Corals are a group of small, tropical marine animals that attach themselves to the seabed and form extensive reefs, commonly in shallow, warm-water seas. These reefs are made up of the calcium-carbonate (limestone) skeletons of dead coral animals. Coral reefs form the basis of complex marine food webs that are richer in species than any other ecosystem (community of plants and animals).
Biology of corals
A coral, or polyp, lives inside a cup-shaped skeleton that it secretes around itself. Resembling a sea anemone, a coral is a jelly-like sac attached at one end in its skeleton. The open end, the mouth, is fringed with stinging tentacles. A coral feeds by sweeping the water with its tentacles and stunning microscopic prey, which it then draws inside itself. Individual corals that gather together in large colonies are usually under one-eighth inch (3 millimeters) long. Living corals are often beautifully colored.
Corals reproduce two ways. Fertilized eggs released by the corals hatch to form larvae. After settling on a suitable surface, the larvae secretes its own limestone cup and grows into a mature coral, thus beginning a new colony. Corals also reproduce by budding, or forming new corals attached to themselves by thin sheets of tissue and skeletal material. In this way, corals grow into large, treelike structures.
Formation of coral reefs
Coral reefs are formed mainly by the hard skeletons of corals and the limestone deposits of coralline algae and other marine animals. Reefs grow upward as generations of corals produce limestone skeletons, die, and become the base for a new generation. Coral reefs lie in a zone of water 30°N to 30°S of the equator. Reef-forming coral animals flourish only in water under 100 feet (30 meters) deep and warmer than 72°F (22°C).
Coral reefs are classified into three main types. Fringing reefs grow close to the shore of a landmass, extending out like a submerged platform. Barrier reefs also follow a coastline, but are separated from it by wide expanses of water. Atolls are ring-shaped reefs surrounding lagoons.
wide and about 1,250 miles (2,010 kilometers) long, and is separated from the shore by a lagoon 10 to 150 miles (16 to 240 kilometers) wide.
Ecology of coral reefs and the damage caused by humans
With it numerous crevices and crannies, a coral reef is a home and feeding ground for countless numbers of fascinating marine life-forms. No ecosystem on Earth plays host to the diversity of inhabitants as found in and around a coral reef. Except for mammals and insects, almost every major group of animals is represented. More than 200 coral species alone are found in the Great Barrier Reef.
Coral reefs also benefit humans by protecting shorelines from the full onslaught of storm-driven waves. Humans, however, are responsible for causing severe damage to coral reefs. Reefs are often destroyed by collectors, who use coral to create jewelry, and fisherman, who use poison or dynamite to catch fish. Because corals need sunlight and sediment-free water to survive, water pollution poses a grave danger. Oil spills, the dumping of sewage wastes, and the runoff of soil and agricultural chemicals such as pesticides all threaten the delicately balanced ecosystem of coral reefs.
The extent of the damage done to the world's coral reefs was made clear by a report issued at the end of the year 2000. The Global Coral Reef
Monitoring Network, an international environmental monitoring organization, issued the report with data gathered from scientists around the globe. According to the report, the world has lost 27 percent of its coral reefs. Some of those reefs can never be recovered, while some could possibly come back. Most of the damaged reefs were found in the Persian Gulf, the Indian Ocean, the waters around Southeast and East Asia, and the Caribbean and adjacent Atlantic. The report pointed out that global warming was the biggest threat facing coral reefs, followed by water pollution, sediment from coastal development, and destructive fishing techniques (such as using dynamite and cyanide). If nothing is done to stop the destruction caused by humans, 60 percent of the world's coral reefs will disappear by 2030.
coral, small, sedentary marine animal, related to the sea anemone but characterized by a skeleton of horny or calcareous material. The skeleton itself is also called coral. Although most corals form colonies by budding, there are some solitary corals; in both types the individual animals, called polyps, resemble the sea anemone in form.
Corals grow in warm and temperate climates and in the cold water found at greater depths, but they are most abundant in warm, shallow water; over 200 coral species are found in the Great Barrier Reef of Australia. In many shallow-water species the polyps contain unicellular plants, which may provide the high oxygen concentration required by such corals.
