Coral reefs are structures built by tiny marine animals called coral polyps. These polyps, related to jellyfish and sea anemones, secrete hard external skeletons made of calcium carbonate, which accumulate over thousands of years. Though they cover less than 0.1% of the global ocean area, these habitats provide a home for approximately a quarter of all marine species. Understanding their global location requires examining the environmental conditions that allow the polyps to survive and grow.
Environmental Requirements for Coral Survival
The distribution of coral reefs is constrained by the needs of reef-building organisms. These corals thrive exclusively in tropical and subtropical waters, where temperatures remain consistently stable. Reef-building corals cannot tolerate water temperatures below 64° Fahrenheit (18° Celsius); optimal growth occurs between 73° and 84° Fahrenheit (23°–29° Celsius).
The corals form a symbiotic relationship with microscopic algae called zooxanthellae, which require sunlight to photosynthesize. This necessity restricts reefs to clear, shallow waters, typically less than 50 meters deep. The water must also maintain a specific salinity level, generally between 32 and 42 parts per thousand, and be low in suspended sediment. High sediment loads block the light needed by the symbiotic algae and can also physically smother the coral polyps.
Global Map of Reef Distribution
The stringent environmental requirements confine coral reefs to a tropical zone between 30 degrees North and 30 degrees South latitude. This global distribution is divided into two distinct biogeographic regions: the Indo-Pacific and the Wider Caribbean/Atlantic.
The Indo-Pacific region is the most extensive and diverse area, stretching from the East African coast and the Red Sea across the Indian Ocean and into the Central Pacific. This region contains the greatest number of coral species globally, with diversity nearly ten times greater than that of the Atlantic. Major systems include the reefs surrounding Southeast Asia, Australia’s Great Barrier Reef, and numerous Pacific archipelagos.
The Atlantic region includes the Wider Caribbean, the Gulf of Mexico, and the Caribbean Sea. While it has fewer coral species than the Indo-Pacific, its reefs contain unique species assemblages. Reefs can also occur outside the typical tropical band where warm ocean currents, such as the Gulf Stream, carry warm water farther north to places like Bermuda. The absence of major reefs along large landmasses in otherwise suitable latitudes is often explained by large river systems introducing freshwater and high sediment loads.
The Coral Triangle: Center of Marine Biodiversity
The Coral Triangle, located within the Indo-Pacific, is the global epicenter of marine biodiversity. This roughly triangular region encompasses the tropical waters of six countries: Indonesia, Malaysia, the Philippines, Papua New Guinea, Timor-Leste, and the Solomon Islands. It is often referred to as the “Amazon of the Seas” due to its unparalleled concentration of life.
The Coral Triangle contains approximately 76% of all known coral species, including over 600 types of reef-building corals. This high diversity is attributed to the region’s complex oceanography, which promotes species overlap and larval dispersal. The epicenter of this diversity is found in the Bird’s Head Seascape, located in Indonesian Papua. The region also hosts more than 37% of the world’s reef fish species and six of the seven global marine turtle species.
Different Types of Coral Structures
Coral reefs are classified based on their structural relationship to a nearby landmass. The three main categories are fringing reefs, barrier reefs, and atolls.
Fringing Reefs
Fringing reefs are the most common type, growing directly from the shore of a continent or island. These structures form a border along the coastline and are separated from the land by either a very narrow, shallow lagoon or no lagoon at all.
Barrier Reefs
Barrier reefs are situated farther offshore, running parallel to the coast but separated from it by a deeper, wider channel of water called a lagoon. The Great Barrier Reef off the coast of Australia is the largest example of this structure.
Atolls
Atolls appear as circular or ring-shaped reefs that enclose a deep, central lagoon. They form when a volcanic island, initially surrounded by a fringing reef, slowly subsides beneath the ocean surface. As the island sinks, the coral grows upward toward the sunlight, leaving only the ring of coral where the island once stood.