August 7, 2004

Coral-reef ecosystems benefit humans commercially, recreationally, aesthetically and environmentally and are among of the most diverse, complex, and beautiful ecosystems on earth. The abundant biological diversity of the coral reef ecosystem, not only includes coral and the commercially important species associated with the reef but also tens of thousands of other plant and animal species. The declining state of coral reef ecosystems has sparked concern by scientists.

Coral reef ecosystems are under increasing pressure, and the threats are primarily from human interactions. In some cases, natural disturbances further compound the effects of anthropogenic stress. Of the approximate 617,000 km2 of coral reefs worldwide, it is estimated that about 10 per cent have already been degraded beyond recovery and another 30 per cent are likely to decline significantly within the next 20 years. In addition, unless effective integrated coastal zone management is implemented more than two-thirds of the world’s coral reefs may become seriously depleted of corals and associated biota within two generations. Coral reef ecosystems at the greatest risk are located in South and Southeast Asia, East Africa, and the Caribbean. However, people have damaged or destroyed reefs in more than 93 countries.

Coral reefs are massive structures made of limestone that is deposited by living beings. Interestingly, although thousands of species inhabit coral reefs, only a fraction produce the limestone that builds the reef. The most important reef building organisms are corals. Although coral is often mistaken for a rock or a plant, it is actually composed of tiny, fragile animals called coral polyps. When we say “coral” we are actually referring to these animals and the skeletons they leave behind after they die.

Almost all corals are colonial organisms. This means that they are composed of hundreds to hundreds of thousands of individual animals, called polyps. Each polyp has a stomach that opens at only one end and this opening is called the mouth. A circle of tentacles surrounds it. The polyp uses these tentacles for defence, to capture small animals for food, and to clear away debris. Food enters the stomach through the mouth. After the food is consumed, waste products are expelled through the same opening.

Although there are hundreds of different species of corals, they are generally classified as either “hard coral” or “soft coral”. Not all coral species build reefs. The actual architects of coral reefs are hard or stony corals, which are referred to as hermatypic or reef-building corals.

As the polyps of stony corals grow, they produce limestone for their skeletons. When they die, their skeletons are left behind and are used as foundations for new polyps, which build new skeletons over the old ones. An actual coral mound or tree is composed of layer upon layer of skeletons covered by a thin layer of living polyps.

Other types of animals and plants also contribute to the structure of the reef. Many types of algae, seaweed, sponge sediment and even mollusks like giant clams and oysters, add to the architecture of a coral reef. When these organisms die, they also serve as foundations for new corals.

Corals grow at different rates, depending on water temperature, salinity, turbulence, and the availability of food. The massive corals are the slowest growing species, adding between 5 and 25 millimetre (0.2 inches to an inch) per year to their length.

Soft corals such as sea fingers and sea whips are soft and bendable and often resemble plants or trees. These corals do not have stony skeletons but, instead, grow wood-like cores for support and fleshy rinds for protection. Soft corals are found in both tropical seas and in cool, dark regions.

A coral polyp is a spineless animal. Coral polyps can be the size of a pinhead while others are larger, sometimes a foot in diameter. One coral branch or mound is covered by thousands of these animals. They are invertebrates (spineless animals). When thousands of these animals are grouped together, they are referred to as coral colonies. Each coral “tree” or “mound” is one colony of coral polyps. A polyp has a sac-like body and an opening or mouth encircled by stinging tentacles called cnidae. The polyp uses calcium carbonate from seawater to build itself a hard, cup-shaped skeleton. This limestone skeleton protects the soft, delicate body of the polyp. Coral polyps are usually nocturnal, meaning that they stay inside their skeletons during the day. At night, polyps extend their tentacles out to feed.

Coral polyps eat in two different ways, depending on their species. Many coral polyps are nourished in a unique way by tiny algae called zooxanthellae. The algae live within coral polyps, using sunlight to make sugar for energy, just like plants.

Zooxanthellae process the polyp’s wastes to retain important nutrients and in turn provide the polyp with oxygen. Meanwhile, the coral polyps provide the algae with carbondioxide and a safe, protected home. Zooxanthellae living within the tissue of hard corals can supply them with up to 98 per cent of their nutritional needs. Another way that corals eat is by catching tiny floating animals known as zooplankton. At night the polyps come out of their skeletons to feed, making the reef look like a “wall of mouths.”

