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Coral reefs are vast underwater ecosystems often called the "rainforests of the sea" due to their immense biodiversity. They are built by of tiny animals called polyps form hard skeletons calcium carbonate. vibrant structures shelter food for 25 percent all marine . Reefs thrive in warm, clear, shallow waters, primarily in tropical and subtropical regions around the world. They also protect coastlines from erosion and support local economies through fishing and tourism.