A coral reef is a diverse marine ecosystem composed primarily of calcium carbonate structures secreted by corals, which are marine invertebrates. These reefs thrive in shallow, warm tropical waters and provide habitats for countless species, making them biodiversity hotspots. Coral polyps, the tiny organisms that build reefs, form symbiotic relationships with zooxanthellae algae, which provide them with energy through photosynthesis. Reefs act as natural barriers, protecting coastlines from erosion and supporting complex food chains in the marine environment.