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.
