Coral - biology. I INTRODUCTION Coral, marine animals found in all the world's oceans, although they are best known and most widespread in the tropics. There are many types of coral, but all have a simple body plan: a tubular body capped by a ring of stinging tentacles surrounding a mouth. This body plan is referred to as a polyp. It is the living part of a coral animal and resembles an anemone. Coral polyps are usually tiny, but many species have developed the ability to live in colonies, and such coral colonies can reach considerable sizes. Corals originated in the early history of life on Earth, during the Cambrian Period about 500 million years ago. In the tropics many coral colonies build spectacular and beautiful skeletons by secreting calcium carbonate (limestone) from the bottom half of the polyp's tubular body, forming skeletal cups. These cups anchor the polyps, and the polyps also can withdraw into them for protection. These coral colonies, known as stony corals, are the architects of coral reefs, perhaps the most beautiful and among the most diverse habitats in the sea. Other corals, such as soft corals and sea fans, lack stony skeletons, but still build colorful colonies. Corals belong to the phylum, or group of animals, known as Cnidarians (the C is not pronounced). Some experts also refer to the group as Coelenterates. Within the Cnidarians there are four main groups. Two groups, the jellyfish (Scyphozoans) and the box jellyfish (Cubozoans), are free-swimming creatures that do not include corals. The other two groups both include some corals. Altogether there are about 5,000 to 6,000 species of coral. II WHAT ARE CORALS? The basic form of a coral animal is called a polyp. A polyp has a tubular body with one end attached to or resting on the sea bottom, or the polyp may be attached to a coral reef. At the other end is a single opening, a mouth, which is surrounded by tentacles. The main structure of the body consists of only two layers of cells, an outer wall or skin called the epidermis and an inner wall known as the endodermis or gastrodermis. Between these two layers is a binding gluelike layer of material known as the mesogloea or coenenchyme. The tentacles have some specialized stinging capsules, known as nematocysts. These capsules can be "fired," turning rapidly inside out to release a whiplike thread that is often tipped with poisonous spikes. Since they are very simple animals, corals lack a brain. But they have muscles and a very simple nervous system that enable them to move their tentacles to capture food and also to retract into the body if there is a threat. Inside the mouth is a throatlike pharnyx and then a wide body cavity, the coelenteron. In most corals this coelenteron has long vertical folds of body tissue reaching toward the center. The vertical folds are known as mesenteries. Generally speaking coral polyps do not move about. Many are fixed by a solid skeleton to the seafloor. The types of skeleton used by corals are highly varied. III HOW CORALS OBTAIN FOOD All corals capture at least some of their food with their tentacles. However, because they cannot travel, they do not actively seek prey. For the most part they rely on microscopic particles from the plankton floating or swimming past their tentacles. The nematocysts are then rapidly fired to hold and kill their prey. Many corals obtain most of their food energy from a close relationship with algae known as zooxanthellae. These tiny algae resemble simple plants. Like all plants they use photosynthesis to transform energy from sunlight into sugars and other compounds. The zooxanthellae live within the cells of the polyps in incredibly large numbers. Typically about 1 million algae live in each square centimeter of coral tissue. Both the algae and the corals benefit considerably from this symbiotic relationship. The algae can thrive without the danger of being eaten, and they make use of the waste products of the coral. The corals, in turn, use the excess sugars and other compounds produced by the algae as a source of food. Some corals get more than 80 percent of the food they need directly from these algae, enabling the corals to grow quickly and to thrive in areas where little food is available in the surrounding waters. IV LIFE CYCLE AND REPRODUCTION The life cycle of a coral is relatively simple. Within the body, gametes (eggs and sperm) are formed. In some species the eggs are fertilized within the polyp, but in the majority the gametes are released into the water, where fertilization takes place. The fertilized eggs develop into larvae, known as planulae, which are typically oval or pear-shaped and are covered in cilia (fine, movable hairs). A planula is capable of swimming, but is also carried by water movements. After some days or weeks in the water, the planula swims toward the bottom and attaches itself, whereupon it metamorphoses into a polyp. Most coral polyps divide and build colonies. New polyps are produced from the original in a process of budding or splitting. Budding is a form of reproduction known as asexual reproduction. The offspring are genetically identical, and in the case of most coral colonies they remain attached to the parent polyp. In time, as the new polyps all begin to grow and divide, they form large colonies, often numbering many thousands of polyps. One of the most spectacular events in nature, which was only discovered in the early 1980s, is the mass coral spawning on Australia's Great Barrier Reef. In November 1982, after some years of detective work, a group of scientists entered the water for a night dive and witnessed hundreds of corals simultaneously releasing millions of egg-bundles and clouds of sperm into the water where the eggs were fertilized. The effect