The longest barrier reefs occur off the coasts of Australia and Belize. Atolls rest on the tops of submerged volcanos. They are usually circular or oval with a central lagoon. Parts of the atoll may emerge as islands. Over 300 atolls are found in the south Pacific.
Coral reefs provide habitats for a large variety of organisms.
These organisms rely on corals as a source of food and shelter. For more background on corals and coral reefs, see the section of the links page with General Coral Reef Information . Coral reefs and their associated communities of seagrasses, mangroves and mudflats are sensitive indicators of water quality and the ecological integrity of the ecosystem. They tolerate relatively narrow ranges of temperature, salinity, water clarity, and other chemical and water quality characteristics. Reefs thus are excellent sentinels of the quality of their environment.