How Symbiotic Relationships Shape the World

Biology, the study of life, includes the intra- and interspecific relationships among and between species. One type of interspecific relationship that commonly occurs is called a symbiosis. A symbiosis is a relationship between two different species where both species are in close physical proximity to each other and impact each other in physical and chemical manners. There are three different types of symbiosis: mutualism, commensalism, and parasitism. Mutualism is when the two different species positively impact each other. Commensalism is when one species positively benefits from the symbiosis while the other species is neutrally impacted. Parasitism, the most commonly known symbiotic relationship, is when one species benefits while the other species is negatively impacted. Symbiotic relationships are so important that they have shaped the evolution of life as we know it, and continue to shape our world.

Illustration by Isabel Alcantara.

Terrestrial life wouldn’t be the same without the symbiotic relationships between plants and certain fungi. These mutualistic relationships, called mycorrhizae, form when fungi colonize the root systems of plants. The colonizing fungus has a high surface area to volume ratio, which allows the plant to absorb more nutrients and water. In return, the fungus receives carbohydrates made as a product of photosynthesis. When plants began to colonize the terrestrial landscape, nitrogen and phosphorous proved to be the two most limiting nutrients for plant success (1). An arbuscular mycorrhiza formed between these plant pioneers and fungi belonging to the phylum Glomeromycota, which allowed for increased uptake of phosphorous, thus eliminating one of the two biggest limiting factors for colonization (1). Presently, approximately eighty percent of plant species have an arbuscular mycorrhizal association (1).

Another important, as well as eye-catching, mutualistic symbiosis occurs between corals and dinoflagellate algae called zooxanthellae (2). The zooxanthellae live in the tissues of their host coral (2) and up to ninety percent of the nutrients acquired by that coral are provided by them (3). Zooxanthellae are divided into three clades, and it has been found that one of these clades, clade D, increases the thermal tolerance of their respective symbiotic coral by 1.0-1.5°C (2). As ocean temperatures rise, those corals with type D zooxanthellae may be able to survive better than those corals with types A and C zooxanthellae. Fun fact: the pigments in zooxanthellae are what give corals their vibrant and beautiful colorations!

One really prevalent aquatic parasitic relationship that exists in the more local waters of Yellowstone Lake, WY, is the one that exists between the protozoa Trichophrya and two fish species, cutthroat trout (Salmo clarki) and longnose suckers (Catostomus catostomus) (4). Trichophryans reside in and feed on the gills of these fishes, much to the detriment of the fishes and the benefit of the protozoans (4). The waters of Yellowstone Lake are infested with these parasites, as sixty percent of all longnose suckers and one hundred percent of all cutthroat trout with a total length of greater than or equal to fourteen centimeters were found to be victims of trichophryans (4).

One extreme form of parasitism is seen in multiple species of wasps. The subfamily Microgastrinae is full of obligate endoparasitoid wasps, which means that wasp females lay their eggs into live insect hosts, who continue to live until the eggs hatch and eat their way out of the live hosts (5). They exclusively lay their eggs into Lepidoptera caterpillars, who can survive up til the emergence of the wasp larvae from their bodies (5).

Illustration by Isabel Alcantara.

One example of commensalism is the relationship between whales and the barnacles that adorn them. The humpback whale (Megaptera novaeangliae) is the host for the obligate commensal whale barnacle, Coronula diadema (6). Larval barnacles are able to chemically sense humpback whales, attach themselves onto their hosts’ bodies, and filter feed as the whales travel. The humpback whales aren’t negatively or positively impacted by their little travel companions (6).

These examples of symbiotic relationships range from mild to extreme, from having a passive travel partner to being eaten alive from the inside out. One common theme in nature is the interconnectedness among species, with each species having its own unique relationships with others. These relationships are along a spectrum of beneficial to detrimental, and symbioses in particular and their corresponding effects hold significant importance in the lives of the species involved.

Written by Samantha Duven, Colorado Parks and Wildlife.


1. Lambers, H., Raven, J. A., Shaver, G. R., & Smith, S. E. (2008). Plant nutrient-acquisition strategies change with soil age. Trends in Ecology & Evolution, 23(2), 95-103. https://dx.doi.org/10.1016/j.tree.2007.10.008

2. Berkelmans, R., & van Oppen, M. J. H. (2006). The role of zooxanthellae in the thermal tolerance of corals: a ‘nugget of hope’ for coral reefs in an era of climate change. Proc Biol Sci, 273(1599), 2305-2312. https://dx.doi.org/10.1098/rspb.2006.3567

3. Muscatine, L., & Porter, J. W. (1977). Reef corals: mutualistic symbioses adapted to nutrient-poor environments. BioScience, 27(7), 454-460. https://dx.doi.org/10.2307/1297526

4. Heckmann, R. A., & Carroll, T. (1985). Host-parasite studies of Trichophrya infesting cutthroat trout (Salmo clarki) and longnose suckers (Catostomus catostomus) from Yellowstone Lake, Wyoming. Great Basin Naturalist, 45(2), 255-265.

5. Arias-Penna, D. C., Whitfield, J. B., Janzen, D. H., Hallwachs, W., Dyer, L. A., Smith, M. A., Hebert, P. D. N., & Fernandez-Triana, J. L. (2019). A species-level taxonomic review and host associations of Glyptapanteles (Hymenoptera, Braconidae, Microgastrinae) with an emphasis on 136 new reared species from Costa Rica and Ecuador. ZooKeys, 890, 1-685. https://dx.doi.org/10.3897/zookeys.890.35786

6. Nogata, Y., & Matsumura, K. (2006). Larval development and settlement of a whale barnacle. Biology Letters, 2(1), 92-93. https://dx.doi.org/10.1098/rsbl.2005.0409

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