How Do Coral Reefs Form?

Coral Reefs Are Made of Stony Corals

Colorful coral reef, Similan Islands, Thailand
kampee patisena/Moment/Getty Images

Reefs are centers of biodiversity, where you'll find many types of fish, invertebrates and other marine life. But did you know that coral reefs are also alive?

What Are Coral Reefs?

Before learning how reefs form, it's helpful to define a reef. Acoral reef is made up of animals called stony corals. The stony corals are made up of tiny, soft colonial organisms called polyps. Polyps look a lot like a sea anemone, as they are related to these animals. They are invertebrates in the Cnidaria phylum.

In stony corals, the polyp sits within a calyx, or cup that it excretes. This calyx is made of limestone, also known as calcium carbonate. The polyps are interconnected to form a mass of living tissue over the limestone skeleton. This limestone is why these corals are called stony corals. 

How Do Reefs Form?

As the polyps live, reproduce, and die, they leave their skeletons behind. A coral reef is built up by layers of these skeletons covered by living polyps. The polyps reproduce either through fragmentation (when a piece breaks off and new polyps form) or sexual reproduction through spawning.

reef ecosystem may be made up of many species of corals. Healthy reefs are typically colorful, highly biodiverse areas made up of a mishmash of corals and the species that inhabit them, such as fish, sea turtles, and invertebrates such as sponges, shrimp, lobsters, crabs, and seahorses. Soft corals, like sea fans, may be found within a coral reef ecosystem, but do not build reefs themselves. 

The corals on a reef are further cemented together by organisms like coralline algae, and physical processes like waves washing sand into spaces in the reef. 


In addition to the animals living on and in reefs, the corals themselves host zooxanthellae. Zooxanthellae are single-celled dinoflagellates that conduct photosynthesis. The zooxanthellae use the waste products of the coral during photosynthesis, and the coral can use the nutrients provided by the zooxanthellae during photosynthesis. Most reef-building corals are located in shallow water where they have plenty of access to the sunlight needed for photosynthesis. The presence of the zooxanthellae helps the reef to thrive and become larger.

Some coral reefs are very large. The Great Barrier Reef, which stretches more than 1,400 miles off the coast of Australia, is the world's largest reef.

3 Types of Coral Reefs

  • Fringing reefs: These reefs grow close to the coast in shallow waters.
  • Barrier reefs: Barrier reefs, like the Great Barrier Reef, are large, continuous reefs. They are separated from land by a lagoon.
  • Atolls: Atolls are ring-shaped and located near the sea surface. They get their shape from growing on top of underwater islands or inactive volcanoes.

Threats to Reefs

An important part of coral reefs is their calcium carbonate skeleton. If you follow ocean issues, you know that animals with calcium carbonate skeletons are under stress from ocean acidification Ocean acidification causes a lowering of the ocean's pH, and this makes it difficult for corals and other animals that have calcium carbonate skeletons.

Other threats to reefs include pollution from coastal areas, which can affect reef health, coral bleaching due to warming waters, and damage to corals due to construction and tourism.

References and Further Information:

  • Coulombe, D.A. 1984. The Seaside Naturalist. Simon & Schuster. 246pp.
  • Coral Reef Alliance. Coral Reefs 101. Accessed February 22, 2016.
  • Glynn, P.W. "Corals." In Denny, M.W. and Gaines, S.G. Encyclopedia of Tidepools and Rocky Shores. University of California Press. 705pp.
  • NOAA Coral Reef Conservation Program. Coral Anatomy and Structure. Accessed February 22, 2016.
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Kennedy, Jennifer. "How Do Coral Reefs Form?" ThoughtCo, Feb. 16, 2021, Kennedy, Jennifer. (2021, February 16). How Do Coral Reefs Form? Retrieved from Kennedy, Jennifer. "How Do Coral Reefs Form?" ThoughtCo. (accessed March 21, 2023).