Symbiosis: How do Animals Interact?

Author: Patrick Lin

Editors: Simran Gohel and Ken Saito

Artist: Susan Wu

Animals interact with other animals all the time. Despite being from distinct species, animals can prey on, compete with, or even help each other. The word symbiosis does not have a single, definite meaning. Since the term was coined, it had two different definitions: one is a close physical association between two organisms. The other is a mutually beneficial relationship between two organisms. With both definitions, symbiosis can apply to most long-term biological interactions such as mutualism, commensalism, parasitism, and amensalism.

The most common use for the word symbiosis, mutualism, is an interaction in which both animals benefit. One form of mutualism is cleaning symbiosis, a relationship where one organism removes parasites from the client. An example of this is the relationship between Caribbean cleaning gobies and their clients. Other marine creatures frequently visit the goby to rid themselves of parasites, and in return, the gobies get easy access to their primary source of food; parasites. Another form of mutualism is protection mutualism, a relationship in which two organisms protect each other. A classic example of protective mutualism is the relationship between ants and trees. Cordia alliodora trees provide Azteca pittieri ants food and shelter, while the ants act as tree defenders. If the ants effectively defend the tree, the tree can grow and provide more food and shelter for the ant colony to grow, allowing both parties to flourish together.

Commensalism and amensalism are both similar in that it is a relationship where one side of the party is either unaffected or if the effect is negligible. In commensalism, one organism benefits from the relationship, while in amensalism, one organism is harmed from the relationship. There are four basic forms of commensalism: Inquilinism - one organism uses the host as shelter, Metabiosis - the (usually dead) host turns into a shelter for the other organism, Phoresy - one organism uses the host for transportation, and Microbiota - organisms form communities or microbiomes within a host. An example of a commensalism relationship is the burdock plant. The plant produces spiny seeds that stick onto animal fur. The animal helps the plant by dispersing the seed but receives nothing in return. Amensalism has two forms: competition - a larger or stronger organism deprives another organism of resources, and antibiosis - an unaffected organism secretes a chemical that harms another organism. An example of antibiosis is penicillium. The fungi produce penicillin, which kills various bacteria.

The final form of symbiosis, parasitism, is an interaction in which a parasite benefits at the host's expense. Parasites harm the host by either slowly eating the host or stealing its food and nutrients. The most common forms of parasites are ectoparasites and endoparasites. Ectoparasites live on the surface of the host, while endoparasites live inside the host's body. Other forms of parasitism include brood and sexual parasitism. Brood parasitism is practiced mostly by a few bird species, such as cuckoos and cowbirds. These birds do not make nests of their own and instead leave their eggs in the nest of another bird so that the other bird will take care of it. Sexual parasitism is a form of reproduction unique to anglerfish. Male anglerfish are substantially smaller than their female counterparts, and they attach onto females, fusing the tissue and sometimes even blood vessels with each other. The male essentially becomes a sperm-producing organ that relies entirely on the female for nourishment.

Symbiosis plays a fundamental role in biology. The discovery and study of symbiosis revealed the impact it has on life itself. Further research will allow us to apply symbiosis to benefit fields such as agriculture, medicine, and the study of evolution.


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