Some coral is valued for decoration or jewellery, for example, Mediterranean red coral Corallum rubrum . The algae obtain carbon dioxide from the coral polyps, and the polyps receive nutrients from the algae. Corals also have a relationship to the fish that rest or take refuge within their branches, and which excrete nutrients that make the corals grow faster. The majority of corals form large colonies although there are species that live singly. Their accumulated skeletons make up large coral reefs and atolls.
Coral reefs provide a habitat for a diversity of living organisms.
In 1997, some 93,000 species were identified. One third of the world's marine fishes live in reefs. Diseases Since the 1990s, coral reefs have been destroyed by previously unknown diseases. The white plague attacked 17 species of coral in the Florida Keys, USA, in 1995. The rapid wasting disease , discovered in 1997, affects coral reefs from Mexico to Trinidad.