What Makes a Coral Reef?

This infographic describes the anatomy of a coral polyp and how polyps form reef structures.

This infographic was made in collaboration with the University of Maryland Center for Environmental Science Integration and Application Network.

Visit the National Coral Reef Monitoring Program's page for all U.S. coral reef status reports.

What Makes a Coral Reef

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Infographic Transcript: What Makes a Coral Reef?

Coral reefs are living, ocean-dwelling animals. Each individual coral exists as a colony consisting of multiple small, identical coral polyps. Wherever corals make up the foundation of an underwater habitat, a coral reef occurs. Coral reefs are complex marine ecosystems that include diverse collections of colorful fish and other sea creatures. But what is it that allows corals to grow and support so much wildlife?

Anatomy of a Coral Polyp
  • Zooxanthellae = symbiotic algae
  • Nematocyst = stinging cells
  • Tentacles
  • Mouth
  • Digestive sac
  • Calyx = skeleton
1. One coral is made of many polyps

Polyps are the basic building block for all coral colonies. They are small, colorful, and essential for corals to grow, eat, reproduce, and recover if ever injured.

2. Corals build coral reefs

Corals build their skeleton from calcium and carbonate in seawater. This skeleton not only gives corals their structure, but also provides the architecture for the coral reef overall.

3. Coral reefs provide food and shelter

With small animals seeking shelter in the coral—and herbivorous fish keeping corals clean of nuisance algae—corals lie at the heart of a complex food web system that allows marine life to thrive in a coral reef.