Found in shallow, clear marine waters, tropical coral reefs are among nature's most diverse ecosystems, composed of thousands of species of fish, plants, corals, invertebrates, and microorganisms. Their natural beauty attracts thousands of tourists and divers, but their importance goes far beyond aesthetics. They control the erosion of shorelines. They serve as a source of income and food for local residents, providing as much as 25% of their dietary protein. And current research indicates that coral reef organisms produce biologically active compounds that may play a role in medicines.
These reefs, often thousands of years old, have been built up slowly by algae and corals which leave behind calcified deposits. Many reefs, however, are threatened by both natural and man-made dangers. Hurricanes tear reefs apart, and gradually warming waters bleach and weaken corals. Diseases often become a problem for the weakened organisms. Humans pose additional hazards to the reefs by polluting the waters and by the use of poor fishing practices.
There are many web sites that examine the complex ecosystems of the reefs, and their place in the modern environment:
- An Introduction to Coral Reefs 
- SeaWorld - Corals and Coral Reefs 
- NOAA's Coral Reef News 
- NOAA Reef Photo Library 
- Coral Reef Alliance 
- NOAA's National Ocean Service 
- Planetary Coral Reef Foundation 
- United States Coral Reef Task Force 
- Reefbase