Coral reefs are built by tiny animals called polyps, each no larger than a pinhead. The polyps secrete calcium carbonate, and over thousands of years their skeletons pile up into vast limestone structures. Most reef-building corals live in partnership with microscopic algae, which provide food through photosynthesis and give the coral its brilliant color. In return, the coral offers the algae shelter and the nutrients they need. Although reefs cover less than one percent of the ocean floor, they support roughly a quarter of all known marine species.
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