Coral Reefs
Living cities built, over centuries, by tiny animals.
Shallow tropical structures built by corals, crowded with fish, invertebrates and color.
Overview
Coral reefs are built by colonies of small animals — coral polyps — that lay down limestone skeletons over hundreds or thousands of years. Inside their tissues live microscopic algae that use sunlight to make food, which is why reefs grow in clear, shallow, warm water. The resulting structure offers shelter, nurseries and feeding grounds to a remarkable range of marine life.
Climate
Warm, clear and sunlit tropical water with stable conditions.
Where it is found
Tropical seas, mainly between the Tropics of Cancer and Capricorn.
Vegetation
Coral’s partner algae, seaweeds and nearby seagrass meadows provide most of the food.
Living conditions
- Clarity: Corals need clear water so sunlight can reach their partner algae.
- Temperature: A narrow temperature range — prolonged heat causes bleaching.
- Wave energy: Reefs break waves and shelter the coasts behind them.
Key facts
- Area: Less than 1% of the ocean floor (NOAA)
- Water temperature: About 23–29 °C (Typical for reef-building corals (NOAA))
- Depth: Mostly shallower than about 45 m
- Salinity: Normal seawater
How the ecosystem works
- Symbiosis: Corals and algae, clownfish and anemones — partnerships are everywhere.
- Nurseries: Many fish spend their early life sheltered in the reef.
- Grazers: Fish and urchins keep seaweed from smothering the corals.
Food web
- Producers
- Coral algae, Seaweed, Seagrass
- Grazers
- Parrotfish, Green sea turtle, Sea urchins
- Predators
- Groupers, Reef sharks
- Decomposers
- Bacteria, Sea cucumbers
Biodiversity
Reefs support a disproportionate share of marine species for the small area they cover — fish, corals, molluscs, crustaceans and countless partnerships like the clownfish and its anemone.
Threats
- Marine heatwaves — Heat stress makes corals expel their algae and bleach.
- Acidification — More acidic water makes it harder to build skeletons.
- Runoff — Sediment and fertilisers cloud the water and feed seaweed.
- Destructive fishing — Overfishing removes the grazers that keep reefs healthy.
Conservation
Reefs protect coastlines, feed coastal communities and are among the most diverse places on Earth. Their future depends on limiting warming and reducing local pressures.
- Marine protected areas help reefs recover between heatwaves.
- Cleaner rivers mean clearer reef water.
- Protecting grazing fish keeps algae in check.