What is Commensalism?
Commensalism is a type of symbiotic relationship between two living organisms in which one of the organisms benefits from the other leaving it unharmed. The word commensalism is actually derived from the word commensal which means eating at the same table in human social interaction. Hence Pierre-Joseph van Beneden introduced the term commensalism in 1872. He, however, defined mutualism as a reciprocal relationship and commensalism as a type of sharing, not unlike a gracious host serving a friend’s dinner.
In commensalism, one organism benefits from the host organisms by obtaining locomotion, food, shelter, or support without harming it. Commensalism can be contrasted with other types of symbiosis, like mutualism and parasitism. However, the difference between commensalism and other types of symbiosis is that the second party or host remains unharmed in commensalism.
Additionally, there are several cases of commensalism existing in our ecosystem. As many hosts of commensal organisms appear to be unaffected or even bothered about the presence of the commensal organism. The commensal however benefits in a commensal relationship gaining transportation, protection, nutrition, or a variety of other benefits.
What are Commensals?
Commensals are the organisms or species that benefit from the commensal relationship. The commensal can obtain nutrients, shelter, support, or locomotion from the host organism which is substantially unharmed. In commensalism, it is often between a smaller commensal and a larger. However, the commensal species may show great morphological adaptation, a best-known example of a commensal is the remora (family Echineidae) that rides attached to sharks and other fishes.
The remoras have evolved a flat oval sucking disk structure on the top of their heads that adheres to the bodies of their hosts. Remoras and pilot fishes feed on the leftovers of their hosts’ meals. Other common examples of commensals are the great egret that feeds on insects turned up by grazing mammals or on soil organisms stirred up by plowing.
Examples of Commensalism
- Caribou (Reindeer) and Arctic fox
- Beetles/Large insects and Pseudoscorpions
- Aspergillus and Humans
- Staphylococcus and Humans
- Birds and Army ants
- Nitrosomonas spp and Nitrobacter spp
- Whales and Barnacles
- Orchids that grow on branches
- Livestock and Cattle egrets
- Milkweed and Monarch butterfly
- Nesting birds and Trees
- Sharks and Remora fish
- Sharks and Pilotfish
- Burdock seeds on Animals
- Sea cucumbers and Emperor shrimp
- Baitfish and Manta rays
- A microorganism modifying an environment to suit another microorganism
- Nurse plants and seedlings
- Golden jackals and Tiger
- Goby fish and Sea animals
- Hermit crab and Dead gastropods
Caribou and Arctic fox
The relationship between the Reindeer and the arctic fox in the tundra (cold frozen landscape) is commensal. As the Reindeer prowls for food the fox trails it for a reason, keeping its distance to avoid spooking the reindeer. This is because as the reindeer digs up the soil, it exposes lichen plants. Eventually, subnivean mammals get attracted to the site because of these lichen plants which then make these mammals easy targets for the fox to prey on.
Beetles/ Large insects and Pseudoscorpions
Pseudoscorpions are tiny scorpion-like insects. These insects grow to about half an inch in length. However, their lack of stingers distinguishes them from real scorpions.
Pseudoscorpions hide on exposed surfaces of host animals like the fur of mammals, and beneath the wings of bees and beetles.
At times the pseudoscorpion attaches to the leg of a much larger fly. Since the pseudoscorpion does not have a stinger like a traditional scorpion it does not kill the insect it hitches a ride on. Oftentimes, it only hitches a ride from one place to another and once it has hitched a ride, the relationship is over.
Hence the pseudoscorpion gains transportation. Also, it gains protection from unfavorable weather and predators. However, as harmful as the Pseudoscorpions sound, it actually causes minimal intrusion and doesn’t even harm the host insect. Also, they are too small to be of any benefit to the host due to their small size.
