Mutualism, Commensalism, Parasitism: Types Of Symbiosis With Examples

Some plants are pollinated by hummingbirds through mutualism, a form of symbiosis. Photo: Charlesjsharp via Wikimedia

Symbiosis is broken down into mutualism, commensalism, and parasitism based on how two species interact in their ecosystem. Mutualism is where both organisms benefit, commensalism is where one benefits but the other organism isn’t harmed, and lastly, parasitism is where one organism benefits and the other is harmed.

The various species found within a single ecosystem can relate to each other in a variety of ways. The terms mutualism, commensalism, parasitism and symbiosis all refer to the various ways that species within an ecosystem can interact with one another. What are the distinctions between these different terms?

Two different organisms which live in the same locale can have a wide range of interactions with one other. One organism can benefit from the other while the other isn’t harmed, they can help each other to their mutual benefit, or one organism can feed off of the other organism. These ways of interacting are all referred to as symbiosis. Symbiosis is an umbrella term referring to any long-term interaction between two organisms that share a close physical space. Mutualism, parasitism, and commensalism are all different types of symbiosis. Let’s take a closer look at the three different kinds of symbiosis.

Mutualism

Photo: Public domain

A mutually symbiotic relationship is any relationship between two organisms where both organisms benefit. It is mutually beneficial. Mutually symbiotic relationships can even extend to the point where both organisms need each other to survive. Mutualistic relationships confer a number of benefits to the organisms in them, including protection and nutrition.

There are two different types of mutualistic relationships: obligate or facultative. The survival of either one or both organisms is dependent upon the relationship in obligate relationships. By contrast, facultative relationships just confer benefits to both organisms, so the organisms could survive without the relationship. Many mutualistic relationships consist of scenarios where one organism receives nutrients while the other organism receives a service, like cleaning or locomotion.Mutualistic symbiotic relationships may have even played a role in the development of the first complex cells.

Examples of mutualistic relationships include oxpeckers and cattle, and sea anemones and clownfish. In the case of clownfish and sea anemones, the sea anemones provide the clownfish with protection by using their stinging tentacles. In return, the clownfish cleans the sea anemone and frightens off preying animals like the butterfly fish. Regarding the oxpeckers and cattle, cattle allow the small birds known as oxpeckers to eat ticks and other harmful insects off them. The oxpeckers receive nourishment while the cattle get cleaned of parasites and other nuisances. The clownfish and sea anemones represent an obligate mutualistic relationship, while the cattle and oxpeckers represent a facultative mutualistic relationship.

Commensalism

Photo: Duncan Wright via Wikimedia

In commensalism, one organism benefits from the relationship with the other is unharmed by it. Very frequently these scenarios involve one organism depending on another for transport, nutrients, or shelter while the host organism doesn’t benefit and is more or less unharmed.

Examples of commensalism are remora, golden jackals, and goby fish. Remora are fish which have a large disk on their head that allows them to cling onto larger sea creatures such as manta rays and sharks. When the host animal feeds on something, the remora detach themselves and eat any extra food. Golden jackals will often follow larger predators like tigers around to eat the remains of their kills. Meanwhile, goby fish are small fish that are capable of changing color. They attach themselves to larger fish, much like Remora do, and blend in with the larger fish, gaining protection from predators.

There are often debates as to what counts as mutualism or commensalism, as it isn’t always clear what relationships benefit both organisms and which relationships only benefit one organism. For example, some scientists believe the bacteria that live in the human gut represent an example of commensalism, while other scientists argue it is an example of mutualism.

Subtypes of commensalism include inquilinism, metabiosis, phoresy, and microbiota. Inquilinism happens when one organism depends upon another organism for permanent shelter, like plants which grow on trees. Metabiosis happens when one organism creates a habitat for another organism to use, as is the case when dead gastropods leave their shells behind which are used by hermit crabs as homes. Phoresy refers to when one organism uses another for travel. Microbiota are those organisms that create communities within a specific host. An example is how bacterial flora live within the guts of humans. As mentioned, there is disagreement over whether this is truly a type of commensalism.

Parasitism

Photo: CDC

Parasitic relationships are those which involve one organism living off of another organism, to the detriment of the other organism, possibly including the death of the other organism. Frequently the parasite actively lives on the body of the host, consuming nutrients from its blood or other parts of its body.

Common examples of parasites include fleas, ticks, and tapeworms. Ticks and fleas live on the body of their hosts for a while, sucking their blood for nutrients. Tapeworms burrow into the intestines of animals like cows and pigs and eat the partially digested food of the host. This ends up depriving the host of vital nutrients. Some parasites live off of plants instead of animals. For example, Aphids cling to plants and eat the sap of the plant. Some plants and fungi can even turn the tables on animals or insects and parasitize them.

Parasites don’t usually aim to kill their host, as this simply deprives them of a source of nutrients. Killing their host means they will have to search for a new host, potentially dying while they search for one. Frequently hosts are killed unintentionally. While this is true, some parasites do end up killing their host intentionally. If a parasite kills its host intentionally, it is a necrotrophic parasite. Parasites which thrive off a living host are called biotrophic parasites. Parasites and hosts usually evolve alongside one another. Parasites try to evolve better ways of finding hosts and getting past their defenses while hosts evolve better ways of defending themselves against parasites. Many incidences of mutualism arise from the interaction of two organisms, one of which removes parasites from their host and get nutrients by doing so.

Different Types and Aspects of Symbiosis:

  • Mutualism: Both organisms involved benefit.
    -Obligate: The symbiosis is essential to the survival of both organisms.
    Facultative: The symbiosis benefits both organisms, but isn’t necessary to their survival.
  • Commensalism: One organism benefits, the other is unharmed.
    -Inquilinism: One organism uses the other for permanent shelter.
    -Metabiosis: One organism creates a habitat for the other organism.
    -Phoresy: One organism uses another for travel.
    -Microbiota: Organisms which create communities inside of a host.
  • Parasitism: One organism benefits, the other organism is harmed.
    -Necrotrophic: A parasite which kills its host.
    -Biotrophic: A parasite which sustains itself with a living host.

The species that inhabit a an ecosystem together have various complex relationships with one another, all driven by the forces of evolution acting on different species over different amounts of time.

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