What Is a Symbiotic Relationship?

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Nature is full of symbiotic relationships, such as the honey bee and flower, the clown fish and anemone, and your gut and the prokaryotic intestinal bacteria living inside it. Symbiosis defines three basic relationship types (with multiple subgroups) occurring between living entities: mutualism, where both species benefit; commensalism, where one organism benefits and the other experiences no harm; and parasitism, in which one entity benefits, sometimes at the cost of the other.

The word symbiosis comes from the Greek sym and bios, which translated means together and life, or life working together. To understand how these relationships evolved, researchers developed a system to classify all life based on the distinct characteristics of individual organisms.

TL;DR (Too Long; Didn't Read)

Biologists and ecologists define a symbiotic relationship as an intimate interaction between two or more species, which may or may not be beneficial to either.

Biology's Classification System

The system for classifying species – taxonomy – uses different classification levels to sort where an organism fits in the biological scheme of things, as well as helping researchers to understand the relationships between organisms and across classifications. At the top of biological's organizational chart sit the broadest categories – the domains archaea, bacteria and eukarya – followed by kingdoms, phylum, class, order, family, genus and species at the tip of an upside-down triangle. The archaea and bacteria domains include only single-celled organisms, while the eukarya kingdom includes protists, fungi, plant and animals.

Mutualism: Relationships With Benefits for Both

Mutualistic relationships defined under symbiosis are those relationships where both species benefit from the association. The honey bee and the flower represent this kind of relationship. The bee collects nectar from the flower using a long, straw-like proboscis to suck the sweet fluid into a separate sac called a nectar or honey sac for later use in the colony as food. While the bee moves about the flower, pollen collects on its furry legs and body. When the bee leaves the flower to land on the next one, the pollen falls or rubs off onto the next flower, resulting in pollination. The flower helps the bee by giving it nectar, and the bee helps pollinate the flower by moving pollen from flower to flower.

Defensive Symbiosis: A Mutualistic Relationship

The relationship between ants and aphids, for example is a mutualistic one defined as defensive symbiosis. The ant acts like shepherds over the aphids. Aphids provide honeydew for the ants, and the ants herd the aphids into their shelter at night for protection against predators, escorting them back outside in the morning. Some ant species are even known to take aphid eggs into the nest's storage chambers during the cold winter months. Often called ant cattle, sometimes ants remove the wings from aphids to keep them from flying away. The ants may also release chemicals that cause the aphids to become more docile.

Obligate Mutualism: One Organism Cannot Survive Without the Other

Another type of mutualistic relationship – obligate mutualism – exists when each individual species cannot survive without the other. An example of this occurs between termites and their intestinal flagellate symbionts – prokaryotic organisms with whip-like flagella or appendages that help them move. The organisms within the termite help break down the dense sugars in wood so that the termite can digest it. But termites also have other symbionts in their innards that work in cooperation with each other and the termite. Without this relationship, termites and their inner guests would not survive.

Protocooperation Symbiosis: Not Obligatory, but Beneficial to Both

The clown fish and the anemone represent protocooperation symbiosis, a relationship that benefits both, but unlike the termite's and its symbionts, both can survive independently of the other. The fish has a home within the fat, wavy arms of the anemone that protects the fish from predators; the fish also protects the anemone from its predators and sometimes even brings it food.

Endosymbiosis: Cells Living in Other Cells

When one organism lives inside the tissue or cells of another, biologists define that as endosymbiosis. For the most part, these relationships are the norm for many unicellular entities. For example, a unicellular eukaryotic (a cell with an encased nucleus inside it) organism Paramecium bursaria serves as a host to eukaryotic Chlorella algae cells. The alga produces energy via the photosynthesis process, and the paramecium benefits as it receives some of that energy or food. Additionally, the algae reside inside a protected, mobile home – the body of the paramecium.

Ectosymbiosis: Organisms That Live on the Surface of Another

Another kind of mutualistic symbiosis involves one organism living on the skin or surface of another in a mutually beneficial relationship. Leaf cutter ants have a special symbiont, a type of unicellular bacteria that lives on their skin. Leaf cutter ants bring the cut foliage back to the colony where they inject it with a special type of fungus. The fungus serves as a food source for the colony, which the bacteria protect from other invading fungi species.

Phoresy Relationships: Transport Hosts and Food Sources

A phoresy symbiotic relationship occurs when one organism lives on or near the body of another, but not as a parasite, and performs a beneficial service to the host and itself. A species of marine life, the remora fish, attach themselves to the bodies of whales, manta rays, sharks and turtles (and even ships) via sucking discs atop their heads. The remora, also called shark suckers, don't harm the host nor take anything from it other than eating the parasitic sea creatures that infest it. Remora fish also use the disc to hitchhike a ride from the host. Oxpecker birds are common sites atop the backs of rhinoceros where they eat the parasites and ticks living there. They also fly in the air and scream when danger nears, providing a warning for the rhinoceros or zebra host.

