxpecker, wherein the oxpecker cleans the imapala's body of parasites and gets food in return for this service.

Commensalism
As opposed to mutualism, in commensalism only one of the two species involved gets benefited from the interaction, while the other remains unaffected. Explaining how commensalism works can be a bit difficult considering that it is easier to show that an organism is affected than to prove that it is unaffected. As in case of mutualism, even commensalism is broadly categorized into three different types on the basis of the underlying reason for interaction.
  • Phoresy - One species uses another for the purpose of transportation.
  • Inquilinism - One species uses another for the purpose of housing/shelter.
  • Metabiosis - One species indirectly creates environment suitable for another.
One of the best examples of commensalism is the relationship between the tiger and jackal, wherein the jackal follows the tiger at a safe distance and feeds on whatever is left after the tiger is done with his kill. Yet another example of the same is the relationship shared by the cattle egret and cattle, wherein the egret feeds on the insects that come to the surface as a result of foraging by cattle. In these examples, jackal and egret are beneficiaries of the interaction, while the tiger and cattle remain unaffected.

Parasitism
The term parasitism refers to a symbiotic relationship wherein one of the two species involved gets benefited from the interaction, while the other is harmed. Simply put, one organism benefits at the expense of the other organism involved in the interaction. In this case, the organism which benefits from the interaction is referred to as a 'parasite', while the one which is adversely affected is called the 'host organism'.

In a typical parasitic relationship, the parasite sticks to the host organism and derives the nutrients that it requires to survive. Even though the host organism is harmed to some extent, it is not killed as a result of this interaction. As opposed to this is the concept of parasitoidism, wherein the parasite attaches itself to the host organism for a specific period and eventually kills it.

When we talk about the examples of parasitism, one of the apt species to take into consideration is the ocean sunfish (Mola mola) which is known to carry several parasites, including sea lice and shark tapeworm, on its body. On the other hand, one of the best examples of parasitoidism is the relationship between the phorid fly and leaf-cutter ants, wherein the phorid fly attacks a leaf-cutter ant and lays an egg in the crevice in its head. After hatching, the larvae burrows into the ants body and feeds on the ant from within, eventually killing it.

At times, symbiotic relationships are also categorized into obligate and facultative relationships. In 'obligate symbiotic relationships', one organism is totally dependent on its interaction with the other for survival. In 'facultative symbiotic relationship', on the other hand, the organism is dependent, but can also do without this interaction.