Formation

• Coral reefs are made up of the limestone skeletons of small invertebrates. These animals create hard outer skeletons to protect themselves, and when they die they leave behind their skeletons. The skeletons left behind accumulate for thousands of years and become a reef.
  
  

Types

• There are three basic types of reef. A fringing reef extends from the shore to the sea, while a barrier reef develops with a lagoon between the shore and the reef itself. An atoll is a ring of coral reef surrounding a place where a land mass used to be before it sank into the water.
  
  

Function

• The Ecological Defense Fund states that coral reefs are essential to the global calcium cycle, fixing roughly half of all the calcium that enters the sea. It turns the calcium into the calcium carbonate of the reefs. Reefs are also important habitats for a wide variety of sea life, including fish, sea urchins and starfish.