Coral reefs are underwater structures made from calcium carbonate secreted by corals. Corals are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. These polyps secrete hard carbonate exoskeletons which support and protect their bodies. Reefs grow best in warm, shallow, clear, sunny and agitated waters.
Coral reefs are ecologically important ecosystems that support a diversity of life that surpasses even that of tropical rainforests. Coral reefs are home to an estimated 25 percent of all the kinds of life found in the oceans. Like rainforests, they serve as a rich store of genetic resources. They protect islands and continental coastlines from storm surges and erosion. They also provide fisheries resources, food, and income for indigenous people as well as large commercial operations. One coral reef forms the largest continuous living organism on the planet, the Great Barrier Reef in Australia.
Because they need sunlight to survive, coral reefs generally occur in clear tropical oceans to depths of up to only about 150 feet. Tropical corals prefer waters from 68 to 82oF (20 to 28oC). Consistent ocean temperatures in this range occur between the Tropic of Cancer and the Tropic of Capricorn. Corals that occur outside of this range are individual or colonial forms only and do not form the great reefs found in topical zones. Coral reefs occur in three different forms: fringing, such as in Hawaii, where the corals border the shoreline; barrier reefs, which form offshore with a lagoon between the reef and the land; and atoll reefs that form islands surrounding a central lagoon. Atolls are the tops of volcanic seamounts that lie near enough to the surface of the ocean for coral reefs to form. In this case the island is formed entirely by reefs. Atolls are low, only 7–9 feet above sea level, circular, and surround a central lagoon.
Islands in tropical zones are dependent on coral reefs for many services. The reefs protect the islands from ocean storms and provide a livelihood for many of the island residents. Sea level rise and coral loss due to bleaching exposes tropical islands to seawater encroachment. Many islands are under threat, including the Pacific’s Marshall Islands, Kiribati, Tuvalu, Tonga, the Federated States of Micronesia, and the Cook Islands. Antigua and Nevis in the Caribbean Sea and the Maldives in the Indian Ocean are also threatened. The population of these small islands totals more than 800,000 people. Islands that have supported human populations for thousands of years must now contend with inundation. Because they are generally already so low in elevation, atolls are under the most serious threat, but other islands with higher elevations may find their low-lying farmlands and fresh water supplies ruined by salt water invasion. Sea level is rising by 1 inch per year in much of the Pacific. In 30 years the lowest islands are predicted to be underwater.
A better understanding of the process of coral bleaching may uncover solutions to help slow its progress.Biotechnology offers new tools that can help scientists discover the biological basis of coral bleaching.