Aspergillus and Humans
There are several genera of bacteria and fungi that live on and in the human body as part of its natural flora. The fungal genus Aspergillus, for instance, is capable of living under considerable environmental stress. Hence they are capable of colonizing the upper gastrointestinal tract in humans. The Gastrointestinal gut is where relatively few examples of the body’s gut flora can survive due to highly acidic or alkaline conditions produced by gastric acid and digestive juices. Usually, Aspergillus produces no symptoms. For people with healthy immune systems, breathing in Aspergillus isn’t harmful to them. However, a condition called aspergillosis can occur when populations of Aspergillus grow out of control in individuals who are immunocompromised or suffering from existing conditions such as tuberculosis.
Staphylococcus and Humans
Staphylococcus aureus is a common bacterial species that is best known to cause numerous illnesses and conditions. However, many strains of S. aureus are metabiotic commensals. These bacteria are present as part of the human skin flora in roughly 20-30% of the human population. Also, Staphylococcus aureus benefits from the ambient conditions that are created by the mucous membranes in the body. So they can be found in the oral and nasal cavities, as well as inside the ear canal.
Birds and Army ants
The commensal relationship between army ants and birds is weird since both can prey on the other. However, birds trail army ants not to feed on the army ants but to feed on insects escaping the ants as they move across the forest floor. The birds will catch their prey easily while the army ants remain unharmed. Birds avoid eating army ants due to their aggressive nature, painful bites, and poison.
Nitrosomonas spp and Nitrobacter spp
Commensalism between microorganisms includes situations where the waste product of one microorganism is a substrate for another species. A good example is a nitrification (oxidation of ammonium ion to nitrate).
Nitrification happens in two steps. The first step involves bacteria such as Nitrosomonas spp. and certain crenarchaeotes oxidizing ammonium to nitrite. In the second phase, nitrite is oxidized to nitrate by Nitrobacter spp. and similar bacteria. In the end, Nitrobacter spp. benefit from their association with Nitrosomonas spp. This is because Nitrobacter spp. uses energy from the oxidation of nitrite ions into nitrate ions.
Barnacles are crustaceans that cant move on their own. So, during their larval stage, they stick to other organisms like whales or attach to shells, ships, and rocks. They eventually grow and develop on these surfaces without affecting the host negatively. These barnacles feed on plankton and other food materials as the host whale moves.
However, as barnacles do not feed on blood or flesh, they cause no harm to the whale. Hence, they benefit from transportation and nutrition in the relationship.
Orchids that grow on branches
Orchids are a group of flowering plants that grow on the trunks and branches of trees. These epiphytic plants are mostly found in dense tropical forests and they depend on the host plant for sunlight and the water that flows on branches.
Orchids do not grow to be large plants. So, they do not harm the host tree in any manner. However, orchids have their photosynthesis process and do not extract any nutrients from the host plant apart from the water that flows on the outer bark. Additionally, the host plant gains no benefits from the orchids.
Livestock and Cattle egrets
Livestock and cattle egrets have a commensal relationship. The egret is a bird species of heron that moves along with cattle or horses. Sometimes the egret can be seen on the back of the animal.
Initially, it was believed that the birds fed on ticks and other parasites, but it was later discovered that the birds feed on insects hiding in the vegetation. These insects get stirred when the livestock animals feed.
However, when the birds are not working alongside the animal, they hop on the back of the animal for a ride. They do not limit the movement of the host as they are light birds. However, the cattle are unaffected, while the birds gain food. Also, the egrets actually do this with a few large species such as rhinos and elephants.
Milkweed and Monarch butterfly
The monarch butterfly is endemic to North America. During the larval stage of the monarch butterfly, it attaches to a specific species of milkweed. This milkweed contains the toxic chemical, cardiac glycoside and is poisonous to vertebrates. So most animals avoid contact with the plant.
These Monarch butterflies extract and store the toxin throughout their lifespan. Hence, birds find monarch butterflies distasteful and avoid eating them. However, monarch larvae are not affected by the poison because they are resistant to it. Since the milkweed is not a carnivorous plant, it causes no harm to the developing butterfly.
