Commensalism characteristics, types, examples

Simon Doyle

The commensalism It is a type of interaction between two species, where one of them benefits, while the other does not suffer any type of damage, but neither is it favored. In other words, it is a one-way relationship..

Generally, the individual who obtains the benefit is physically anchored or within the organism with which they are related. Commensal relationships are classified according to the benefit obtained in relationships of phoresis, tenancy and chemical commensalism..

Source: Carlos Fernández San Millán [CC BY-SA 2.0 (], via Wikimedia Commons

In the first case the species benefits in terms of transport, physically anchoring itself to the body of a larger animal. This association usually provides, in addition to displacement, a safe place against predators.

The tenancy represents advantages in terms of the availability of habitats. Some species are capable of modifying the environment, and the result is the creation of new niches available for other species..

Chemical commensalism involves the degradation of chemical compounds by one species, into useful products - energetically speaking - for a second species. It has been reported mostly in bacteria.

Furthermore, commensalism can be of the optional or compulsory type. In the first case, the beneficiary species can survive, even though its symbiotic partner is absent. In contrast to the mandatory, where the beneficiary species survives for a short time without the other species.

In nature, we find several examples of this type of interaction, involving both animals and plants as well as microorganisms. Tree epiphytes, small fish that live anchored to the surface of larger fish, and our gut flora are prominent examples of commensalism..

Article index

  • 1 Characteristics of commensalism
    • 1.1 Biological interactions
    • 1.2 Definitions of commensalism: theoretical and practical
  • 2 Kinds
    • 2.1 -According to the benefit
    • 2.2 -According to the need for the interaction
  • 3 Examples
    • 3.1 Clownfish and anemones
    • 3.2 Epiphytic plants
    • 3.3 Marine crustaceans
    • 3.4 Remoras
    • 3.5 Microorganisms and humans
    • 3.6 Pseudoscorpions
  • 4 References

Characteristics of commensalism

Biological interactions

In biological communities, the species that are part of these interact in different ways, in extensive and intricate networks of interaction.

The relationship can have benefits or have negative consequences for the species involved in the interaction. Biologists have classified these series of relationships based on how the organisms involved are affected..

When two species enter into a very close and long-term relationship, it is known as symbiosis. This “couple” lifestyle can be presented in three different ways: as parasitism, mutualism or as commensalism..

Note that although the term symbiosis popularly has connotations of positive interactions between organic beings, it also includes the relationships between parasites and their hosts..

Definitions of commensalism: theoretical and practical

Commensalism is an interaction where only one of the organisms derives some kind of direct benefit from the relationship. His partner, however, is not affected in any way.

Theoretically, one of the most important characteristics to consider an interaction as "commensalism" is that the relationship between the species is unidirectional.

However, putting this into practice is difficult - and in some cases, impossible. For this reason, a broader and more useful definition of commensalism is the interaction between two species, where one is benefited and the other is very little affected, either positively or negatively..


-According to profit

Commensalism can involve different benefits for one of the species, such as transport, obtaining food or protection against predators or unfavorable abiotic conditions..

The first classification system that we will mention for commensal relationships is based on the type of relationship obtained by one of the species. We will describe each one below:


The term phoresis is used when a species is mechanically transported by another species, called a host. The relationship does not involve any type of penalty (nutritional or energy) for the species that takes the role of host.

The phoresis is a way to locate a temporary micro habitat with a high degree of predictability, and provides a potential migration when the temporary habitat disappears..

This phenomenon compensates for their small size in these groups, allowing them to travel greater distances. In addition to compensating for the lack of adaptations, such as the absence of wings, among others.

Being transported on another much larger living being carries another series of advantages. As protection against predators, unfavorable environmental conditions, and in some cases, the transported species can consume the remains of food hunted by the host..

The most notable example of phoresis in nature occurs in arthropods. Adaptations of this type underwent impressive radiation in the mite group (Acari).


This type of commensalism describes the use of a secondary species as a kind of platform or cavity that will serve to house the species that will enjoy the benefit..

The concept of tenancy is even broader, and involves any species that uses as a place to spend the night any structure built by another species, such as nests or burrows..

Conceptually, the term overlaps with other types of commensalism, such as metabiosis, also called thanatocresis. In these cases, one species modifies the habitat, and this modification is used by a second species.

In general, earthworms and other soil-dwelling organisms are considered metabionts, since they are responsible for modifying conditions in a positive way, and a wide range of species are capable of colonizing the environment thanks to the initial reforms.

The thanatocrecia refers to the use of any type of resource left by a dead animal. For example, the hermit crab uses the shells left empty by certain species of snails.

Chemical commensalism

Chemical commensalism is a very particular type of commensalism that is usually applied - but not in all cases - to an interaction system composed of two bacteria. It has also been reported for yeasts.

