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What is a reef?

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Welcome to CoreFacts, where we're always short on time and big on science. I'm Jessica Robertson. Today's question is an interesting one.

What is a reef?

The word "reef" has different meanings to different people. To some, the term can apply to shallowly submerged navigational hazards. To a surfer, a reef is an undersea obstruction that can make waves (and surfboards) break. Geologists and biologists generally restrict the definition of a reef to rigid biological constructions that have formed in shallow water. The Earth's first reefs were built about 2.5 billion years ago. From fossil remains, it is known that a variety of organisms have constructed reefs, including bivalves (clams and oysters), bryozoans (coral-like animals), and sponges. Corals have been found in fossil reefs as old as 500 million years.

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