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ANATOMY OF CORAL
Coral reefs are well known for their underwater splendor, vivid colors and rich species diversity in which 2/3rds of all ocean life exist. The brilliant coloring of many coral species, combined with their 'radial symmetry,' often creates a beauty that is unsurpassed. Reef-building corals are colonies of tiny individual animals called 'polyp.' Each polyp secretes a calcium carbonate exoskeleton (corallite) which protects the soft, sack-like body inside.
A polyp’s tubular body has only one opening, which is surrounded by tentacles. The closed end of the tube forms the base of the polyp where an exoskeleton structure is created, called a basal plate, which has an annular thickening around the edge. In addition, the basal plate is reinforced at six points by separate radial calcareous ribs. These ribs grow upwards, so that they and the basal plate which surrounds them finally extend as the so-called septa into the stomach cavity of the polyp. By means of this exoskeleton structure, the polyp creates a shelter into which it can retire if danger threatens. Finally, once the septa have reached a given height, the polyp creates a new basal plate above them. Whether, when this occurs, the lower part of the polyp is ligatured, and over a continuous series of new levels is created and the stem of coral grows in height. Over thousands of years, this leads to the formation of huge reefs. Such reefs form the foundation of certain mountains in the European Alps. The living conditions of the polyp In spite of its primitive structure, the coral polyp is an organism with very specific requirements. In order to grow, it requires clean, warm water at a temperature of at least 20° C (68° F). In addition it requires ample supplies of oxygen, which it finds primarily in shallow water in constant movement. If a flow of fresh water carries dissolved substances and dirt into the seawater, the coral stops growing or dies off. Changes in temperature, lack of light or the slow deposition of small particles can also cause the death of the polyps. Most polyps are self-cleaning and remove, by means of small cilia, all the foreign particles which come into contact with their body surface. When a tarred road was built in 1969 along the coast of the Sinai Peninsula in the Gulf of Aqaba, causing large quantities of dust, sand and other polluting material to enter the sea, many corals stopped growing after some time or died. The self-cleaning capacity of the polyps was inadequate to cope with such a flow of impurities. Formation of the calcareous skeleton The coral polyp secretes the lime (calcium carbonate) on the external surfaces of its basal plates. The calcium is present as soluble ions in the seawater and scientists have used radioactively labeled calcium to prove that this is in the source from which the polyp extracts the calcium. The rate of lime deposition varies with the different species of coral and is markedly dependent on the illumination. This process takes place most rapidly in the middle of the day in unimpeded sunlight, dropping by half that rate if the sky clouds over, and by 90 per cent in darkness. Corals are able to produce daily some 10 grams of lime per square meter of polyp surface (0.3 ounce per square yard). This requires considerable quantities of energy, which must be provided by the metabolic process of the polyps. Scientists investigating coral have attempted to determine the metabolic rates of the various species of coral. The measurable variable used was an index as the polyps' consumption of oxygen. Results show that the intensity of metabolism of the polyp is greater than that of a human being at rest. In one hour a man requires about 8 milligrams of oxygen for each gram of body weight, while the average figure for the coral is around 20 milligrams. Thus the structurally simple coral polyp has a far higher requirement than that of a highly organized vertebrate. This clearly illustrates the considerable quantities of energy which the polyps must transform in building coral reefs. The source of energy for the metabolic process Coral polyp obtains some of its metabolic energy from processing the plankton on which it feeds. Its tentacles are provided with complicated trapping and 'shooting' mechanisms specially adapted to trap plankton; small spirally-wound arrows are very rapidly expelled from the nematocysts (nettle cells), reacting to the slightest touch like a coiled spring; these missile penetrate into the body of the plankton; at the same time there is released a poison which paralyses the prey. Since plankton is particularly plentiful at night, most types of coral polyp have become nocturnal in habit. Experiments have shown that the polyps do not feed solely on plankton and that certain types of coral actually dispense with it altogether. During these experiments, when all the plankton was filtered out of seawater, some species of coral nevertheless continue to grow. We know now that corals are also dependent on single-celled algae which live in the soft parts of the polyp's body. This algae is also what gives the coral its vibrant colors. These algae belong to the class Dinophyceae, which manufacture oxygen and carbohydrates by photosynthesis under the action of sunlight, and furnish these substances to the polyps. In doing so, the algae also process the carbon dioxide produced by the breathing of the polyps and break down nitrogenous products of metabolism. Without sunlight these symbiotic algae could not exist, and without these algae, coral reefs could not be formed. The birth of a new coral stock Let us follow in sequence the stages in the life of a coral polyp. Polyps can propagate themselves either asexually by cell division, or sexually by fusion of male and female germ cells. A few corals are hermaphrodites, being of both sexes at once. The ripe egg cells are fertilized either within or outside the body. Small free-swimming larvae hatch out of the fertilized eggs and are transported over wide areas by the water currents. Once they find a suitable support--generally an existing reef--they establish themselves there and rapidly change into small polyps. The young polyp now grows rapidly. It secretes lime intensively and surrounds itself with a cup-shaped wall. It can withdraw into this exoskeleton as a protection against it numerous enemies--fish and invertebrates. As we discussed earlier, the pedestal of the polyp is reinforced at six points by separate radial bands of lime and that these bands or septa extend into the large internal stomach cavity. Once the septa have reached a certain height, the poly begins to secrete a new foot, thus producing a further story. The full-grown polyp now divides asexually; new polyp are produced in the