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Why are reefs only found in shallow waters?Coral reefs are found mostly in warm, shallow waters (less than 30 meters (90 feet)) because they contain algae that need light to live. The algae help keep animals called polyps alive by providing oxygen and food in the form of simple sugars. The polyps, in turn, provide the algae with needed carbon dioxide and ammonia. This partnership is known as a symbiotic relationship, where each organism benefits from the other. Anatomy of El Niño | Chasing El Niño | El Niño's ReachDispatches | Resources | Mail | Site Map | El Niño Home Editor's Picks | Previous Sites | Join Us/E-mail | TV/Web Schedule About NOVA | Teachers | Site Map | Shop | Jobs | Search | To print PBS Online | NOVA Online | WGBH © | Updated November 2000 |