Coral reefs are underwater ecosystems formed by the secretion of calcium carbonate skeletons by tiny animals called coral polyps.

Types: Classified as Fringing, Barrier, and Atolls.

Significance: Often called the "rainforests of the sea" due to their high biodiversity. They also protect coastlines from erosion and storm surges.

Threats: Coral Bleaching (expulsion of symbiotic zooxanthellae algae due to high sea temperatures), ocean acidification, pollution, and destructive fishing practices.

Indian Location: Major reef areas are found in the Gulf of Mannar, Palk Bay, Lakshadweep, and Andaman & Nicobar Islands.