immediate vicinity and these too can multiply asexually. All these polyps ultimately form the coral stock, which continues to grow upwards. There are, however, certain species of coral, such as the Fungia or mushroom coral which occurs widely in the Indo-Pacific region, in which the building is done by only a single individual. At certain times the polyps again form asexual cells and swarms of small larvae are distributed all over the reef; thus, the cycle between sexual and asexual propagation recommences. A coral does not continue its growth indefinitely, but has a particular maximum growth. For instance, the hemispherical brain corals have a diameter of 2 meters (6.5 feet), the large madrepore plates 3 meters (almost 10 feet), whereas the gorgonian fans (Lophogorgia) can reach a height of 3 to 5 meters (10 to 16 feet) and the massive Porites coral stocks as much as 6-8 meters (20-30 feet) in height. Sunken wrecks offer ideal possibilities for establishing the approximately rate of growth of corals, provided that the date on which the ship sank is known. Acropora corals grow 10-25 centimeters (4-10 inches) per year; the bulky Porites and the brain corals, on the other hand, grow at only 1 centimeter (0.4 inch) of diameter per year. Measurements of the increase in weight of a coral stock showed that the massive corals have the lowest annual rate of increase. In general, the larger the surface area of the coral the greater the rate of growth. Measurements showed that Pocillopora and Porites species have an annual rate of increase of weight of 100-200 per cent, many species of Acropora 400 per cent, while species of Montipora have a rate as high as 1,200 per cent. A species of coral will establish itself the more successfully on the reef (compared with other species), the higher its rate of secretion of lime and the more intensive its metabolic activity, in other words, the more efficient its polyp colonies are. In addition, when the coral has a large surface area, it can secrete more lime. The convolutions of a brain coral or the multi-branched structure of an Acropora thus serve to provide the species with a larger metabolically active surface, so achieving a higher rate of growth. The species of coral which are most widely distributed over the reef are those which are evolved most recently; their efficient equipment enables them to gain an advantage in the struggle for survival. However, it remains true that certain risks are attached to such specializations. If the ecological conditions are unfavorable, it is precisely these younger species of coral which die off first. Individual polyps whose metabolism is less intensive are less demanding and have a significantly higher degree of resistance. This is why they also occur in the regions inside the reef, which can no longer be colonized by the fast-growing coral species. Polyps also feed on other species of coral which they find in their immediate vicinity. After making contact with their tentacles, they extrude filaments containing digestive juices, spreading these filaments over their prey and gradually consuming it. In this way clearly visible boundaries are drawn at the point of contact between the polyps. This behavior has been labeled 'aggression.' In certain coral reefs off Panama, the fast-growing species of coral are less 'aggressive' than the slower-growing types. The differing requirements in living conditions, peculiarities of metabolism and behavior of the polyps lead to segregation of the species within the reef, so that the living space is divided into a series of zones. Let us assume that a few corals have been able to settle in the vicinity of the coast. The first event is the formation of a narrow band of coral running parallel to the beach. Other colonists arrive: algae, jellyfish, sponges, snails, bivalves, etc. The surf brings in the plankton and oxygen which are indispensable to enable the coral to grow further. In the course of time, this band becomes wider and forms a horizontal ridge of reef, continually growing outwards towards the open sea. The species of coral which have the highest metabolic activity are located along the leading edge of the reef, since they must have optimum living conditions to maintain their intensive growth, and consequently require ample quantities of light, oxygen and plankton. On the coast side, the ridge of the reef slowly dies out, since the conditions obtaining there gradually becomes less and less favorable for the corals. The sediments stirred up by the action of the surf are deposited in the areas of the reef ridge near the beach; the movement of the water has now become sluggish and exchange is inefficient, so that the quantities of oxygen available are no longer adequate. The dead corals leave behind calcareous debris, which is slowly ground down to form sand and becomes overgrown with algae. The areas near the coast are regularly exposed and dried out at low tide. Depressions in the rock contain tidal pools which are subject to wide variations in temperature and salinity. Between the old reef ridge and the rocky tidal zone, sandy lagoons often form and scattered new growths of coral occur here. The boundary between the land and the sea finally becomes no more than a spray zone, which is wetted only by wave spray at very high tides. Let us look again at the edge of the reef. The nearer we get on the ridge of the reef to the open sea, the more living corals we find. Right at the leading edge, where the surf breaks incessantly, there is a particularly lush growth of coral and the variety of fish species is particularly great and colorful. Along this line the ridge of the reef drops steeply into the water. The upper areas of this relatively steep precipice exhibit equally rich colonies of coral, and numerous fissures, cavities and small crevices offer refuge to a large number of organisms. The deeper we dive, the weaker the light. The movements of the water become less and less marked and the supply of oxygen falls off. As a consequence of all this, coral growth becomes progressively reduced. Generally the only species to grow in these deeper zones are brightly-colored gorgonian fans, soft and fan corals, which require less light and oxygen. At fairly greater depths, say about 40 meters (130 feet) on, there is no further coral growth at all. At this point, there is generally a heap of the debris of dead coral at the foot of the reef, which slowly gives place to extensive deposits of sand or mud. The fish and other marine creatures have adapted to the various reef zones in the most varied manners possible. Each zone has its own species that exhibit special adaptations in body structure and behavior which are specialized for that zone. Coral reef zones are explained in detail in the VITAL CORAL REEF file. |
