What Is A Coral Reef?These mounds and ridges found in warm, shallow seas all over the world were formed over thousands of years by coral polyps. There are two types of corals, hard coral and soft coral. Hard coral, tiny, colonial organisms that secrete a hard, outer skeleton made of calcium, lay the foundation of, and build up, reef structures. As the old coral polyps die, new communities of living polyps help build up the coral reef, little by little, every year. The top layer of coral reefs therefore contains living polyps. The soft corals, tiny, bendable, plant-like marine organisms, are also found in coral reefs; however, since they do not leave behind a hard outer skeleton of calcium carbonate, soft corals cannot help build up the reef. Soft corals therefore are most often found at the top, living layer of coral reefs.
The result of this accumulation of a variety of species of hard corals, as well as soft corals are thousands of colorful and unique coral reefs all over the world, extending over 110 000 square miles and providing shelter to billions of fish. As these coral reefs expand, each one develops into one of three different characteristic structures. They become fringing reefs, barrier reefs, or atoll reefs. Fringing reefs form borders along the shoreline and around surrounding islands. Barrier reefs also border shorelines, but they extend greater distances away from the shore. They are separated from the land mass by a deep lagoon. An Atoll forms when a fringing reef surrounding a volcanic island sinks below sea level, while the coral continues to grow upwards. Where are Coral Reefs Located?The location of coral reefs depends greatly upon the temperature, salinity and depth of the water.
In addition to requiring very saline water, reef-building corals cannot tolerate water temperatures below 18 degrees Celsius. Therefore, coral reefs flourish in shallow, marine areas (less than 120 feet) in tropical latitudes where the sun is able to reach the coral and warm the area up. These areas where most corals are restricted to, where sunlight penetrates to a depth of approximately 70 meters, make up the euphotic zone. In deeper areas (disphotic and aphotic zones), not enough light penetrates the depths, which means the reefs' main food producers, algae, and plankton, cannot photosynthesize. Therefore, coral reefs are most often found in shallow, but also tropical zones, near the equator, like the Caribbean Sea, the western Indian Ocean and the western reaches of South Pacific. However, some coral reefs live in non-tropical zones; They live in the Red Sea, where a lot of heat from the sun caused by the surrounding desert climate provides the needed warmth, and Australia's Great Barrier Reef, which is kept warm by a tropical Pacific Ocean current. 
World map indicating in red coral reef zones
|
Diagram of an individual coral polyp
.
Diagram of different ocean sunlight zones
|