Commensalism: One Organism Benefits, the Other Is Unharmed

Commensalistic relationships are those where one species receives all the benefit from its relationship with the other, but the other receives no benefit or harm. A good example of this type of relationship occurs between grazing cattle and cattle egrets. As the cattle graze in the grass, they stir up the insects living there, allowing the cattle egret a tasty meal. The cattle egrets get a meal, but the cattle receive nothing in return from the long-necked birds, nor are they harmed by the relationship.

Parasitism: One Benefits, the Other May or May Not Suffer

The world is full of parasitic relationships where a living entity makes a home in or atop a host entity. Most of the time, the parasite feeds on the host's body but does not kill the host. Two types of hosts exist in these relationships: the definitive host and the intermediate host. A definitive host provides a home to an adult parasite, while an intermediate host unknowingly offers a home to a juvenile parasite. Ticks are examples of parasitic symbiosis, because as blood-sucking insects that thrive on the blood of its victims, they can also harm the host by transferring an infectious disease to it taken in from the blood of another organism.

Parasitoidism: A Symbiotic Relationship Where the Host Dies

Science fiction is replete with examples of parasitoidism, but so is everyday life. In this type of symbiotic relationship, the host usually dies. Many science fiction movies feature this type of relationship between humans and aliens, like in the "Alien" movie series. In parasitoidism, the host serves as a home for the larvae of the parasite. As the larvae mature, they escape the body of the host, killing it in the process. In nature, braconid wasps lay their eggs atop the body of a tomato hornworm, and as the wasp larvae grow, they feed off the body of the hornworm, killing it during metamorphosis.

Predation: A Type of Symbiotic Relationship

A well-known symbiotic relationship exists between a predator and its prey. In an ecological community, some entities live by eating the bodies of other organisms. Thought not considered a parasitic relationship because the predator does not live in or on the body of the animal it eats, it is still a symbiotic relationship because the predator would not survive without the other organism giving up its life. The predator usually sits above its prey in the food chain, like the lion and the gazelle, the coyote and the rabbit (or a household pet), and the wolf and the bison or other cloven hoof animals – ungulates – like deer and antelope. Predation is also responsible for all kinds of evolution in the prey: developing means to hide from predators via mimicry, camouflaging and warning colors.

Competition: Where One or Both Inhibit the Population of the Other

Competition between species occurs when both entities vie for the same resources in the ecosystem. This type of symbiotic relationship works in reverse; one or both organisms suffer because of the existence of each other. Invasive species upset the delicate balance in ecological communities when they procure the resources meant for the native organisms. Yellow starthistle, for example, a native species of Europe, more than likely hitched a ride to the U.S., where it invades ecological communities and pushes out natural grasses. Because starthistle is a rapid-growing plant, it roots suck up all the water and nutrients, stealing these resources from the natural grasses, which often wither and die. Even organisms of the same family can experience competition, like when the green anole lizard, a native of many Southern states, has to compete with the brown anole lizard for food sources and habitat, originally introduced to the region from Cuba.

Neutralism: Both Species Unaffected

The planet is replete with symbiotic relationships where two different species or organisms may interact, but neither experiences any type of evolutionary affect because of the other. An extreme example – stretching the limits of neutralism – and offered by the University of Miami, includes the Bacterian camel and the Long-Tailed Tadpole Shrimp, both of whom may come in contact in the Gobi Desert with negligible effects on either.

Symbiotic Relationships Keep a Delicate Balance

The importance of symbiotic relationships to all living organisms on the Earth cannot be understated. All across the globe, in every ecological community in the world, from those viewable with the naked eye to those only seen under the lens of the microscope, symbiotic relationships remain crucial to maintaining balance in nature's multiple processes.

Symbiotic relationships cross taxonomies and species and involve most all living creatures on the planet in some way or another. Symbiotic relationship help to provide people with food, populate the planet with trees and plants, and keep animal and plant populations in balance. Symbiotic relationships can help individual species to evolve or change and even thrive. Without symbiotic relationships, there would not be any coral reefs, trees might not proliferate as far and wide as they do, aided by the birds and insects that transport seeds afar, and even human beings might not have survived long enough to evolve into Homo sapiens – Earth's modern humans.

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About the Author

As a journalist and editor for several years, Laurie Brenner has covered many topics in her writings, but science is one of her first loves. Her stint as Manager of the California State Mining and Mineral Museum in California's gold country served to deepen her interest in science which she now fulfills by writing for online science websites. Brenner is also a published sci-fi author. She graduated from San Diego's Coleman College in 1972.

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