Massive coral reefs are built by animals a few millimeters across. A coral reef is built of thin layers of calcium carbonate, laid down one tiny polyp at a time — and the scale comes from numbers and time, not size.
Coral polyps form a living mat over a calcium carbonate skeleton. Credit: NOAA.
The builders
The reef builders are the stony corals, or scleractinians. They lay the foundations of reefs and build them up. Each individual stony coral — each polyp — secretes a skeleton of calcium carbonate, and massive reef structures form from those skeletons.
Small animals, heavy colonies
Most stony corals have very small polyps, averaging one to three millimeters (0.04 to 0.12 inches) across. But a colony can grow very large and weigh several tons. It contains millions of polyps, growing on top of the limestone remains of earlier colonies, and over time those layers become massive reefs.
How fast a reef grows
| Conditions | Growth per year |
|---|---|
| Massive corals, in general | 0.5 to two centimeters (0.2 to 0.8 inches) |
| Some species, in favorable conditions — lots of light, consistent temperature, moderate wave action | as much as 4.5 centimeters (1.8 inches) |
Growth that slow means a reef is a long record of the colonies that came before it.
Sources
Based on "What is a coral reef made of?" from NOAA's National Ocean Service, a work of the United States government in the public domain.
许可协议： CC0 1.0（公有领域） · 改编自 oceanservice.noaa.gov
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