In the large group known as stony corals, or true corals (Madreporaria), each polyp secretes a cup-shaped skeleton, the theca, around itself. Some solitary corals of that group may reach a diameter of 10 in. (25 cm); in the colonial forms the individual polyps are usually under 1/8 in. (3 mm) long, but the colonies may be enormous. The body of each polyp is saclike, consisting of a wall of jellylike material surrounding a digestive cavity, with a single opening, the mouth, at the unattached end. The mouth is surrounded by tentacles used to capture small prey and is invaginated to form a pharynx leading into the body cavity. Thin sheets of tissue (mesentaries) extend radially from the wall to the pharynx, dividing the cavity. A second set of radial divisions is created by folds (septa) of the outer skeleton and body wall, which extend upward from the floor of the body cavity. Reproduction occurs both sexually and by budding. Sexual reproduction is by means of eggs and sperm, which are produced in the mesentaries and shed into the water. Fertilization results in a free-swimming larva, which attaches to a surface and secretes a skeleton, becoming (in colonial forms) the parent of a new colony.
As new polyps are produced by budding they remain attached to each other by thin sheets of living tissue as well as by newly secreted skeletal material. The great variety in the form of various colonial corals, which may be treelike and branching, or rounded and compact, depends chiefly on the method of budding of the particular species. In the brain corals, for example, each theca merges with the one next to it on either side, forming long rows of polyps separated by deep channels. In some of the branching corals the polyps occupy small, discrete pits on the surface of the skeleton. As a colonial coral produces more polyps the lower members die and new layers are built up on the old skeleton, forming a large mass. In tropical and subtropical regions these massive corals, along with other plants and animals, may form a coral reef. Most of the reef-forming corals belong to the stony coral group.
The soft corals (Alcyonaria) are a group of soft, often feathery forms, with skeletons composed of calcareous or horny particles imbedded in the body wall. Each polyp of a soft coral has eight tentacles. Among the well-known soft corals are the sea pen, sea pansy, whip coral, and organpipe coral. The precious red coral (Corallium) of the Mediterranean Sea, used for jewelry, also belongs to that group. The spicules of its skeleton are fused together.
Stony and soft corals are classified in the phylum Cnidaria, class Anthozoa.
cor·al / ˈkôrəl/ • n. 1. a hard stony substance secreted by certain marine coelenterates as an external skeleton, typically forming large reefs in warm seas. ∎ precious red coral, used in jewelry. ∎ the pinkish-red color of red coral. 2. a sedentary, typically colonial coelenterate (class Anthozoa) of warm and tropical seas, with a calcareous, horny, or soft skeleton. 3. the unfertilized roe of a lobster or scallop. DERIVATIVES: cor·al·loid / -ˌloid/ adj. ( chiefly Biol. Zool. ).
The ancients regarded coral as wood, because of its tree-like appearance. Only at the beginning of the 18th century was it discovered to belong to the animal kingdom and to consist of the skeletons of marine polyps. Stone corals, found mainly in southern waters including the Red Sea and the Bay of Eilat, are the skeletons of the six-armed polyps (Hexacorallia), and are distinguished by their variety of shapes and their beautiful colors. To another group belong the eight-armed corals (Octocorallia), which include the Red Coral (Corallium rubrum). Found in the vicinity of Sicily and along North African shores, the red skeleton of the coral colony, which is extremely hard, is used for making ornaments. Red coral is probably to be identified with the biblical peninim, the color of which is red (Lam. 4:7). The identification of peninim as "pearl" is apparently wrong. The Talmud (rh 23a) tells of Arameans who brought up coral (Aramaic: kesita) from the bed of the sea. In Maimonides (Yad, Kelim 13:6) and in modern Hebrew the word almog is used to designate coral, but the identification is mistaken (see *Algum). Red coral was an important article in the commerce of Jews, especially those of Leghorn in the 17th–18th centuries.
J. Margolin, Zo'ologyah, 1 (1962), 56f.; J. Feliks, Animal World of the Bible (1962), 141. add. bibliography: Feliks, Ha-Ẓome'aḥ, 204.
An organic substance formed from the hard skeletons of marine organisms, consisting mainly of calcium carbonate and magnesium carbonate. In addition to its value as a source of lime, coral has been used for jewelry and personal ornamentation, but from ancient times it has also been used in medicine and magic.
It was believed to stop bleeding, preserve houses from thunder, and protect children from goblins, evil spirits, and sorcery. It was supposed to strengthen digestion and, if taken in powder form, to protect young children from epilepsy. Coral was worn by children from Roman times.
It has also been used for rosaries as well as for bead necklaces and bracelets.