Underwater ecosystems are complex and diverse environments that include coral reefs. Coral reefs are made up of colonies of tiny animals called polyps, which secrete a hard calcium carbonate skeleton. These skeletons form the foundation of the reef, providing habitat for a variety of marine life. Reefs are found in shallow, warm waters around the world and are home to a wide range of species, from fish to invertebrates. They are also important for protecting coastlines from erosion and providing food and resources for humans.