UPSC CAPF AC 2017 Paper

Corals is limestone rock made by accumulation of -skeletons of tiny marine organisms called coral polyps.
The following are five conditions which favour occurrence of coral reefs.
Sunlight Coral reefs need to grow in shallow water where sunlight can reach them. Corals depend on the Zooxanthellae (algae) that grow inside of them for oxygen and other things, and since these algae need sunlight to survive, corals also need sunlight to survive.
Clean Water Corals need clean water that lets the sunlight through. They do not thrive well in muddy like near river mouth. Sediments and plankton can cloud water which decrease the amount of sunlight that reaches the Zooxanthellae.
Warm Water Reef building corals require warm water condition to survive. Different corals living in different regions can withstand various temperature fluctuations. However, corals generally thrive in water temperature of 20-30° C.
Salty Water Corals need salty water to survive and require certain balance in the ratio of salt to water. This is why corals do not live in areas where rivers drain fresh water into the sea.