Commensalism is a pseudo-partnership in which one species (the commensal) extracts benefits from another (the host) without causing it harm or benefit. The commensal organism obtains food, shelter, locomotion, or support. Commensalism can either be a brief interaction or a lifelong symbiosis.
This contrasts with mutualism, whereby both species prosper; parasitism, in which one species prospers at the cost of another; and amensalism, in which one organism is harmed while the other remains no better or worse for the exchange. The term “commensalism” was coined by Belgian zoologist and paleontologist Pierre-Joseph van Beneden to describe the activity of scavengers trailing predators to eat the remains of their kill. There are four types of commensalism, depending on the relationship between the organisms.
- Inquilinism is a relationship where one body depends on the other for permanent housing, such as a bird living in a tree hole.
- Metabiosis is a relationship where one organism forms a habitat for the other. These include maggots living in dead bodies or hermit crabs using the shells of dead gastropod for protection.
- Phoresy is commensalism relationship where an organism attaches to the other for transport. Examples are millipedes on birds and pseudoscorpions on mammals.
- Microbiota organisms that form communities within the host organism. These include bacteria on human skin.
The following are ten intriguing examples of commensalism in the wild.
Examples of Commensalism
Orchids Growing On Tree Branches
Orchids are a family of flowering plants that grow on trunks and branches of other trees. The epiphytic plants are commonly found in dense tropical forests. Orchids rely on the host plant for sunlight and nutrients that flow on branches. They do not grow to be large plants and do not harm the host tree in any manner. 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. On the other hand, host plants gain no benefits from the orchards.
Golden Jackals And Apex Predators
The term commensalism was initially coined by a Belgian scientist, Pierre-Joseph van Beneden, in 1876, specifically to refer to scavengers that tailed apex predators to dine on the leftover prey. One such example of this is in Golden jackals. When an unlucky individual is banished from the pack, it is no longer capable of hunting on its own. Instead, it will take to following tigers, staying back in the shadows so as not to disturb the hunter-in-action, and then moving in once the feline has had its fill.
Sharks and Remora Fish (Sucker Fish)
Another curious example of commensal scavenging is amongst remora fish (eight species that belong to the Echeneidae family). These "suckerfish" painlessly attach themselves to large marine animals, such as sharks, mantas, and whales, wait for them to feed, and then detach to snag some scraps. Though this is an example of commensalism, sometimes remora feed on their host's external parasites, becoming an example of mutualism.
Whales and Barnacles
Barnacles are crustaceans that are unable to move on their own. During the larval stage, they stick to other organisms such as whales or attach to shells, ships, and rocks. They grow and develop on these surfaces without negatively affecting the host. Barnacles feed on plankton and other food materials as the whales move. This way, they benefit from transportation and nutrition. They do not feed on blood or flesh; therefore, they cause no harm to the whale.
Sea Cucumbers and Emperor Shrimp
The emperor shrimp is a crustacean that is common in the Indo-pacific region. It is often seen attached to sea cucumbers where they benefit from transportation and protection from predators without spending energy. The shrimp get off the host cucumber to feed and attaches to another when it wants to move to a different area. The emperor shrimp is small and light to affect the movement of the cucumber.
Milkweed and Monarch Butterfly
The monarch butterfly is common in North America. During the larval stage, it attaches to a specific species of milkweed that contains toxic chemical cardiac glycoside. The poison is harmful to vertebrates, and most animals avoid contact with the plant. Monarch butterflies extract and store the toxin throughout the lifespan. Birds find monarch butterflies distasteful and thus avoid eating them. Monarch larvae are resistant to the poison and are therefore not affected, and the milkweed is not a carnivorous plant; therefore, causes no harm to the developing butterfly.
Livestock and Cattle Egrets
Unlike the remora, Cattle egrets hang out on the backs of large grazing animals, such as cattle and horses, to capitalize on food. When the beasts of burden stir up the ground, the small birds hop down and snatch up exposed insects. They also follow farm machines for the same reason and even poke around airport-adjacent fields, waiting for planes to stir up the grass during takeoff and landing. These forms of opportunistic feeding account for about 50% of a Cattle egret's diet and about one-third of the energy expenditure.
Burdock Seeds on Animals
If you have been hiking through the thick brush in North America, chances are you have picked a few of this weed's prickly brats off your clothes. The Burdock plant utilizes its spiky, globular flower heads to disperse its seeds across the land. When furry animals pass by, they naturally collect these bundles, which are sharp enough to attach to but not enough to penetrate the thick animal hides. This perfectly represents a special form of commensalism known as phoresy, whereby the commensal species uses a host specifically for transportation.
Beetles and Pseudoscorpions
Pseuroscorpions also exhibit phoresy in that these arachnids attach themselves to exposed surfaces of host animals like the fur of mammals or beneath the wings of beetles and bees. In this way, these scorpion-like creatures, but without stingers, receive protection from predators. Their small size hardly creates any nuisance to the sheltering animal.
Hermit Crabs And Gastropod
Hermit crabs famously seek shelter in acquired shells. Oftentimes, these squishy crustaceans rely on expired gastropods (snails) to protect themselves. Therefore, this qualifies as an example of metabiosis (another category of commensalism), whereby one species forms a habitat from a dead organism.
Birds and Army Ants
The commensal relationship between army ants and birds is unusual since both can prey on the other. Birds trail army ants not to feed on them but to feed on insects escaping the ants as they move across the forest floor. The birds easily catch the prey while the ants remain unaffected. Due to their aggressive nature, painful bites, and poison, birds avoid eating ants.
Maggots Living In Dead Bodies
A less-appealing instance of metabiosis commensalism occurs when maggots inhabit a carcass. They not only feed on the available nutrients but utilize the moist environment to lay their eggs. The next generation of maggots relies on the immediate food source before making their way out of the dead animal's remains.
Arctic Fox and Caribou
This frisky North American tundra-dweller will sometimes follow caribou around, which helps to indirectly summon preferred food sources. The stocky herbivores dig into the snow to scrounge up lichen plants. After the activity settles down, rodents may come to the surface, at which point the fox will swoop in for the kill. Arctic Foxes are also known to scavenge for remains left behind by polar bears.
Owls And Abandoned Nests
Most owls are known to nest in the natural cavities in trees, cliffs, or buildings, as well as the abandoned nests of hawks or crows. In the first case, since they are not themselves burrowing into the woody plant, they cause no harm to the stalwart specimen. This approach is known as inquilinism, which is a form of commensalism whereby one organism uses another for permanent housing. By this definition, many birds exhibit inquilinism when they build their nests amongst the branches of trees.
Tree Frogs And Leaves
Tree frogs also rely on arboreal friends for shelter and protection. Unlike nesting birds, these amphibians set up shop underneath large leaves in order to camouflage from predators and hide from the heavy rains. Tree frogs are found on every continent not called Antarctica, so wherever you are, take a peek at the underside of leafy areas, and you might just spot a few of these jumpy creatures.
The plants, animals, and critters of the earth have formed many creative partnerships amongst themselves. Though these can manifest in many ways, the aforementioned low-impact, net-beneficial interactions represent the pleasant side of the natural world.