Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups.

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An isthmus is a narrow strip of land connecting two larger land areas, usually with water on either side. A strait is the sea counterpart of isthmus.

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