What Is a Coral Reef?

Coral reefs are massive underwater structures made from the calcium carbonate skeletons of coral polyps — tiny soft-bodied animals related to sea anemones. Each polyp secretes a hard limestone cup around itself; over centuries and millennia, successive generations build upon earlier skeletons, creating vast reef structures. Most reef-building corals host symbiotic algae called zooxanthellae, which photosynthesize and provide the coral with up to 90% of its energy, and give reefs their vibrant colours. Coral reefs cover less than 0.1% of the ocean floor yet support about 25% of all marine species, earning them the nickname "rainforests of the sea."