## TL;DR

Coral reefs are underwater ecosystems built by calcium carbonate skeletons of coral polyps — tiny animals with symbiotic algae (zooxanthellae). Cover <0.1% of ocean floor but host 25% of marine species. Great Barrier Reef (2,300 km, Australia) is the largest living structure on Earth. Bleaching: corals expel algae under heat stress → turn white → die.

## Core Explanation

Coral bleaching: triggered by 1-2°C above normal summer temperatures. Recovery possible if stress is brief. Threats: climate change (ocean warming + acidification), overfishing, pollution, coastal development. Acidification: CO₂ dissolves in ocean → carbonic acid → dissolves coral skeletons. Great Barrier Reef: 50% coral loss since 1995. Restoration: coral gardening, assisted evolution, marine protected areas.

## Further Reading

- [The Biology of Coral Reefs (Sheppard, Davy, Pilling, 2nd Ed)](https://global.oup.com/academic/product/the-biology-of-coral-reefs-9780198787358)