Over 500 million years ago in warm tropical climates coral reefs formed. Reefs are formed when hundreds of hard coral colonies grow next to and on top of each other. The hard corals look like stones and the soft ones resemble plants, an example being sea fans.
All coral is made up of polyps, tiny animals that resemble upside-down jellyfish.
Polyps secrete a skeleton to support themselves and the term coral is used to describe the skeletal remains of these animals, particularly those of the hard corals, which form a limestone base that becomes the foundation of the reef. It is after hundreds of these hard coral colonies grow next to each other that a reef is formed. Astonishingly enough, the whole coral reef ecosystem has been built upon a tiny animal that is smaller in diameter than a common pencil eraser. Since coral polyps grow in nutrient-poor water, they have great ecological significance because they provide a habitat for communities of fish and other marine life in waters that might otherwise be desolate and unproductive. Despite their great importance and beauty, coral reefs are still being threatened around the world by human activities such as pollution, over fishing, boat groundings, and general carelessness of snorkelers and divers.