The Coral reef are massive limestone structures that provide shelter for nearly one quarter of all marine life. As one of the most complex ecosystems on the planet, coral reefs are home to over 4,000 different species of fish, 700 species of coral, and thousands of other sea plants and species. Corals are tiny animals called Polyps that generally group together by the thousands forming colonies, which attach to hard surfaces of the sea floor. A colony of coral polyps form a coral tree or mound. A polyp has a sack-like body and an opening or mouth encircled by numerous stringing tentacles called Cnidae. The polyps extract calcium carbonate from the seawater to construct a hard cup-shaped skeleton in an infinite variety of sizes. This limestone skeleton protects the soft and delicate body of the polyp. Coral polyps are nocturnal, remaining inside their skeletons during the day. At night, however, they feed and can be seen by their protruding tentacles.
Polyps eat in different ways depending on their species,. Hard coral polyps are nourished by tiny algae called Zooxanthellae. Zooxanthellae live within the tissue of hard corals, and supply them with up to 98% of their nutritional needs. The tiny algae uses energy from the sun to convert carbon dioxide into oxygen, commonly known as Photosynthesis. Hard corals only live in clear and shallow waters. Coral generally live in warm shallow water, (temperatures ranging from 70 to 85 degrees Fahrenheit), as sunlight can easily filter through to their symbiotic algae, Zooxanthellae.
Fresh water can kill corals so they are not found near coastal areas with excessive run-off.