Corals are characterized by their polypoid shape, sessility and slow growth and secretion of a solid skeleton  made of calcium carbonate. Numerous coral individuals associate in intraspecific harmonic ecological interaction forming colonies with hundreds, thousands and even millions of beings. Water-permeated solid structures of these colonies, known as reefs, work as true ecosystems for other living beings. The biggest known coral colony is the Great Barrier Reef in the northeast coast of Australia. There are however many coral species whose individuals live alone and do not form colonies.