Nesting birds and Trees
Nesting birds and trees show a commensal relationship, too. These birds build their nests high up in the trees for protection and support. Meanwhile, the supporting tree is not really changed by the parent birds building their nest and it just goes on living its big tree life. The tree is not even affected by the baby birds that will live there.
Sharks and Remora fish
The Remora or suckerfish is a small fish that is a member of the ray-finned fish. It grows to about 3feet. This fish forms a commensal association with large sea organisms, especially sharks, turtles, and whales. The remora has specially designed suckers that attach to the fins of the host animals.
Remora (Echeneis remora) is a sucking fish with a flat disk on its head which it uses to stick to larger fish like sharks.
Remora fish have a disk on their heads to attach to larger and when the animal feeds, they detach themselves to eat the extra food. Hence they benefit from transportation and protection from predators. Also, feeding on the leftover of sharks. However, the shark barely feels the remora’s presence as its small size makes it less intrusive.
Sharks and Pilotfish
Another example also is the pilot fish that follow sharks and eat leftover scraps that the bigger fish leaves behind.
Burdock seeds on Animals
A lot of plants have evolved different dispersal features like sticky, barbed, or hooked curved spines. This is why if you’ve ever walked through a natural grassland, you notice that on the other side your clothes will be covered with a variety of sticky seeds.
However, Burdock plants are mostly found along roadsides because their seeds are equipped with long curved spines. These long curved spines attach to the fur of animals and are transported to other areas. Also, burdock seeds are very light that animals barely recognize their presence and their long hooks are not strong enough to pierce the skin of animals. In summary, the burdock plant produces spiny seeds that cling to the fur of animals or the clothing of humans, relying on this method for reproduction and dispersal, while the animals are unaffected.
Sea cucumbers and Emperor shrimp
Emperor shrimp is a crustacean that is endemic to the Indo-pacific region. They are often seen attached to sea cucumbers where they benefit from transportation and protection from predators without exerting energy. However, the shrimp does get off the host cucumber to feed and attach to another when it wants to move to another area. Since the emperor shrimp is small and light, it doesn’t affect the movement of the cucumber.
Baitfish and Manta rays
Small baitfish and manta rays often show a form of commensalism in that the baitfish are protected simply by their proximity to the larger fish. The Large manta rays will often be seen with huge schools of small fish underneath their enormous fins. However, it is believed that these small fishes are protected from birds that could dive in and eat them. Also, the manta ray is completely unaffected by the baitfish, and may not even notice their presence.
In fact, most large marine animals have some or many smaller animals following or attached to them. In some cases, the animals are parasitic but still many cases of commensalism exist as well, where the host is unharmed. Such include fishes that follow sharks and feed on the left-overs of their kills.
A microorganism modifying an environment to suit another microorganism
Commensal associations also occur when one microbial group modifies the environment to make it better suited for another organism. For instance, the synthesis of acidic waste products during fermentation stimulates the proliferation of more acid-tolerant microorganisms, which may be only a minor part of the microbial community at neutral pH. A typical example is the succession of microorganisms during milk spoilage. Biofilm formation is another example. Hence the colonization of a newly exposed surface by one type of microorganism (an initial colonizer) makes it possible for other microorganisms to attach to the microbially modified surface.
Nurse plants and seedlings
Nurse plants and seedlings have a commensal relationship. Since they are larger plants, they offer protection to seedlings from the weather and herbivores. This relationship gives the seedlings an opportunity to grow and the nurse plant is not harmed and doesn’t benefit from the seedlings either.
Tree frogs and Plants
Tree frogs use plants as protection from predators and unfavorable weather conditions. Most times they blend in with the plant, disguising themselves from predators.