To capture their food, corals use stinging cells called nematocysts. These cells are located in the coral polyp’s tentacles and outer tissues. If you’ve ever been “stung” by a jellyfish, you’ve encountered nematocysts. Nematocysts are capable of delivering powerful, often lethal, toxins, and are essential in capturing prey. The polyps stretch out there long, stinging tentacles to capture the zooplankton that is floating by. The captured plankton is then put into the polyps’ mouths and digested in their stomachs.

Coral reefs are found in over 100 countries. Worldwide, Coral reefs cover an estimated 617,000 square kilometres (0.2 per cent of total ocean and 15 per cent of shallow seafloor area). Reef account for one third of all calcium carbonate deposited in oceans of world. Coral reefs grow best in waters with a temperature of between 20oC and 29oC (70oF and 85oF). It is possible for soft corals to grow in hotter and colder places, but growth rates under these conditions are very slow. Most also require very saline water ranging from 32 to 42 parts per thousand, which must also be clear so that a maximum amount of light penetrates it. The corals’ requirement for high light also explains why most reef-building species are restricted to the euphotic zone, the region in the ocean where light penetrates to a depth of approximately 70 metres.

Corals prefer clear and shallow waters, where lots of sunlight filters through to their symbiotic algae. It is possible to find corals at depths of up to 91 metres, but reef-building corals grow poorly below 18-27 metres. Corals also grow poorly near river openings or coastal areas with excessive run-off, because corals need salt water to survive. They are also generally absent in turbid, or murky waters, because high levels of suspended sediments smother them, clogging their mouths, impairing feeding and decreasing the depth to which light can penetrate.

With such stringent environmental requirements, reefs generally are confined to tropical and semitropical waters. The number of species of stony corals decreases in higher latitudes up to about 300 north and south. Beyond these latitudinal boundaries, reef corals are usually not found.

Coral reefs support over 25 per cent of all known marine species. As the most complex ecosystem, coral reefs are home to over 4,000 different species of fish, 700 species and thousands of other plants and animals.

Coral reef area is an important ecosystem on the global basis. They protect coastline from erosion and supply calcareous sediments to near shore. They are recreational area for million of peoples. The potential yield of fish from coral reef ecosystem is about 10 per cent of the total commercial ocean fish landing.

Coral reefs begin to form when free-swimming coral larvae attach to submerged rocks or other hard surfaces along the edges of islands or continents. As the corals grow and expand, reefs take on one of three major characteristic structures — fringing, barrier or atoll.

Fringing reefs, which is the most common, project seaward directly from the shore, forming borders along the shoreline and surrounding islands. Barrier reefs also border shorelines, but at a greater distance. A lagoon of open, often-deep water separates them from their adjacent landmass.

In addition to being some of the most beautiful and biologically diverse habitats in the ocean, barrier reefs and atolls also are some of the oldest. With growth rates of 0.3-2cm per year for massive corals, and up to 10cm per year for branching corals, it can take up to 10,000 years for a coral reef to form from a group of larvae. Depending on their size, barrier reefs and atolls can take from 100,000-30,000,000 years to fully form.

All three reef types, fringing, barrier and atoll share similarities in their biogeographic profiles. Bottom topography, depth, wave and current strength, light, temperature, and suspended sediments all act to create characteristic horizontal and vertical zones of corals, algae and other species. These zones vary according to the location and type of reef. The major divisions common to most reefs, as they move seaward from the shore, are the reef flat, reef crest or algal ridge, buttress zone, and seaward slope.

Millions of people depend on reefs for food and livelihood. Reefs also create sheltered lagoons and protect coastlines and mangroves against wave damage. Mangroves in turn protect reefs from sedimentation and eutrophication. Mangroves and seagrasses also play an important role in coastal protection and provide spawning and nursery areas for reef and offshore fishes.

In the Pacific, over 2.5 million people live on islands built by or surround by coral reef ecosystems. Over 300,000 people live on coral islands in the Indian Ocean and many more in the Caribbean. Coral reefs provide 10-12 per cent of the harvest of finfish and shellfish in tropical countries and about 20-25 per cent of the fish catches of developing countries. As much as 90 per cent of the animal protein consumed on many pacific islands comes from marine sources. The potential sustainable yield of fishes, crustaceans and molluscs from coral reefs could be some nine million tons (12 per cent) of the world fisheries catch. According to one estimate, coral reefs provide goods and services worth about $375 bn per year — a staggering figure for an ecosystem that covers less than 1 per cent of the Earth’s surface. Nearly a third of the world’s fish live in coral reefs.

