Coral reefs grow in shallow tropical water where sunlight still reaches the sea floor. T reef its is bu by ti animals cal polyps, wh draw cal from t water a harden i into stone. Inside every polyp lives a microscopic alga that makes sugar from sunlight and passes most of it to its host. When the sea grows too warm the polyps expel these algae and the reef turns white. A bleached reef is not dead, but it is starving, and it rarely survives a long heat wave.

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