A is the underwater buildup of limestone skeletons from certain small animals?

The underwater buildup of limestone skeletons from certain small animals is called a coral reef. Coral reefs are formed by the accumulation of calcium carbonate skeletons secreted by tiny marine animals called coral polyps. The polyps live in colonies and secrete a hard exoskeleton that builds up over time to form a reef. Coral reefs provide important habitat for a wide variety of marine life and are considered to be one of the most biodiverse ecosystems on Earth.