The Coral Kingdom
IN THE CRYSTAL CLEAR, WARM WATERS of the tropics, coral reefs flourish, covering vast areas. Made of the skeletons of stony corals, coral reefs are cemented together by chalky algae. Most stony corals are colonies of many tiny, anemonelike individuals, called polyps. Each polyp makes its own hard limestone cup (skeleton) which protects its soft body. To make their skeletons, the coral polyps need the help of microscopic, single-celled algae that live inside them. The algae need sunlight to grow, which is why coral reefs are found only in sunny, surface waters. In return for giving the algae a home, corals get some food from them but also capture plankton with their tentacles. Only the upper layer of a reef is made of living corals, which build upon skeletons of dead polyps. Coral reefs are also home to soft corals and sea fans, which do not have stony skeletons.
INSIDE A CORAL ANIMAL
In a hard coral, a layer of tissue joins each polyps to its neighbor. To reproduce, they divide in two or release eggs and sperm into the water.
Colorful hydrocorals are related to sea firs and, unlike horny and stony corals, produce jellyfishlike forms that carry their sex organs. Known as fire corals, they have potent stings on their polyps.
In living black corals, the skeleton provides support for the living tissues and the branches bear rows of anemonelike polyps. Black corals are mainly found in tropical waters, growing in the deep part of coral reefs. Although they take a long time to grow, the black skeleton is sometimes used to make jewelry.
Sea fans arc gorgonian corals that have soft tissues growing around a central horny or chalky skeleton. They are more closely related to sea pens, organ-pipe coral, and soft corals than to true stony corals. Most kinds live in tropical waters where they often grow on coral rcefs. Some sea fans form branching, treelike shapes (left), but in others the branches join together to form a broad, fan-shaped network. Prom this structure the anemonelike polyps emerge to strain food from the water's currents.
ATOLL IN THE MAKING
An atoll is a ring of coral islands formed around a central lagoon. Charles Darwin (1809-82) thought atolls were formed by a reef growing around a volcanic island which then subsided beneath the surface, a theory later proved to be correct.
Dull green -colored tissue covers the bright red skeleton of living organ-pipe coral. Its anemonelike polyps emerge from each of the tiny pipes in the skeleton. Organ-pipe cora1 is not a true stony coral, but a relative of sea fans, soft coral, and sea pens.
A CORAL BY ANY OTHER NAME
Rose coral is moss animal and grows in colonies on the seabed. Each colony is made of millions of tiny animals, each living in one unit in its leaflike structure.
BIGGEST AND BEST
Here Australia's Great Barrier Reef shows fish feeding on plankton. Over 1,200 miles (2,000 km) long, it is the largest structure in the world made by living organisms. Of the 400 kinds of coral, many spawn on the same night after a full Moon, the water resembling an underwater snowstorm.
Living brain coral's surface is covered with soft tissue. Anemonelike polyps grow in rows along the channels in its skeleton. Brain corals are slow-growing stony corals, increasing in width just over an inch each year.