What Are Coral Reefs?
Coral reefs are underwater structures built by colonies of tiny animals called coral polyps. Each polyp secretes a calcium carbonate (limestone) skeleton that accumulates over centuries into massive reef formations. Reef-building (hermatypic) corals thrive in warm (23-29°C), clear, shallow (less than 50 meters), well-lit tropical waters. The three main reef types are fringing reefs (directly bordering a shoreline), barrier reefs (separated from shore by a lagoon—the Great Barrier Reef extends 2,300 km along Australia's coast), and atolls (ring-shaped reefs surrounding a lagoon where a volcanic island has subsided). Though covering less than 0.1% of the ocean floor, coral reefs support an estimated 25% of all marine species, earning them the nickname 'rainforests of the sea.'