Coral animals or corals are marine, mostly colonial polyploid coelenterates, looking like miniature sea anemones and living in a skeleton of their own. Their calcareous or horny skeleton is also commonly known as coral. The coral colony grows continuously in size by budding of the polyp and often forms coral reefs and coral islands. According to Vaughan (1917), “A coral reef is a ridge or mound of lime stone, the upper surface of which is near the surface of the sea and it is formed of Calcium carbonate by the action of organisms, chiefly corals,” The coral reefs are banks of coral rocks built upon the sea-bottom, about the shores of tropical islands. They are confined to zones extending about 28 degree on either side of the equator. They secrete calcareous skeleton, which along with the shells of molluscs, echinoderms, annelids, and foraminifera’s get cemented together into a compact rock by encrusting organisms and by depositing lime in course of time assume huge size.
Most reefs grew at the rate of 10-200 mm each year. Most of the existing reefs could have been formed within a period of 15,000 to 30,000 years.
THE CORAL POLYP AND CORALLITE
The coral polyp that secretes huge calcareous skeletons is about 1 cm long with whorl of tentacles surrounding the mouth that leads to pharynx and columella. The coelenteron is divided by septa into chambers in which septal filaments with nematocysts are present to paralyse and kill the prey. Polyps that are connected together by gastrodermal tubes multiply by budding. Each polyp secretes a cup-like calcareous skeleton around itself into which it withdraws for protection. Fused calcareous mass of many polyps is called Corallum which form part of extensive coral reefs.
CONDITIONS SUITABLE FOR CORAL GROWTH
Corals grow in tropical and subtropical seas up to a depth of 50 metres and where temperature is 20-25 degrees Celsius. There should be plenty of light and availability of planktons. Massive coral growth takes place between 10 and 30 metres. With the change in the sea level, growth of corals also changes, making coral reefs of different sizes and depths. Reef formation involves constructive and destructive phases of corals. Reefs are destroyed by environmental forces and by boring sponges and bivalves and are rebuilt by polyps.
KINDS OF CORAL REEFS
The coral reefs are mainly of three types:
Fringing Reef: Coral reefs lying close to the shores of some island or part of a continent are called fringing reefs. A fringing reef may extend out to a distance of a quarter mile from the shore with the most active zone of the coral growth facing the sea. This seaward zone is commonly called the edge or front. A shallow water channel, 50 to100 meters broad, lies between the reef-edge and shore. At low tide, water of channel recedes, quickly exposing a flat bottom surface, called reefflat.
Barrier Reef: Barrier reefs are like fringing reefs but they are located some distance away from the shore. The stretch of water, separating the barrier reef from land, may be half a mile to 10 miles or more in width. It is called a lagoon. It is 10 to 50 fathoms deep and suitable for navigation. Most notable example of barrier reef is the Great Barrier Reef along the North-eastern coast of Australia. It is about 2000 Km long and up to 150 km away from the shore.
Atoll: An Atoll is also termed a coral island or lagoon island. It is a ring–like or horse-shoe shaped reef that encircles a lagoon but not an island. The lagoon varies from a few kilometres to about 90 km. across. It may be complete or broken by a number of channels, of which only a few are navigable. Outer side of the reef slopes off rather steeply into the depth of ocean. The Atoll of Bikine, famous for atomic and hydrogen bomb tests, lies in the Pacific Ocean.
FORMATION OF CORAL REEFS
Following important theories have been put forward to explain the formation of coral reefs:
Darwin’s Subsidence Theory
According to this theory, all known coral reefs are found in regions where subsidence of land has taken place. He reasons that, initially the corals grew as fringing reef on the sloping shores of an island in a shallow tropical sea. Subsidence of the islands commenced so that the fringing reef turned into barrier reef, separated from the island by a wide, deep water channel or the lagoon. The island, while sinking became smaller and smaller and finally disappeared entirely beneath the surface of the ocean.
Sir John Murray’s Theory of sea bottom rise
Sir John Murray (1874-1876) advanced the view that the corals grow on the highest peaks of the ocean bottom. The deposition of sediments brings them to an optimum level for coral growth. The barrier reefs and atolls are produced by the better growth of the corals on the edges of the coral deposition and through solution of inner coral rock.
Penck-Daly’s Glaciation control Theory
The theory was propounded by Albrecht Penck (1909) and Reginald Daly (1917) and advocates that during glacial period formation of ice-caps lowered the level of ocean by 60-70 m. At that time extreme cold temperature prevailed. Subsequently ice melted and the temperature rose. The corals began to grow on the flat platform of the ocean bottom and kept pace with rising levels of the ocean forming gigantic reefs.
IMPORTANCE OF CORAL REEFS
Corals of the remote geological past formed coral reefs that became favourable sites for accumulation of petroleum deposits. Thus coral reefs are of much importance to oil industry. Large quantities of corals are shipped every year for the curio trade. The coral reefs serve as habitats for many plants and animals like sponges, molluscs, echinoderms, fishes etc. Some coral islands are used as habitation by man as well. Some corals are highly prized for their decorative value. Coralliumrubrum is considered to be a precious stone in India and China and is treated as auspicious. The red coral and organ pipe coral are used in some indigenous system of medicine in India. Chunks of coral skeletons belonging to species of Porites are used as building material. Coral skeletons serve as raw material for the preparation of lime, mortar and cement because of their calcium carbonate and magnesium carbonate content. Coral skeletons are also helpful in making ridges that may act as natural barriers against sea erosion and cyclonic storms. Coral reefs serve as good nursery grounds for commercially important fishes. Reef fish varieties are more colourful than others.
DESTRUCTION OF CORAL REEFS
Coral reefs are destroyed by pollution and by the increase in CO2 concentration in atmosphere. Warming of the sea surface causes coral bleaching due to the death of symbiotic algae living inside polyps. Water pollution by oil, gas and pesticides is destructive to corals. Soil erosion and sedimentation covers the coral reefs and chokes them. Overfishing and blast fishing destroys coral reef ecosystem. Coral mining by man also destroys coral reefs.