Underwater structures built from calcium carbonate secreted by colonies of tiny marine animals called coral polyps, found in warm, shallow, clear tropical seas.
- Grow best in warm (about 20 to 25° Celsius), shallow, clear, salty, sunlit waters between roughly 30° north and south of the equator.
- Corals live in symbiosis with algae (zooxanthellae) that give them colour and food; loss of this algae causes coral bleaching (whitening), often due to warming seas.
- Three classic types (Charles Darwin's classification): fringing reefs (attached to the shore), barrier reefs (separated by a lagoon), and atolls (ring-shaped around a lagoon).
- The Great Barrier Reef off Australia is the world's largest; in India, reefs occur in the Gulf of Mannar, Gulf of Kachchh, Lakshadweep, and the Andaman and Nicobar Islands.
- Often called the "rainforests of the sea" for their high biodiversity; threatened by warming, ocean acidification, and pollution.
The reef types (fringing, barrier, atoll), growth conditions, the bleaching mechanism, and Indian reef locations are recurring oceanography and environment facts.
Fringing (shore-attached) versus barrier (separated by a lagoon) versus atoll (ring around a lagoon); bleaching is loss of symbiotic algae, not the death of the reef itself.
Calcium-carbonate structures built by coral polyps in warm shallow seas; types are fringing, barrier, and atoll.