Liz Austin, age 13, of Austin, Texas, for her question:
HOW IS A CORAL REEF FORMED?
Coral is a limestone formation that is made by millions of tiny animals in the sea. Coral formations look like domes, tiny organ pipes and branching trees. The coral forming animals color the formations in beautiful shades of green, purple, yellow, tan and orange.
When the tiny sea animals die, they leave limestone "skeletons" that actually form the foundations of barriers and ridges in the sea which are called coral reefs.
Most coral reefs are found in warm and tropical seas because the reef forming corals cannot live in water colder than 65 degrees Fahrenheit. Reefs exist in great numbers throughout the South Pacific, the East Indies and the Indian Ocean to Sri Lanka, and around Madagascar on the southeastern African coast.
Coral reefs can also be found along the tropical eastern coast of Brazil, through the West Indies, along the Florida coast and the shores of Bermuda.
Coral reefs do not develop on the East Coast of North America north of Florida and Bermuda. But small patches of coral grow as far north as New England. Certain kinds of coral grow as far north as the Arctic Circle.
There are three types of coral reefs: fringing reefs, barrier reefs and atolls.
Fringing reefs are submerged platforms of living coral animals that extend from the shore out into the sea.
Barrier reefs follow the shoreline, but are separated from it by water. Often a barrier reef will be made up of a long series of reefs separated by channels of open water.
An atoll is a ring shaped coral island in the open sea. It forms when coral builds up on a submerged mudbank or on the rim of a crater of a sunken volcano.
The animals that form coral belong to the same animal group as the hydras, jellyfish and sea anemones. Most individual coral animals, called polyps, measure less than an inch in diameter.
A coral polyp has a cylinder shaped body. At one end is a mouth surrounded by tiny tentacles. The other end attaches to hard surfaces on the sea bottom.
Most coral polyps live together in colonies. The stony corals attach themselves to each other with a flat sheet of tissue that connects to the middle of each body. Half of the coral polyp extends above the sheet and half below.
Coral polyps build their limestone skeletons by taking calcium out of the seawater. Then they deposit calcium carbonate, or limestone, around the lower half of the body. As new polyps grow, the limestone formation become larger and larger.
Tiny swimming animals, such as the young of many kinds of shellfish, provide the coral polyps with food. Also, coral reef animals cannot live without algae. They use some food manufactured by algae that live in the polyps' own tissue.
Coral polyps reproduce either from eggs or by budding. Small growths called buds appear on the body of an adult polyp, or on the connecting sheet, from time to time. These buds grow larger, separate from the parent and start to deposit their own limestone. With some corals, eggs grow into tiny forms that swim away to form new colonies.