Maximum range of biodiversity is noticed at Coral Reefs. Coral reefsare underwater structures made fromcalcium carbonatesecreted bycorals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built fromstony corals, which in turn consist ofpolypsthat cluster in groups. The polyps are like tinysea anemones, to which they are closely related.