A marine ecosystem is a complex system of living organisms and their environment, which includes the ocean and its inhabitants. A REEF is an underwater structure made up of coral, rocks, and other materials that provide a habitat for a variety of marine life. Reefs are important to the health of the marine ecosystem, as they provide food and shelter for many species of fish, invertebrates, and other organisms. They also help protect shorelines from erosion and provide a nursery for young fish.