Coral reefs are underwater ecosystems built by tiny animals called coral polyps. These polyps secrete hard calcium carbonate skeletons, which accumulate to form the structures recognized as reefs. Often called “rainforests of the sea,” these biodiverse habitats provide a home for a quarter of all marine species, despite occupying less than 0.1% of the global ocean area.
Global Zones of Coral Reefs
Coral reefs thrive under specific environmental conditions, limiting their global distribution primarily to tropical and subtropical waters. The vast majority of large reef systems are found within a zone roughly 30 degrees north and 30 degrees south of the equator.
Corals require warm ocean temperatures, ideally ranging between 20°C and 29°C (68°F and 84°F). They also depend on shallow, clear, and sunlit waters, typically less than 50 meters deep, because corals engage in a symbiotic relationship with microscopic algae called zooxanthellae, which perform photosynthesis. Additionally, coral reefs need stable saline water and thrive in waters with low nutrient levels, which prevents excessive algal growth. Areas like the “Coral Triangle” in the Indo-Pacific exemplify a region where these conditions converge, fostering exceptional marine biodiversity.
Prominent Coral Reef Regions
The Indo-Pacific region contains the largest concentration of coral reefs, accounting for about 92% of the world’s total reef area. Within this expansive zone lies the Coral Triangle, a marine area spanning Indonesia, Malaysia, Papua New Guinea, the Philippines, Solomon Islands, and Timor-Leste. This region is recognized as the global center of marine biodiversity, hosting over 76% of the world’s coral species.
Australia’s Great Barrier Reef, located off the coast of Queensland, is the world’s largest coral reef system, stretching over 2,300 kilometers and comprising more than 2,900 individual reefs. Other significant Indian Ocean reefs include those found in the Maldives, Seychelles, Mauritius, and the Chagos Archipelago, as well as the Red Sea. The Pacific also features extensive reef systems around islands such as French Polynesia and New Caledonia, the latter boasting the second-longest double barrier reef globally.
The Caribbean Sea and Atlantic Ocean host about 7.6% of the world’s coral reefs. The Mesoamerican Barrier Reef System, stretching along the coasts of Mexico, Belize, Guatemala, and Honduras, is the second-largest barrier reef system on Earth. Notable reef areas also exist around various Caribbean islands, including the Bahamas, and off the coast of Florida, home to the third-largest barrier reef in the world, the Florida Reef Tract. Even Bermuda, outside the typical tropical band, sustains coral reefs due to the warming influence of the Gulf Stream.
How Reefs Form in These Locations
Coral reefs begin when free-swimming coral larvae settle and attach to stable, submerged surfaces. Once attached, these larvae develop into polyps, which then secrete calcium carbonate to build their hard exoskeletons. As successive generations of polyps grow and die, their calcium carbonate skeletons accumulate, forming the foundational structure of the reef over thousands to millions of years.
The specific geological features and proximity to land influence the development of different reef types. Fringing reefs are the most common type and grow directly from the shoreline, often separated by a narrow, shallow lagoon. Australia’s Ningaloo Reef exemplifies a large fringing reef.
Barrier reefs, in contrast, parallel the coastline but are separated from land by a deeper, wider lagoon. They often originate as fringing reefs that grow outwards. The Great Barrier Reef is a prominent example.
Finally, atolls are circular or oval-shaped reefs that enclose a central lagoon. They typically form around volcanic islands that have completely subsided beneath the ocean surface, leaving only the ring of coral. The numerous atolls in the South Pacific, like those found in the Maldives, illustrate this formation.