BharatNotes What is Coral Reefs?
Coral reefs are limestone structures built over thousands of years by colonies of marine invertebrates called coral polyps. Each polyp secretes a skeleton of calcium carbonate (CaCO3); as generations pile up, these skeletons form the reef. Reef-building corals survive through a symbiosis with zooxanthellae (microscopic algae of the genus Symbiodinium) living inside their tissues. The algae photosynthesise and supply the coral with up to about 90% of its energy, while the coral provides shelter and nutrients. This partnership also gives corals their colour.
Often called the "rainforests of the sea", coral reefs cover less than 1% of the ocean floor yet support about 25% of all marine species (NOAA).
Conditions for Growth and Types
Reef-building (hermatypic) corals need specific conditions:
| Factor | Ideal condition |
|---|---|
| Temperature | ~23-27 degrees C (warm tropical/subtropical seas) |
| Depth | Shallow, generally less than ~60 m (sunlight for algae) |
| Water | Clear, sediment-free, salty (oceanic salinity) |
| Light & oxygen | Abundant sunlight; well-oxygenated water |
Charles Darwin (1842) identified three reef types, each a stage in his subsidence theory around a sinking volcanic island:
- Fringing reef — grows directly along the shoreline (most common type).
- Barrier reef — separated from the coast by a wide, deep lagoon (e.g. Australia's Great Barrier Reef, the world's largest at about 2,300 km).
- Atoll — a ring-shaped reef enclosing a central lagoon after the island has fully submerged (e.g. Lakshadweep).
Coral Reefs in India
India's coral reefs are concentrated in four regions (National Biodiversity Authority):
| Region | Reef type | Coral diversity |
|---|---|---|
| Andaman & Nicobar Islands | Fringing | Richest (~177 species) |
| Lakshadweep Islands | Atolls | ~91 species |
| Gulf of Mannar & Palk Bay | Fringing | ~82 species |
| Gulf of Kachchh | Fringing | Poorest (~36 species) |
The Andaman & Nicobar group has the highest coral cover, while the Gulf of Kachchh is least diverse owing to high turbidity and temperature fluctuation.
Current Status: The Bleaching Crisis
Coral bleaching occurs when stressed corals expel their zooxanthellae — temperatures even ~1 degree C above the summer maximum can trigger it. Bleached corals are not dead but are starving and highly vulnerable.
The world is recovering from the fourth global mass bleaching event, which NOAA confirmed and which it determined likely ended in mid-2025. Between early 2023 and mid-2025, bleaching-level heat stress affected about 84% of the world's coral reef area across the Pacific, Atlantic and Indian Ocean basins — the most extensive event on record (NOAA, 2025). By comparison, earlier events affected 21% (1998), 37% (2010) and 68% (2014-17). Beyond warming, reefs face ocean acidification, sedimentation, pollution, destructive fishing and coastal development.
UPSC Angle
Treat coral reefs as a high-yield cross-cutting topic. For Prelims, memorise growth conditions, the three reef types, and India's four reef sites. For Mains GS3, frame answers around climate change (warming + acidification), ecosystem services (coastal protection, fisheries, tourism), and conservation — protected areas, restoration projects, and global initiatives such as the International Coral Reef Initiative (ICRI). Link reef loss to the Sustainable Development Goal on life below water (SDG 14).
