What are coral reefs also called?

A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups.

Sometimes called ‘rainforests of the sea’, shallow coral reefs form some of the Earth’s most diverse ecosystems. They occupy less than 0.1% of the world’s ocean area. Additionally, shallow coral reefs provide a habitat for at least 25% of all marine species, including fish, mollusks, worms, crustaceans, echinoderms, sponges, tunicates and other cnidarians.

Found mostly at shallow depths in tropical waters, they are also present in deep water and cold water but on a smaller scale.

Because of the sensitivity to water conditions, coral reefs have declined by 50% since 1950. They are under threat from excess nutrients including nitrogen and phosphorus, rising temperatures, oceanic acidification, overfishing, and harmful land-use practices, including runoff.

Considering the larger picture, coral reefs cover 284,300 km2 (109,800 sq mi). They are located in the Indo-Pacific region, including the Red Sea, Indian Ocean, Southeast Asia and the Pacific Ocean, collectively accounting for 91.9% of this total.