Golden jackals and Tiger
Once the Golden jackal has been expelled from a pack, it will trail a tiger to feed on the remains of its kills. The Golden jackals will follow tigers on their hunt for prey so that they can feed off of the tiger’s scraps. Although the tiger does all of the work by actually catching and killing its prey, it doesn’t seem to mind that the jackal cleans up after it. However, since the jackal benefits and the tiger isn’t affected, we can say that this relationship between the golden jackal and the tiger is an example of commensalism.
Goby fish and Sea animals
The Goby fish live on other sea animals. They change their color to blend in with the host. This eventually helps them gain protection from predators.
Hermit crab and Dead gastropods
Hermit crabs have a commensal relationship with sea snails and other creatures that leave their shell behind when they die. After a sea snail dies, it decomposes and leaves behind the hard shell it made and lived in. Then, the hermit crab comes along and once it can fit into the abandoned shell, it moves in. The shell ends up providing safety and a home for the hermit crab and the sea snail is not affected because it was already dead.
Types of Commensalism
Like all Symbiotic relationships, commensalisms vary in strength and duration. The association could vary from intimate, long-lived symbiosis to brief, weak interactions through intermediaries.
In phoresy, one organism attaches to another organism exclusively for transport. This type of commensalism is easily and often seen mainly in arthropods. Typical examples are mites on insects (such as beetles, flies, or bees), beetles on birds, anemone attachment to hermit crab shells, pseudoscorpions living on mammals, and millipedes traveling on birds. However, phoresy may be either obligate or facultative (induced by environmental conditions).
This type of commensalism is solely about shelter dependency. In Inquilinism, an organism uses another organism for permanent housing. Typical examples are Tillandsia bourgaei growing on an oak tree in Mexico, orchids that grow on trees, and a bird that lives in a tree hole. However, sometimes epiphytic plants that grow on trees are considered Inquilinism, although others might consider this to be a parasitic relationship. This is because the epiphyte might weaken the tree or take nutrients that would otherwise go to the host.
In metabiosis, it is a commensal relationship that is more like an indirect dependency. This is a type of commensalism in which one organism creates or prepares a suitable environment for another organism. Typical Examples include maggots, which feast and develop on corpses, and hermit crabs which use shells from a dead gastropod to protect their bodies.
Microbiotas are commensal organisms that form communities within a host organism. This type of commensalism is still under criticism. An example is a bacterial flora found on human skin. However, scientists argue on whether the microbiota is truly a type of commensalism. Like in the case of skin flora, for example, there is evidence that the bacteria confer some protection on the host which could be seen as mutualism, instead.
Commensalism vs Mutualism
Commensalism and mutualism are both symbiotic relationships. The symbiotic relationship between two organisms is known as symbiosis. However, symbiosis is any type of close and long-term biological relationship between two different biological organisms. This relationship could be mutualism, commensalism, or parasitism. However, the main difference lies in whether one or both of the organisms benefits from the relationship.
Mutualism is a symbiotic relationship between two organisms where both organisms benefits. A typical example of mutualism involves goby fish and shrimp. The nearly blind shrimp and the goby fish spend time together. In this relationship, the shrimp maintains a burrow in the sand in which both the fish and shrimp live.
Interestingly, the fish touches the shrimp with its tail as a warning when a predator approaches. Then, both the fish and shrimp will retreat to the burrow. However, they both hide in the burrow until the predator leaves. From this relationship, the shrimp benefits by getting a warning of danger and the fish get a safe place to hide and lay eggs. However, mutualism is also called interspecies reciprocal altruism. These mutualistic relationships can be obligate for both species in that they can’t live without each other. Also, the mutualistic relationship can be facultative for both species in that they can live without each other.
However, contrary to mutualism, in commensalism, only one organism benefits, and the other doesn’t benefit or get harmed. Therefore, commensalism is defined as a symbiotic relationship in which one species benefits while the other species is not affected either positively or negatively. One organism typically uses the other for a purpose of transportation or protection. For instance, mites attach themselves to larger flying insects to get a free ride, and hermit crabs use the shells of dead snails for shelter.