This type of feeding occurs mainly at night when the density of plankton is the highest, while during the day the polyp remains retracted inside its skeleton. This calcification by corals enables scientist to infer considerable information about past marine environmental conditions like sea level, siltation and temperature. There are basically three tyres of coral reef formation. The patch reefs Fringing reefs develop around oceanic islands close to the shore. The reef flat tends to be shallow on the land ward side and gives rise to a reef crest emerging out of the water.
Barrier reefs tend to develop along the edge of the continental shelf.
A wide and deep lagoon separates the coast from the reef. Patch reefs tend to form on the sea bed of the continental shelf that is raised and close to the surface. Biological Diversity and Economical Value Coral reefs are biologically very diverse ecosystems, as diverse as tropical rain forests. The reef's structure provides shelter to a wide range of organisms like fish, crabs and invertebrates. Furthermore, wave action and different organisms, in search of food and shelter, attack the whole structure.