In this case, one type of bacteria metabolizes a chemical that is not useful for the second. The metabolic product of the reactions carried out by the first bacterium is now useful for the second bacterium.

-According to the need of the interaction

Additionally, there is another classification system. Commensalism can be classified in terms of the relationship participants' need for their partner. Thus, we have the obligatory commensalism and the facultative.


In compulsory commensalism, one of the organisms depends entirely on the relationship with the other species. When its mate is absent, the survival of the beneficiary species decreases significantly..

This type of commensalism can be found in the relationship that certain species of mollusks and annelids establish with a species of chelicerate commonly known as horseshoe crab or pan crab (Limulus).

Small invertebrates are usually found attached to the surface of the saucepan crab and obligatorily reside in its shell..


In facultative commensalism, the species that gets the benefit in the relationship can survive when its partner is absent.

The arctic wolf, for example, benefits from the remains of food married by the polar bear. This decreases the energy used by the wolf to capture its prey, and greatly facilitates the process. However, in the absence of the bear, the wolf can survive by hunting its own food..

Another example closer to us is the presence of rodents, such as rats and mice, in domestic environments. The presence of man has favored an increase in the populations of these small mammals, providing them with food and safe resting places..

However, rodents are capable of living outside these sites and finding their own food..


Clown fish and anemones

One of the most popular animal interactions is the relationship between small tropical fish known as "clownfish" and sea anemones..

Anemones are characterized by having a series of stinging tentacles that they use to defend themselves. Anemones can survive without problem in the absence of the clownfish, but the fish cannot survive for long if it is not housed inside its symbiotic companion..

Clownfish are not affected by these stinging structures. These species have adaptations, such as secreting a mucous substance, that allow them to swim freely inside the anemone without suffering any type of damage..

The fish gets protection, since the anemone is a more than safe place to stay, while the anemone does not benefit from the interaction. In addition, the fish can consume the leftovers of the anemone prey, reducing the energy cost of searching for food..

From this point of view, the relationship is a clear example of commensalism. Some authors claim, however, that the anemone is positively affected. This view suggests that the fish provides defense against potential consumers of the anemone..

Epiphytic plants

There are a series of plants known under the name of "epiphytes", which grow on the branches of other trees. In this interaction there is no type of nutritional parasitism, that is, the epiphytic plant does not take nutrients from the larger tree or plant where it is being hosted..

In contrast, the benefit you get is purely "mechanical." The plants are housed in a relationship where they can capture a greater amount of solar radiation (with respect to the ground). An increase in light uptake translates into an increase in food production for the epiphyte.

Marine crustaceans

Certain species of marine crustaceans grow on other larger aquatic organisms. They do this to obtain protection from the organism, gaining displacement in the ocean and protection against potential predators. Note that the organism that transports them does not suffer any type of damage.


Remoras are a type of fish belonging to the family Echeneidae. Like the previous example, one of the most striking characteristics of the group is its behavior of adhering to other larger aquatic animals, such as rays, sharks, sea turtles, among other species.

Remoras have a suction system on the anterior portion of their body that allows them to efficiently adhere to the bodies of their larger companions..

The benefit obtained by the remora is not limited to obtaining a free movement through the ocean, since the small fish also feeds on the leftovers of the shark's prey.

In addition, the chances of the remora being consumed with a predator decrease exponentially if they are attached to the body of a shark..

The shark, for its part, is not significantly affected by the presence of the remora. However, some authors propose that there may be a certain hydrodynamic disadvantage due to remora.

Microorganisms and humans

Species relationships also involve organisms that we cannot observe with the naked eye. Our digestive system is the habitat of a significant number of commensal microorganisms.

In fact, microorganisms are not restricted to the digestive system. These microscopic inhabitants begin to colonize our body at the moment of birth, when we pass through the birth canal, where lactobacilli become predominant inhabitants of the newborn's digestive system..

Subsequently, the environment and food increase our contact with bacteria and other microorganisms, favoring the colonization process..

An important number of these species will form part of the normal flora of our skin and mucous membranes, establishing a commensal relationship..

Our body provides an optimal environment for their growth, while we are not affected. For example, certain bacteria live in our ears and genitals, feeding on secretions from these regions..

However, we usually hear that the bacterial flora brings us benefits. On a small scale, each species establishes a commensal relationship with our body.

However, the sum of all the species that are part of the normal flora helps to competitively exclude other microorganisms that could be pathogens.


Pseudoscorpions, or false scorpions, are a group of arachnid invertebrates, morphologically reminiscent of a real scorpion, but are only distant relatives of this group.

These small arachnids have the ability to attach to a wide variety of arthropods whose mobility is greater. Among the guests are more than 44 families of insects and 3 of other arachnids.

Additionally, pseudoscorpions have been found in associations with different species of mammals, and even with species of birds..


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