Creature Feature: Reef-building Corals

While people may be in awe of New York City’s skyline or the intricate detailing on Victorian houses that dot the hills of San Francisco, real city architectural wonder is found, not on the horizon, but below it, in tropical waters across the globe. Coral reefs are the largest geological structures on earth built by living organisms, and house more vertebrate (i.e. having a backbone) species, than any other ecosystem in the world[1]. But wait, The Ethogram is about animals and animal behavior; aren’t coral reefs a mix of plants, animals, and rocks? Yes! However, the organisms that are responsible for the large calcium carbonate structures are reef-building corals (Class: Anthozoa) that are in fact animals that house entire underwater ecosystems!

The Great Barrier Reef is the world’s largest coral reef and is home to thousands of marine species. One of the world’s seven natural wonders, this global heritage site stretches over 2300 km off the coast of Australia. [Source]

If you didn’t know that corals are actually animals, don’t worry – neither did early scientists. They described coral as “neither animals nor plants but are possessed of a third nature” and thus labeled them zoophytes (zoo- meaning animal, –phyte meaning plant)[1,2]. Upon closer examination, a coral head is made up of a colony of polyps. These creatures are related to, and look a lot like, miniature sea anemones. Sea anemones and coral polyps are both essentially stomachs with stinging appendages; their tentacles, armed with nematocysts (venomous or barbed cells), reach out each night to capture prey such as zooplankton or tiny fish. Prey is moved into the polyp’s mouth to be digested in the stomach and waste is excreted the same way it came in (#pottymouth). However, unlike sea anemones, which attach themselves to hard objects, corals build their own hard substrate, secreting calcium carbonate beneath them. The coral “skeleton” may form a variety of shapes, which is often used to differentiate one species of coral from another[1]. Aside from genetics, coral architecture is also defined by the amount of light available as well as the degree of wave impact on the rocky substrate.

Shape, size, and color can be used to differentiate the many species of coral. How many different coral architectural variations do you see? [Source]

Light is vital to coral survival because coral polyps are not only architects but also farmers. Coral polyps provide shelter and nitrogen-rich nutrients to single-celled algae called zooxanthellae (pronounced zoo-zan-thell-ee). These microscopic organisms rely on photosynthesis and reside inside polyp cells. In return for living inside safe coral polyp tissue, the zooxanthellae provide their hosts with oxygen (a byproduct of photosynthesis) and carbohydrate nutrients. This relationship is considered mutually symbiotic since both organisms benefit from each other. Thus, in order to survive and grow, coral is often found in calm, clear waters with a lot of sunlight and low turbidity[1,3].

Coral polyps house microscopic algae called zooxanthellae, essentially providing a home to these single-celled organisms. [Source]

In case you didn’t find architect and farmer to be notable coral careers, they are also underwater city planners. The hustle and bustle of daily life on a coral reef–with a multitude of species busily tending to their shelters, searching for food, and avoiding predators–can only exist because of the intricate architectural foundation provided by reef-building corals. As mentioned earlier, coral reefs are the one of the most diverse ecosystems on earth and support more species per unit area than any other marine environment, including up to 4,000 species of fish, hundreds of species of sponges, crustaceans (e.g. shrimp, lobsters, crabs), echinoderms (e.g. sea stars, sea urchins), tunicates, polychaete worms (e.g. Christmas tree worms), sea turtles, and eels[1,4]. Due to the complexity of coral architecture, night life on the reef can be just as busy as during daytime. Nocturnal species come out of the reef to feed under the stars, while their diurnal neighbors are nestled in the nooks and crannies of the coral skeleton.

While fish swimming around coral heads may be the only animals you see, if you take a closer look in coral crevices, there is a whole new world waiting to be discovered. Take a look at this beautiful sea urchin. [Source: Karli Chudeau]

Despite being such small individual animals, reef-building corals’ contribution to ocean biodiversity makes them competitive contenders for the role of mayor of the seas. Their combined architectural and farming abilities allow an entire underwater ecosystem to flourish in waters that would otherwise be unable to sustain life. So, before you admire your favorite city skyline, remind yourself of the sustainable, metropolitan wonders that occur below the ocean’s surface.

Karli Chudeau is in the Animal Behavior Graduate Group and a part of the UC Davis Center for Animal Welfare. She is interested in conservation management and assessing animal welfare in wildlife rehabilitation settings. Her current research examines how we can use behavioral management interventions, such as environmental enrichment, to improve reintroduction success with pinnipeds. She is also an avid ocean nerd.


1 Fenner, D. (2005). Corals of Hawaii: A Field Guide to the Hard, Black, and Soft Corals of Hawaii and The Northwestern Hawaiian Islands, Including Midway. Honolulu: HI: Mutual Publishing, pp. 1-21.

2 Bowen, J. (2015). The Coral Reef Era: From Discovery to Decline: A history of scientific investigation from 1600 to the Anthropocene Epoch. Springer. pp. 5–7.

3 Barnes, R.D. (1987). Invertebrate Zoology (5th ed). Fort Worth, TX: Harcourt Brace Jovanovich College Publishers, pp. 92-96, 127-134.

4 Reaka-Kudla, M. (1997). The global biodiversity of coral reefs: a comparison with rain forests. In M., Reaka-Kudla, D.E. Wilson, & E.O. Wilson (eds.), Biodiversity II: Understanding and Protecting our Biological Resources. Washington, D.C.: Joseph Henry Press. pp. 83-108.