Coral reefs are complex underwater structures built by tiny marine animals known as coral polyps. These polyps, typically small and cylindrical, secrete layers of calcium carbonate to form a hard skeleton, which serves as their foundation and protection. Over long periods, these accumulated skeletons create the vast, intricate reef systems that are recognized as some of Earth’s most diverse ecosystems. Despite covering less than one percent of the ocean floor, coral reefs support approximately 25% of all marine species.
Global Distribution of Coral Reefs
Coral reefs are primarily found in tropical and subtropical regions across the world’s oceans. Their distribution is largely confined to a broad zone between 30° North and 30° South latitudes. These underwater ecosystems are prevalent in major ocean basins such as the Indo-Pacific, the Caribbean Sea, and the Red Sea. The Indo-Pacific region, extending from East Africa and the Red Sea to the Hawaiian and Tuamotu Archipelagos, exhibits particularly high concentrations of reefs.
The Western Atlantic also hosts extensive reef systems, including those found in Bermuda, the Bahamas, various Caribbean Islands, Belize, Florida, and the Gulf of Mexico. The highest concentrations of coral reefs globally are found in Southeast Asia and the Pacific, with over half of the world’s reefs located within six countries: Australia, Indonesia, Philippines, Papua New Guinea, Fiji, and the Maldives.
Conditions for Coral Reef Formation
Reef-building corals require warm waters, ideally ranging from 20°C to 29°C (68°F to 84°F), as temperatures below 18°C (64°F) are not tolerated. Stable climatic conditions are also beneficial, as corals are susceptible to rapid changes in their environment.
Shallow depths are necessary for reef-building corals because they rely on sunlight. Optimal coral growth occurs at depths less than 46 meters (150 feet), where abundant sunlight can penetrate the water. This is due to a mutualistic relationship between coral polyps and microscopic algae called zooxanthellae, which live within the coral tissues. The zooxanthellae perform photosynthesis, converting sunlight into energy and providing the coral with up to 90% of its nutritional requirements.
Clear, clean saltwater is another important factor, as suspended sediments reduce light availability and can smother coral polyps. Corals also require stable salinity levels, between 30 and 40 parts per thousand. Strong water movement from waves and currents helps by delivering food, oxygen, and nutrients to the corals while preventing sediment accumulation. The precipitation of calcium from the water is favored by high water temperature and salinity, along with low carbon dioxide concentrations.
Major Reef Systems Worldwide
The Great Barrier Reef, located off the coast of Queensland, Australia, is the world’s largest coral reef system. It stretches for over 2,300 kilometers (1,400 miles) and encompasses more than 2,900 individual reefs and 900 islands. This expansive system supports diverse marine life, including 400 coral species.
The Mesoamerican Barrier Reef System (MBRS), also known as the Great Mayan Reef, is the largest barrier reef in the Western Hemisphere and the second largest globally. It extends for nearly 1,126 kilometers (700 miles) along the coasts of Mexico, Belize, Guatemala, and Honduras. The MBRS has rich biodiversity, providing habitat for over 500 fish species and more than 65 stony coral species, and features a mosaic of marine environments including mangroves and seagrass beds.
The Coral Triangle, often referred to as the “Amazon of the seas,” is a marine region in Southeast Asia encompassing the waters of Indonesia, Malaysia, Papua New Guinea, the Philippines, Solomon Islands, and Timor-Leste. This area covers 5.7 million square kilometers (2.2 million sq mi) and is a global center of marine biodiversity. It hosts over 76% of the world’s shallow-water reef-building coral species and more than 3,000 species of fish.
The Red Sea has extensive and diverse coral reefs, particularly along its northern and central parts. These reefs are predominantly fringing reefs, known for their unique structural formations and remarkable biodiversity. The Red Sea is home to 346 species of hard corals, with about 6% being endemic, meaning they are found nowhere else.