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Coral reefs are confined to a narrow band of the world's oceans, and for good reason: reef-building corals are demanding. They need water that is warm, salty and clear, and outside those limits they cannot survive. That is why reefs are generally found only in tropical and semi-tropical waters.
Coral reef locations around the world. Credit: NOAA.
Warm, but not too warm
| Water temperature | What it means for reef-building corals |
|---|---|
| below 64° Fahrenheit (18° Celsius) | too cold to tolerate |
| 73° to 84° Fahrenheit (23°–29° Celsius) | where many grow best |
| up to 104° Fahrenheit (40° Celsius) | some can tolerate it, for short periods |
Salty
Most reef-building corals also need very salty water, between 32 and 42 parts per thousand.
Clear — because of their partners
The water must also be clear, to let in as much light as possible. The reason lives inside the corals themselves. Most reef-building corals carry photosynthetic algae, called zooxanthellae, in their tissues, and the two depend on each other:
- The coral gives the algae a protected home and the compounds they need for photosynthesis.
- The algae produce oxygen and help the coral get rid of wastes.
- Most important of all, the algae supply the coral with food.
The algae make that food by photosynthesis, which needs light. Cloudy water means less light — and less food for the coral.
Sources
Based on "In what types of water do corals live?" from NOAA's National Ocean Service, a work of the United States government in the public domain.
Licens: CC0 1.0 (offentligt eje) · Bearbejdet efter oceanservice.noaa.gov
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