was rather like an underwater snowstorm. Scientists calculate that at least half of the billions of corals on the Great Barrier Reef take part in this mass spawning. The timing is incredible. Most species spawn about five nights after the full moon in late spring, and individuals of the same species all spawn within an hour or so of one another. The great advantage of reproducing in vast quantities like this is that the predators of the eggs and larvae are overwhelmed and cannot eat them all, so many can survive and settle to grow into adults. V TYPES OF CORALS The classification of corals is complex. Most belong to a group or class known as Anthozoa, which has two broad groupings, the octocorals and the hexacorals. The octocorals have polyps with eight tentacles and a body cavity separated by eight mesenteries. The octocorals include soft corals and their relatives, the blue coral and the sea pens. The hexacorals typically begin life with six tentacles and mesenteries, but these quickly subdivide. The hexacorals include the stony and hard corals, but also noncoral groups such as the familiar sea anemones. A few other corals, notably the fire corals, belong to the class Hydrozoa. Although the individual coral animal is described as a polyp, many colonial species form distinctive communal skeletons. In some cases scientists have found it more helpful to describe the shape and structure of these skeletons rather than the individual polyps. A Soft Corals, Sea Fans, and Whip Corals Soft corals lack a distinct skeleton. Although they live in colonies, the individual polyps are fused into a complex body, usually strengthened by small lumps or spikes known as sclerites, which are made of protein and calcite. Soft corals come in a variety of shapes, including undulating sheets, upright mushroomlike shapes, and beautiful shapes that form branches. A number of other octocorals have skeletons made from a hard or horny protein, sometimes strengthened with more brittle calcareous deposits. Some octocorals have skeletons that form branches. Branching forms include the red coral from the Mediterranean Sea. The skeleton of the red coral has been used to make jewelry for thousands of years. Sea fans are typically finely branching colonies that form broad sheets, measuring up to 2 m (6.5 ft) or more in diameter. Sea fans thrive where there are strong currents, which the polyps filter for food. Whip corals are a group of species that form long, whiplike strands with few or no branches. The organ-pipe coral is a relative of the soft corals and sea fans. It lays down a stony skeleton of limestone with distinctive red tubes around each polyp. B Blue Corals The blue coral is another octocoral but one that has a limestone skeleton. It is classed in its own group or order known as Helioporacea. Blue coral colonies form pillars or wide ridges. When alive the corals are brownish in color. The limestone skeleton is a highly distinctive blue. C Stony Corals Stony corals are the best-known and one of the largest groups of corals. They produce a limestone (calcium carbonate) skeleton, and most live in close association with zooxanthellae. Nearly 800 species of these corals are builders of coral reefs. Stony corals come in a vast array of shapes and sizes. Massive corals are often bouldershaped, with smooth surfaces, or with deeply complex twisting lines earning them the name of brain corals. These corals may only grow a few millimeters a year, but may eventually reach several meters in diameter, as they can live for several hundred years. Branching stony corals are much faster growing and some may extend their branches by 15 cm (6 in) a year. Other stony corals are also commonly named after their appearance, such as pillar corals, plate corals (resembling broad circular dinner plates), and lettuce corals (with twisting sheets). Encrusting corals tend to grow over the bottom and simply follow the contours of whatever lies beneath. D Black Corals Black corals consist of about 200 species, all of which live in colonies. They have a stiff but flexible skeleton made of protein that is usually either a branching skeleton, as in the bushy black corals, or a long twisting thread, as in the wire corals. Black corals lack zooxanthellae and typically grow at depths of about 30 m (98 ft) since they do not require sunlight. The skeleton in some species is very dense and black in color, and is often collected to make jewelry. These corals are uncommon, however, and are slow growing, so they are quickly decimated unless the harvest collection is well managed. E Fire Corals Fire corals are only distantly related to the other coral groups but can look quite similar to them. Fire corals have limestone skeletons and grow in a range of different shapes, including branching, encrusting, and massive forms. All have zooxanthellae. Up close it is possible to see that the colonies have a fairly smooth surface, pitted with small pores. It is often possible to see fine hairlike tentacles that house a battery of stinging nematocysts. These nematocysts are more powerful than most other corals and can inflict stings on humans. They are typically brown, but with distinctive white tips. Related to the fire corals are the lace corals, another small group of branching corals with limestone skeletons. These are colorful but do not have zooxanthellae. VI RANGE AND HABITAT Corals do not occur in fresh water. They are restricted to the sea and are found throughout the world's oceans. Almost all groups, however, are most abundant in tropical waters. All those with zooxanthellae in their tissues are restricted to shallow areas where sunlight is abundant. In most groups there are a few species that live in deeper water and in cooler latitudes. The lace