The coral reef biome is one of the richest and most productive ecosystems on Earth. Although reefs cover less than one per cent of the ocean floor, they shelter roughly a quarter of all known marine species — which is why biologists often call them "the rainforests of the sea."
What is the coral reef biome?
A biome is a large community of plants and animals shaped by a shared climate and set of physical conditions. The coral reef biome is a marine biome built by colonies of tiny animals called coral polyps. Over thousands of years these polyps secrete calcium-carbonate skeletons that fuse into the vast limestone structures we recognise as reefs.
The living engine of the reef is a partnership: reef-building (hermatypic) corals host microscopic algae called zooxanthellae inside their tissue. The algae photosynthesise and feed the coral; the coral gives the algae a safe, sunlit home. This symbiosis is also why reefs are so sensitive — when stressed corals expel their algae, they bleach and can starve.
Where the coral reef biome is found
Reef-building corals need warm, clear, shallow, sunlit water, so the biome is concentrated in a band roughly 30° north and south of the equator. The great reef regions include the Great Barrier Reef and wider Indo-Pacific, the Red Sea, and the Caribbean. See our full guide to where coral reefs are located.
Conditions that define the biome
- Warm water — most reefs thrive between 23–29°C (73–84°F).
- Sunlight — the zooxanthellae need light, so reefs rarely grow below about 45 m.
- Clear, low-nutrient water — sediment and run-off smother corals and block light.
- Stable salinity — reefs avoid river mouths where freshwater dilutes the sea.
Life in the coral reef biome
The reef supports an extraordinary food web — from algae and seagrasses at the base to reef fish, crustaceans, mollusks, echinoderms and apex predators. Explore how it all connects in our coral reef food web guide, or browse the many coral reef animals that call the biome home.
Why the coral reef biome matters
Reefs protect coastlines from storms and erosion, feed hundreds of millions of people, and underpin fishing and tourism economies worth billions. They are also under serious pressure from warming seas, ocean acidification and pollution. Learn what is being done — and what you can do — in our guide to protecting coral reefs.
