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Corals and the Changing Oceans

Corals are beautiful, living animals that form the base of reef ecosystems worldwide, and have evolved in our oceans over the past 200-300 million years. A single coral animal, called a polyp, consists of a mouth surrounded by tentacles and a simple stomach. Coral polyps reproduce by budding, which forms genetically identical copies of the original animal. Together, the polyps live in large colonies of hundreds to thousands and create what we think of when we hear the word “coral”. However, coral polyps can’t exist on their own; a symbiotic life is the key to their survival. Symbiosis occurs when a close relationship forms between two organisms of different species.

Each coral polyp has a tiny, single-celled plant roommate called zooxanthellae (pronounced “zoa-zan-thel-lay”). The plant lives in the tissues of a coral polyp and shares the nutrients needed for both to survive. Zooxanthellae are also responsible for the vibrant colors seen in different coral reefs around the world. Beyond the individual polyps, colonies of coral provide food and habitat to an estimated 25% of all marine life. They act as nursery grounds, protect coasts from storms, filter the run-off from land, and so much more.

Corals are slow growing animals who are incredibly picky about their surroundings. Ideal temperatures for coral are between 73-84ºF, and when water temperatures change too quickly, they become stressed. Think about it: if you were incredibly stressed, would you want to deal with a roommate on top of everything? When corals feel this change in water temperature, they expel their house plant roommates, exposing the white skeletons underneath. News articles are constantly surfacing showing reefs with little to no color, which is the result of this phenomena called coral bleaching. Without zooxanthellae to provide up to 90% of their nutrients, not only are they hungry, but they are also more susceptible to potentially fatal diseases. As of August 2022, 60% of the famous Great Barrier Reef off the coast of Australia had either moderate or severe levels of coral bleaching.

Left photo by Gary Bell /, right photo by Roger Grace / Greenpeace

Global warming refers to the warming of Earth’s overall temperature. In the last hundred years, we have seen the perpetual rise in fossil fuel burning, which increases the greenhouse gasses in our atmosphere responsible for trapping heat. Temperature changes as small as 1º can lead to bleaching in coral. Corals can survive short-term environmental disturbances, but if they persist, their chances of survival shrink. Since the 1980’s, these bleaching events have become more frequent and more severe.

One 2018 study focused on the association between plastic waste and disease rates in coral reefs. Pathogens and communities of bacteria are able to hitch a ride on plastic debris, and as the waste is transported from land to the ocean, they will find new homes wherever they can. Nearly 125,000 reef-building corals were visually inspected for this report, and the results were frightening. The likelihood of disease on corals free from plastic was less than 5%. In the presence of plastic, this number jumped to 89%. Plastic waste is an entirely human invention, and the pitfalls of its improper disposal are clear.

Coral is classified as a keystone species, meaning it has a disproportionately large effect on its environment compared to its size. More than a quarter of all marine life directly depends on reefs for survival, despite them covering less than 1% of Earth’s surface. Corals hold the potential to change our world as we know it. Bleaching events that lead to significant mortality can entirely change the ecosystem, pointing to significant fish reduction in reef communities. We have seen lowered catch rates for fisherman, as well as decreases in snorkeling and scuba-diving tourists who provide revenue for coastal communities. The common marine animals who frequent reefs, such as sea turtles looking for food and reef sharks looking for sanctuary, are disappearing. As John Hocevar, the director of Greenpeace’s Ocean Campaign so eloquently states, “We need leaders that will work with us to create a world where living sustainably is easier and more fulfilling than one where most of us drive from our coal or gas-powered homes to a coal or gas-powered office. We need leaders with vision, who are not in bed with the fossil fuel industry”.


Hocevar, J. (2016, April 5). What's killing coral reefs? and how can we stop it? Greenpeace USA.

International Coral Reef Initiative. (2020, April 3). What are corals? ICRI.

Lamb, J. B., Willis, B. L., Fiorenza, E. A., Couch, C. S., Howard, R., Rader, D. N., True, J. D., Kelly, L. A., Ahmad, A., Jompa, J., & Harvell, C. D. (2018). Plastic waste associated with disease on coral reefs. Science, 359(6374), 460–462.

NOAA. (2010, March 15). What is coral bleaching? National Ocean Service.,and%20are%20subject%20to%20mortality

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