Several reef organisms build their skeletons and shells out of calcium carbonate. When these organisms die, their skeletal remains are transported to the beach or are cemented into the framework of the reef. Most of the light-colored sand on beaches derives from coral reefs. Photo courtesy of John Pye. Coral reefs are sensitive environments that require pristine coastal water quality.
Corals are very efficient marine organisms that thrive in nutrient-poor environments.
This is because coral polyps contain zooxanthellae -unicellular, symbiotic algae that produce food for their hosts through photosynthesis. Photosynthesis requires sunlight, and the depth and intensity of sunlight penetration is reduced by suspended sediments. Silt can also settle out on corals and interfere with feeding and recolonization. Corals can survive occasional short-term siltation events. When stressed they produce mucus, which helps them shed the fine-grained sediments that have settled upon them.