The Symphony of Symbiosis: Unseen Partnerships in Nature
Introduction
Humans are social creatures: we have family, friends, and sometimes enemies. However, these partnerships are not exclusive to us only. With millions of species roaming the Earth, many species of animals cohabit in the same spaces and compete for the same resources. The interdependence on one another is seen in the relationships formed as they work together for their survival.
Long-Term Partners: Understanding Symbiosis
Symbiosis encompasses any biological interaction between two organisms of different species, ranging from beneficial partnerships to parasitic ones. The definition of symbiosis was debated for 130 years. It was first used by Albert Bernhard Frank to describe a mutualistic relationship in lichens. However, it was later defined by Heinrich Anton de Bary as “the living together of unlike organisms”. Within the scientific community, there was a lot of controversy over whether symbiosis should only refer to mutual relationships or all interactions.
Symbiosis can be described as either obligate or facultative. Obligate symbiosis is when both symbionts (the two species in a relationship) depend on each other completely for survival. Conversely, facultative suggests that the two organisms typically live independently. Symbiosis is also differentiated into ectosymbiosis and endosymbiosis, which explains the attachment of the organisms. In ectosymbiosis, the symbiont lives on the host’s body surface. Some examples of this are barnacles attached to baleen whales. On the other hand, the symbiont lives within the tissues of the other organism in endosymbiosis. This type of symbiosis only occurs due to the host organism lacking certain nutrients that the endosymbiont provides.
Symbiotic relationships can be classified into 4 main categories: mutualism, commensalism, parasitism, and competition. To go over this briefly, commensalism is a relationship where one symbiont benefits and the other is unaffected (neither harmed nor helped). Moving on, in a parasitic relationship, the parasite benefits while the host organism is harmed. Next, competition can be either intraspecific (between organisms of the same species) or interspecific (between organisms of different species) as a result of the struggle for limited resources, such as food and water. The main section of symbiosis discussed today will be mutualism.
Mutualism was coined by Pierre-Joseph van Beneden in his book, describing it as mutual aid among species. Mutualism – otherwise known as reciprocal altruism – essentially means that both species must benefit from the long-term relationship. There are 3 types of mutualistic relationships: resource-resource, service-resource, and service-service. Service-resource interactions are most commonly found, whereas service-service relationships are rare and usually conflated with one of the other two.
Mutualism is crucial to maintaining ecosystem health and diversity. For example, plant-pollinator mutualistic relationships can increase the abundance, productivity, and temporal stability of organisms in the food web, even if not directly connected to the symbionts. They act as foundational interactions to the network that regulate the ecosystem. Hence, mutualism shapes the biodiversity of ecosystems and organisms by allowing for diversification of life (Bascompte, 2019).
Case Study 1: Clownfish & Sea Anemone
This relationship – as expertly depicted in movies like “Finding Nemo” – is a clear example of mutualism. In this case, clownfish are considered to be obligate symbionts, whereas sea anemone are facultative symbionts (Allen, 1972, Fautin and Allen, 1997). Sea anemone, living atop coral reefs, have nematocysts on their tentacles, which are used to sting and trap prey. When animals come in contact with nematocysts, toxins are released to paralyse the organism, thus allowing for easy ingestion. However, clownfish have a mucus covering their bodies, which its chemical composition is immune to the sting of nematocysts. Thus, the anemone provides the clownfish with protection from its predators, as other species of fish cannot tolerate the stings of nematocysts. At the same time, the bright colours of clownfish attract other species, which will be stung and ingested by the anemone. In terms of service, clownfish also protect anemones against their predators, such as butterflyfish and other parasites.
Figure 1: Demonstrates clownfish in sea anemone (Source: Amanda Cotton / Coral Reef Image Bank)
According to research by Modi Roopin and Nanette E. Chadwick in 2008, an additional benefit from this mutualistic partnership was found. Clownfish waste is rich in ammonia, which fertilises the anemone. The clownfish waste provides supplementary nitrogen uptake (ammonia, sulfur, and phosphorus) for the zooxanthellae algae in the tentacles of the host anemone, allowing the zooxanthellae to supply the anemone with energy-rich photosynthetic compounds for growth (Steen, 1988, Achituv and Dubinsky, 1990, Whitehead and Douglas, 2003). Since anemones have no control over the nutrient composition of the water and live on a passive feeding strategy, this ensures that the host anemones maintain health.
Case Study 2: Ants & Acacia Trees
Another example of a mutualistic relationship is the obligate partnership between ants in the genus Pseudomyrmex and acacia trees, which have been reclassified to the genus Vachellia. A study found that the acacia trees develop the necessary traits to host a colony of ants, such as hollow thorns and Beltian bodies (nutrient-rich leaflets). In exchange for shelter, the ants defend acacia trees from herbivores and reduce interspecies competition from other plants by trimming other vegetation. Other than visual and chemical cues from predators, the ants also detect the herbivores by vibrations induced by browsers feeding on the tree. Experiments showed that the arts are able to differentiate browser-induced vibrations from wind-induced vibrations. Once detected, the ants use it as an alarm cue and begin to patrol. Moreover, the ants use tropotactic directional vibration sensing to find the attacked part of the tree (Hager et al., 2019).
Figure 2: Depicts an acacia ant in the thorn of an acacia tree (Source: Dan Janzen)
Additionally, the ants feed on the extrafloral nectaries and Beltian bodies, which are rich in carbohydrates and lipids. Sometimes, depending on the size of the herbivore, the ants may predate on those as well after protecting the tree from the herbivore.
Conclusion
As seen, the natural world thrives on cooperation and collaboration in relationships like the sea anemone and clownfish. In summary, mutualism in symbiosis is central to the ecological diversity and stability of food webs, networks, and communities. As these interdependent relationships underpin larger ecological communities, the loss and disruption of mutualistic relationships suggests broader implications of loss of biodiversity, food security risks, and more. In the face of potential threats like climate change and habitat loss, mutualism is at risk. Sustainable practices and conservation methods that value mutualism must be used to safeguard the community’s ecological network and health.
Article prepared by: Estelle Sia Yu Qi, MBIOS R&D Director 24/25
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References
Hager, F. A., & Krausa, K. (2019). Acacia Ants Respond to Plant-Borne Vibrations Caused by Mammalian Browsers. Current Biology, 29(5), 717-725.e3. https://doi.org/10.1016/j.cub.2019.01.007
Hale, K.R.S., Valdovinos, F.S. & Martinez, N.D. Mutualism increases diversity, stability, and function of multiplex networks that integrate pollinators into food webs. Nat Commun 11, 2182 (2020). https://doi.org/10.1038/s41467-020-15688-w
Leichty, A. R., & Poethig, R. S. (2019). Development and evolution of age-dependent defenses in ant-acacias. Proceedings of the National Academy of Sciences of the United States of America, 116(31), 15596–15601. https://doi.org/10.1073/pnas.1900644116
National Geographic Society. (2023, October 19). Symbiosis: the Art of Living Together | National Geographic Society. Education.nationalgeographic.org; National Geographic. https://education.nationalgeographic.org/resource/symbiosis-art-living-together/
Palmer, T. M. (2023). Acacia ants. Current Biology, 33(11), R469–R471. https://doi.org/10.1016/j.cub.2023.02.002
Roopin, M., & Chadwick, N. E. (2009). Benefits to host sea anemones from ammonia contributions of resident anemonefish. Journal of Experimental Marine Biology and Ecology, 370(1-2), 27–34. https://doi.org/10.1016/j.jembe.2008.11.006
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