The Great Barrier Reef is made up of more than 2500 massive individual reefs. These reefs have been built by tiny coral polyps over thousands of years. Coral polyps live in colonies. Each different shaped and colourful portion of a reef is made by a separate colony representing a different species. There are over 350 species of hard coral along the Great Barrier Reef.
Only the hard corals build reefs. The outer base (bottom) of the coral polyp is anchored by a secretion of limestone or calcium carbonate. This forms the polyp’s skeleton. The polyps keep laying down new layers of limestone. A colony of thousands of polyps over a few years can build large limestone structures. Different species build different shaped structures.