Commensalism: Definition, Types, Facts & Examples

Living organisms are bound together in a web of relationships that can be helpful, harmful or inconsequential to their survival. One way that organisms are symbiotically interlinked is called commensalism, which occurs when one species benefits, while the other is unaffected.

For instance, hermit crabs make their home in the shells of dead snails. This benefits the crabs while the snails are unaffected.

Origin of Commensalism Theory

In 1872, Belgian zoologist Pierre-Joseph van Beneden coined the terms mutualism and commensalism. He defined mutualism as a reciprocal relationship and commensalism as a type of sharing, not unlike a gracious host serving friends dinner.

The species or organism that benefits is known as the commensal. Van Beneden backed up his theory with examples in the natural world such as pilot fish that follow sharks and eat leftover scraps that the bigger fish leaves behind.

Definition of Commensalism

Commensalism (+/0) is defined as a unilateral relationship between two species that benefits one species without consequence to the other. Most of the interactions occurring in the natural world affect both organisms in some way.

However, there are several examples of commensalism relationships that benefit one species alone, without helping or harming the other species. For instance, epiphytic orchids live on trees without affecting the tree in any significant way.

Amensalism (-/0) is a unilateral interaction like commensalism. However, one organism causes harm to another without being helped or harmed in the process.

The harm done to another organism may be incidental. For example, an elephant walking across the savannah may unwittingly crush plants and small animals under its toes.

Types of Symbiotic Relationships

Commensalism, mutualism and parasitism are types of symbiotic relationships. In biology, a symbiotic relationship is defined as a close relationship between two distinct species that persists long-term. Community ecologists study species interactions and develop mathematical models that can predict how changes in one species may affect another in scenarios such as increased global warming.

Mutualism (+/+) refers to longstanding relationships where both organisms benefit without cost to either. Species don’t need to be aware of each other’s presence to receive organism benefits.

You may not know it, but you have a mutualistic relationship with billions of good bacteria in your intestines. In exchange for a habitat inside your body, helpful microflora such as certain strains of E.coli aid in digestion, ward off pathogenic bacteria and make vitamins B and K.

Parasitism (+/-) is an interaction that harms the host species: In cases like the strangler fig, the parasitic species can even kill the host. Many animal parasites such as ticks and fleas suck blood from their host. Vectors are parasites that carry pathogenic bacteria that infect its host.

For instance, blacklegged ticks can infect humans with Borrelia burgdorferi, a bacterium some ticks carry that causes Lyme disease.

Basic Facts About Commensalism

Commensalism in biology is one of many ways in which all living organisms on Earth are interlinked in the web of life. Commensalism examples are often related to transportation perks or housing needs, but the relationship can offer any type of benefit.

One of the most common commensalism examples is a bird nest or a spider web in a tree_._ The habitat of the bird and/or spider does not affect the tree in this type of symbiosis.

True one-sided commensal species are uncommon, according to some scientists. That is because interactions between different species typically affect both species in some way but to varying degrees. Commensal relationships exist in the middle range of a continuum from exclusively mutualistic relationships on one end of the continuum to exclusively parasitic relationships at the other end of the continuum.

In certain situations, commensal interactions can turn into a parasitic or mutualistic symbiotic relationship. Overgrowth of the commensal can adversely affect the functioning of the host species. Or the host species may receive some benefit if the commensal has an appetite for parasites, for instance.


Barnacles are commensal filter feeders that enjoy a free ride on whales that swim through plankton rich waters. Normally, the whale is unaffected by barnacles.

However, too many barnacles can potentially slow the whale down. Conversely, a massive set of barnacles offers the grey whale some protection from the bite of a killer whale.

Other Commensalism Examples

Livestock and egrets: Cattle and horses stir up insects in the grass as they walk through pastures. Egrets follow along eating the airborne insects. This relationships demonstrates commensalism because the birds benefit from the interaction but not the livestock. When egrets and other small birds such as oxpeckers sit on the backs of cattle eating pesky fleas and flies on the animal’s hide, the relationship is mutualistic.

Mimicry in butterflies: Examples of commensalism can include one species imitating another. For example, the viceroy butterfly has evolved to look like the monarch butterfly as a protective strategy. Predators avoid monarch butterflies because they contain poison from feeding on milkweeds. Monarchs are not thought to be significantly helped or harmed by the viceroy’s mimicry.

Animals and seed burs: Burdock and other weeds have seed burs that get stuck on animals that may travel long distances. Burs are an adaptation that helps with wider seed dispersal and plant reproductive success. Assuming the animal transports seed but without it, only the plant species benefits, making this an example of a commensal relationship.

Sea anemones, clownfish and crabs: Colorful clownfish and sea anemones are generally considered commensal organisms. Clownfish are able to hide from predators inside the sea anemone by gradually developing a mucous coating that protects them from the deadly sting of its host. Clownfish keep the sea anemone clean by living off debris from the anemone’s last meal.

The anemone crab enjoys safe, permanent housing inside the sea anemone. This type of crab dwells in the tentacles of its host. The crab catches food in the water while remaining protected from predators by the feared sea anemone making their relationship an example of commensalism.

Shrimp and Sea Cucumbers: Imperial shrimp hitch rides on the unsuspecting sea cucumber, which is a type of echinoderm named for its physical resemblance to a cucumber. Shrimp conserve energy by hopping on sea cucumbers and dropping off to feed in desirable areas. After feeding, the shrimp finds another sea cucumber for a lift. The sea cucumber is not bothered by the shrimp.

Remora and Marine Animals: The Remora fish, commonly known as the brown sucker, has a disc on its flat head that acts like a suction cup. The fish gloms on with its head to sharks, turtles, marine mammals and even deep sea divers. They are not considered parasitic because their only motivation is to ride along feeding on scraps and ectoparasites on the host.

Alternating Commensal Relationships

A living organism can have many different types of relationships going on with different species. In fact, a particular species can engage in parasitic, mutualistic and commensal relationships throughout the day. For instance, the southern stingray has many such engagements.

The southern stingray is a host organism to ectoparasites. Harm is mitigated because the southern stingray has a mutualistic relationship with Spanish hogfish, a cleaner fish that eats parasites off of stingrays.

They also have a commensal relationship with other fish that get some of the prey that stingrays leave behind after churning up the sand. The stingray may also find itself involved in a predator-prey relationship with a hungry Hammerhead shark.

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