Commensalism is a relationship between two organisms in which one benefits from the other without causing harm to it. The commensal organism obtains food, shelter, locomotion, or support. Commensalism can either be a brief interaction or a lifelong symbiosis.
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 form communities within the host organism. These include bacteria on human skin.
Examples of Commensalism
Orchids Growing on 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.
Livestock and Cattle Egrets
A typical commensal relationship is between livestock and cattle egrets. The egret is a species of heron that moves along with cattle or horses. Sometimes it 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 vegetation, which get stirred when the animals feed. When the birds are not working alongside the animal, they hop on the back for a ride. They are light birds and do not limit the movement of the host.
Sharks and Remora Fish
The remora or suckerfish is a small fish that grows to about three feet. It is a member of the ray-finned fish. The remora forms a commensal relationship with large sea organisms, especially sharks, turtles, and whales. Its specially-designed suckers attach to the fins of the host animals and thus benefit for transportation and protection from predators. It also feeds on the leftover of sharks. The small size of the remora makes it less intrusive, and the shark barely feels its presence.
Beetles and Pseudoscorpions
Pseudoscorpions are tiny scorpion-like insects that grow to about half an inch in length. They are distinguished from real scorpions by a lack of stingers. Pseudoscorpions hide on exposed surfaces of host animals such as fur of mammals, and beneath the wings of bees and beetles. They gain transportation and protection from predators and weather elements. Pseudoscorpions cause minimal intrusion and do not harm the host insect due to their small size. They are also too small to be of any benefit to the host.
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.
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.
Burdock Seeds on Animals
Many plants have evolved different dispersal features, including curved spines. Burdock plants are mostly found along roadsides. Their seeds are equipped with long curved spines that attach to the fur of animals and are transported to other areas. Burdock seeds are incredibly light that animals barely recognize their presence while their long hooks are not strong enough to pierce the skin of animals.
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.
Caribou and Arctic Fox
The relationship between the caribou and the arctic fox is an example of commensalism in the tundra. The fox trails the caribou while the reindeer prowls for food. As it digs up the soil to expose lichen plants, subnivean mammals are attracted to the site, making them easy targets for the fox. The fox keeps its distance from the deer to avoid spooking it.