IMPORTANT USER ALERT (2 July 2024): This past weekend, a storage server at NOAA's Center for Satellite Applications and Research, which delivers the NOAA Coral Reef Watch data, suffered a hardware failure. The issue is being addressed, and we hope to have the NOAA Coral Reef Watch data back up and running as soon as possible. (1) The most recent data will be made available first; historical data may take time to be restored. (2) If you need specific data that are not yet available, please contact us at coralreefwatch@noaa.gov. (3) If you use code to pull data, please revise your code to use “…/socd/…” in lieu of “…/sod/…” in all relevant web addresses for all future needs. We apologize for any inconvenience this may cause.




Satellites & Bleaching

Coral Bleaching

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What Are Corals and Coral Reefs?   |   Coral Bleaching   |
   Why Should We Care About Our Coral Reefs?
coral polyps

Close-up view of the polyps on the surface of a coral. Photo Credit: Flower Garden Banks National Marine Sanctuary/George P. Schmahl. Click the image for a larger view and more information.

You can think of a coral colony as a skeleton of rock. On the surface of that rock lives Polly, a tiny coral polyp. Polly is an animal, closely related to sea anemones. She gets some of her food by catching small plants and animals that float by over the reef, but mostly she grows her own food. Her body is basically transparent, but she has thousands of microscopic plant cells growing beneath her skin. Inside Polly, these plants (algae), called zooxanthellae (zo-zan-THEL-ay), get a safe place to live and the nutrition they need to grow. Most of Polly's energy (food) comes from the zooxanthellae, and they also help her build the rocky home where she lives. It is a classic example of a mutualistic symbiosis -- both the zooxanthellae and Polly benefit from their cooperation.

Polly is just one tiny polyp, and most coral colonies have many thousands of polyps covering the surface. Each square meter of coral surface may have more than 10,000 polyps (more than 1,000 per square foot).

A healthy coral reef can have tens of thousands of coral colonies in each square kilometer of reef. So when we talk about a coral reef, think about Polly and millions of her polyp cousins. They have attached to submerged rocks or other hard surfaces along the edges of islands or continents, in tropical and subtropical waters, and have grown and expanded into a field of coral colonies that help form a coral reef.

To learn much more about corals and coral reefs, visit the Corals Tutorial produced by NOAA's National Ocean Service.


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