corals are predominantly found in these waters. The stony corals are best known from the tropics. Two very distinct groupings are found--those of the Atlantic, where there are about 60 species, and those of the remaining tropical areas stretching from the Red Sea to the Pacific (a region known as the Indo-Pacific), where there are nearly 750 species. Most corals need a hard substrate such as rock on which to settle and build their skeletons. They cannot grow where there is shifting sand or mud. The need for sunlight means that murky waters can prevent some coral growth because particles in the water block the light. The same particles may also settle and smother the coral. A large number of corals have also adapted to the stable, warm environment of tropical waters. For them a fairly constant year-round temperature is necessary. Few reef-building corals can survive in waters of less than about 18°C (64°F). VII CORAL REEFS The limestone skeletons of stony corals remain even after a coral dies. Where large numbers of stony corals grow together, the combined bulk of their skeletons begins to change the shape of the seafloor. Over decades or centuries new corals grow upon the old skeletons. Where storms or grazing animals damage the skeletons, they may be broken up to form piles of rubble or sand. This material fills the gaps between the corals. Certain types of algae (calcareous algae) also lay down limestone and can help cement all of this material together. A large mound of limestone begins to build up, with corals growing on its surface. This structure is known as a coral reef. Over time coral reefs begin to form recognizable shapes. For example, fringing reefs form around islands, and barrier reefs form some distance offshore. Many thousands of marine animal species live around the corals, making use of the complex structures, which offer numerous places to live. Because most stony corals are found in the tropics and require sunlight, these coral reefs only form in the warmest and clearest waters of the world, but here they are widespread. Coral reefs provide a home to more types of animals than almost any other habitat on Earth. Some scientists have estimated that more than a million species may exist on coral reefs, making them almost as diverse as tropical rain forests. Coral reefs are highly productive fisheries, and for thousands of years people have used coral reefs as a plentiful source of food. They also provide a barrier between the ocean and the shore, often protecting coastal peoples from the worst ravages of waves and storms. A few types of stony corals grow in deeper and cooler waters. In recent years a number of large coral structures have been discovered in the northern Atlantic Ocean. These resemble true coral reefs, but are built by only a few types of coral and occur at depths of 200 m (660 ft) or more. VIII THREATS TO CORALS Certain types of coral have been used by humans for many years. Red coral and black corals have been fashioned into jewelry. Many stony corals have been used in making building materials, and some have been taken as curios for tourists or for the aquarium trade. Unfortunately this use has had disastrous consequences in some places as corals are slow-growing and will not always return if removed from an area. There are now strict laws in place to control the import and export of coral, and tourists are advised not to buy products made from coral. Apart from endangering the coral animals, purchasing coral products could be breaking the law. In addition to threats to individual corals, recent decades have shown degradation and loss of entire coral reefs as a result of human activities. Much of this damage is related to the sensitivity of the corals themselves. A major threat arises from increased sediments in the water, which have resulted from chopping down forests or changing farming methods inland, leading to soil erosion that causes sediments to enter the rivers and then the sea. Pollution, too, is a problem. Nutrients coming from sewage or from fertilizers threaten corals. They encourage the growth of algae and plankton, which can either out-compete the corals or smother them by blocking out sunlight. More recently a new threat has been observed. Most stony corals have adapted to the very stable temperatures of the tropics and are extremely sensitive to temperature change. When stressed by unusually high temperatures, stony corals (as well as many soft corals) expel or lose their zooxanthellae. The immediate result is that the corals lose their color and turn a bright white (the color of the limestone skeleton underneath). This phenomenon is known as coral bleaching and is most often caused by high temperatures. If conditions improve, the coral takes back its zooxanthellae and recovers. However, if a coral remains bleached for too long, it may die. Since the 1990s an unprecedented number of warm years have led to mass bleaching events where all corals on a reef have bleached and many have died. Scientists predict that global warming in coming decades will further raise temperatures to levels that may threaten coral reefs throughout the world. Scientific classification: Corals belong to the class Anthozoa in the phylum Cnidaria. Corals with eight tentacles make up the subclass Ostocorallia or Alcyonaria. Corals that commonly have six tentacles or multiples of six belong to the subclass Zoantharia (or Hexacorallia). Stony, or true, corals belong to the order Scleractinia (or Madreporaria). Soft corals, sea fans, and whip corals belong to the order Alcyonacea. Blue corals belong to the order Helioporacea. Black corals belong to the order Antipatharia. Fire corals belong to the order Milleporina. 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