Symbiosis
Close, long-term interactions between different species.
Symbiosis refers to any close, long-term biological interaction between two organisms of different species. The term comes from ancient Greek words meaning "living together" or "companionship." In 1879, the German mycologist Heinrich Anton de Bary defined it broadly as "the living together of unlike organisms." For about 130 years, scientists debated the exact definition. Some argued it should only cover persistent mutualisms, while others included all long-term interactions like mutualism, commensalism, and parasitism, but excluded brief encounters such as predation. In 1949, Edward Haskell proposed a classification of "co-actions" that biologists later adopted as "interactions."
Symbiotic relationships can be classified in several ways. They may be obligate, where one or both organisms depend on each other for survival, or facultative, where they can live independently. For example, in lichens, the fungal partner cannot survive alone (obligate), while the algal or cyanobacterial partner usually can (facultative). When the symbiont lives on the host's surface—like head lice on humans or barnacles on whales—it is called ectosymbiosis. When one partner lives inside the other's tissues, such as *Symbiodinium* algae within coral, it is endosymbiosis. Endosymbiosis also includes nitrogen-fixing bacteria in legume root nodules, bacteria inside alder roots, and bacterial endosymbionts that provide essential nutrients to many insects. These endosymbionts often have reduced genomes and accumulate harmful mutations over time.
The interactions themselves vary. Competition reduces the fitness of both parties, often due to limited resources like food, water, or territory. Amensalism harms one organism while the other is unaffected; this can happen through competition (e.g., a large tree shading a sapling) or antagonism (chemical secretion). Commensalism benefits one organism while the other is largely unaffected. Parasitism benefits one at the other's expense. Mutualism benefits both parties.
Symbiosis has often driven evolution. Mutualism has allowed species to colonize new environments. Symbiogenesis is thought to have created eukaryotes when bacteria became mitochondria and chloroplasts inside cells. Major co-evolutionary relationships include mycorrhizae, insect pollination of flowers, ants protecting acacia trees, animals dispersing seeds, nitrogen-fixing bacteria in legume root nodules, and the mutualistic partnership of algae and fungi in lichens.
- field
- Biology
- known_for
- Defining symbiosis as 'the living together of unlike organisms'
Lore & Background
The definition of symbiosis has been debated for over a century. In 1877, Albert Bernhard Frank first used the term to describe the mutualistic relationship in lichens, while Heinrich Anton de Bary broadened it in 1878 to mean any "living together of unlike organisms." Some scientists later argued for a more restricted, mutualistic sense, where both partners benefit, while others included all long-term biological interactions such as commensalism and parasitism, excluding brief ones like predation. Symbiosis can be obligate, where one or both organisms depend on each other for survival, or facultative, where they can live independently. In ectosymbiosis, one organism lives on the host's surface, including its digestive tract; examples include lice and barnacles on whales. In endosymbiosis, one partner lives inside the other's tissues, such as nitrogen-fixing bacteria in legume root nodules or single-celled algae within coral. Endosymbionts often have reduced genomes and lose genes for metabolism and DNA repair, and their populations, when passed to offspring, accumulate harmful mutations due to Muller's ratchet. Symbiosis has driven evolution, with mutualism enabling colonization of new environments and symbiogenesis contributing to the origin of eukaryotes through bacteria becoming mitochondria and chloroplasts. Major co-evolutionary relationships include mycorrhiza, insect pollination, ant-protected acacia trees, animal seed dispersal, nitrogen fixation in legumes, and lichens formed by algae and fungi.
Reader's Guide
Symbiosis is a foundational concept in biology, describing the diverse ways organisms co-exist. It is classified in multiple ways: obligate versus facultative (where one or both depend on each other for survival), ectosymbiosis (living on the surface) versus endosymbiosis (living inside tissues), and by the effect on fitness—competition, amensalism, commensalism, parasitism, or mutualism. Symbiosis has driven major evolutionary events, such as the incorporation of bacteria as mitochondria and chloroplasts in eukaryotic cells. Key examples include mycorrhiza, pollination, nitrogen-fixing bacteria in legume root nodules, and lichens. The definition's historical debate reflects the complexity of these interactions, which range from harmful to beneficial and shape ecosystems worldwide.
Did You Know?
- Symbiosis can be obligate (one or both depend on each other for survival) or facultative (they can subsist independently).
- Endosymbiosis includes bacteria living inside insect cells, often with reduced genomes due to Muller's ratchet.
- Mutualism has enabled species to colonize new environments, such as lichens formed by algae and fungi.
Frequently Asked Questions
What does symbiosis mean in animal behavior?
Symbiosis is the umbrella term for any close, long-term biological partnership between organisms of two different species. It is not limited to one type of relationship but covers a whole spectrum of interactions.
What are the main types of symbiotic relationships?
The three recognized categories are mutualism (both partners gain), commensalism (one gains, the other is neither helped nor harmed), and parasitism (one gains at the other's expense). All three are considered valid forms of symbiosis under the broad definition.
How has symbiosis driven the evolution of species?
Symbiotic interactions have been a powerful evolutionary force, most strikingly in the origin of eukaryotic cells through a process known as symbiogenesis. This demonstrates that sustained cooperation between unlike organisms can give rise to entirely new biological forms.
What is the classic one-line definition of symbiosis?
It is traditionally phrased as 'the living together of unlike organisms,' highlighting two key requirements: the partners must be from different species, and the association must be sustained over time.
Why is symbiosis considered a foundational concept in biology?
It provides the framework for understanding how species across the animal kingdom depend on one another in beneficial, neutral, or harmful ways. Grasping these relationships is essential to studying ecosystems, evolutionary history, and the behavior of interacting species.
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