Alicia Zheng
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Zooxanthellae: The Secret of Coral Reefs

— Alicia Zheng

Did you know that corals are animals? A coral is made up of thousands of tiny organisms called polyps, invertebrates that can be as small as a pinhead to as large as a foot in diameter. Related to anemones and jellyfish, they have a sac-like body with a mouth surrounded by tentacles. The key to their survival is zooxanthellae, a unique type of algae that provides coral polyps with nutrients and vibrant color!

Coral reefs are one of the most diverse ecosystems in the world, supporting a diverse array of marine animals from clownfish to seahorses to sea stars. Nearly 25% of marine life relies on healthy coral reefs. Well-functioning polyps use calcium carbonate (limestone) from seawater to form a skeleton around their delicate bodies. Each polyp relies on zooxanthellae, which live in its gastrodermis. The zooxanthellae provide nutrients and food to the polyp, and the polyp provides the algae with protection and a space to live. Zooxanthellae is also responsible for the rich and varied colors of coral polyps, which can be anything from brown and yellow to orange and pink!

Coral polyps and zooxanthellae have a symbiotic relationship, a term used to describe the continued interaction between two organisms living in the same physical environment. Specifically, they have a mutualistic relationship, a type of symbiotic relationship from which both species benefit. 

Zooxanthellae act as the “light” of coral reefs, helping produce beautiful and amazing underwater rainforests. Unfortunately, these delicate ecosystems experience the effects of climate change, and are harmed by warming and acidifying oceans. Such negative effects are often visible in the form of coral bleaching, as corals release zooxanthellae (and therefore lose their color) when threatened. This could cause further harm to the coral polyps, though, since without the nutrients provided by the zooxanthellae they become more prone to infection. 

A coral reef is just one example of a beautiful ecosystem harmed by our unsustainable practices. By becoming more environmentally-conscious of our actions, we can work to preserve the beauty and biodiversity of our planet!


Bleaching Biology. Reef Resilience Network. (n.d.).

Hagen, R. (2017, November 9). Underwater and Underrated: Coral Reefs and Climate Change. JSTOR.

Hall, D. (2023, October 19). Symbiosis: The Art of Living Together. National Geographic Society.

How does climate change affect coral reefs?. NOAA’s National Ocean Service. (2023, January 20).,to%20the%20smothering%20of%20coral.

National Oceanic & Atmospheric Administration. (n.d.-b). Corals are animals. Florida Keys National Marine Sanctuary.,of%20tiny%20animals%20called%20polyps

NOAA. (2019, February 1). Coral reef ecosystems | National Oceanic and Atmospheric Administration. NOAA., C. (2021). Coral Reefs: A Very Short Introduction (2nd ed.). Oxford University Press.