Coral reefs are marine ecosystems composed of reef-building corals, which are colonies of small, sessile animals known as polyps. These reefs are found in shallow, warm ocean waters and are vital for biodiversity, providing habitat for a vast array of marine life. Coral reefs are also important in the study of marine biology, ecology, and climate change, as they are sensitive indicators of environmental stress.
Reference: Spalding, M. D., Ravilious, C., & Green, E. P. (2001). World Atlas of Coral Reefs. University of California Press.