NATURAL THREATS
Predators, disease, solar radiation, increase sea temperature and weather changes all can damage coral. Corals exposed during daylight hours are subjected to the most ultraviolet radiation, which can overheat and dry out the coral’s tissues. Corals may become so physiologically stressed that they begin to expel their symbiotic zooxanthellae, which leads to bleaching, and in many cases, death.

Weather-related damage to reefs occurs frequently. Large and powerful waves from hurricanes and cyclones break apart or flatten large coral heads, scattering their fragments. Increased sea surface temperatures, decreased sea level and increased salinity from altered rainfall can all result from weather patterns such as El Niño. Together these conditions can have devastating effects on a coral’s physiology. During the 1997-1998 El Niño season, extensive and severe coral reef bleaching occurred in the Indo-Pacific and Caribbean. Approximately 70 to 80 per cent of all shallow-water corals on many Indo-Pacific reefs were killed. In addition to weather, corals are vulnerable to predation. Fish, marine worms, barnacles, crabs, snails and sea stars all prey on the soft inner tissues of coral polyps.

Coral reefs may recover from periodic traumas caused by weather or other natural occurrences. If, however, corals are subjected to numerous and sustained stresses including those imposed by people, the strain may be too much for them to endure, and they will perish. Coral diseases, including white and black band disease, can also damage or kill coral. Corals must compete for space with algae, sponges, and other sedentary, benthic organisms. Tropical storms can break and bury coral, causing mortality, especially to shallow-water coral colonies. Coral reefs generally recover from damage caused by tropical storms, but in varying time frames.

Human-caused activities are major threats to coral reefs. Pollution, over-fishing, destructive fishing practices using dynamite or cyanide, collecting live corals for the aquarium market and mining coral for building materials are some of the many ways that people damage reefs all around the world every day.

One of the most significant threats to reefs is pollution. Land-based runoff and pollutant discharges can result from dredging, coastal development, agricultural and deforestation activities, and sewage treatment plant operations. This runoff may contain sediments, nutrients, chemicals, insecticides, oil, and debris. When some pollutants enter the water, nutrient levels can increase, promoting the rapid growth of algae and other organisms that can smother corals.

Coral reefs also are affected by leaking fuels, anti-fouling paints and coatings, and other chemicals that enter the water. Petroleum spills do not always appear to affect corals directly because the oil usually stays near the surface of the water, and much of it evaporates into the atmosphere within days.

However, if an oil spill occurs while corals are spawning, the eggs and sperm can be damaged as they float near the surface before they fertilize and settle. So, in addition to compromising water quality, oil pollution can disrupt the reproductive success of corals, making them vulnerable to other types of disturbances. .In many areas, coral reefs are destroyed when coral heads and brightly-coloured reef fishes are collected for the aquarium and jewellery trade.

Careless or untrained divers can trample fragile corals, and many fishing techniques can be destructive. In blast fishing, dynamite or other heavy explosives are detonated to startle fish out of hiding places. This practice indiscriminately kills other species and can crack and stress corals so much so that they expel their zooxanthellae. As a result, large sections of reefs can be destroyed.

Cyanide fishing, which involves spraying or dumping cyanide onto reefs to stun and capture live fish, also kills coral polyps and degrades the reef habitat More than 40 countries are affected by blast fishing, and more than 15 countries have reported cyanide fishing activities.

Other damaging fishing techniques include deep water trawling, which involves dragging fishing net along the sea bottom, and muro-ami netting, in which reefs are pounded with weighted bags to startle fish out of crevices. Often, fishing nets left as debris can be problematic in areas of wave disturbance. In shallow water, live corals become entangled in these nets and are torn away from their bases. In addition anchors dropped from fishing vessels onto reefs can break and destroy coral colonies.

Coral bleaching refers to a process in which corals expel the algal cells (zooxanthellae) that normally live within their tissue. These algae give corals their characteristic brownish colour and once they have been expelled, the white skeleton shows through a coral’s transparent tissue, giving it a bleached white appearance. Bleached coral looks very similar to coral that has recently died, but can be distinguished (on close inspection) by the presence of small polyps and tentacles on the coral surface.

Coral bleaching can be caused by stressful environmental conditions such as extreme temperature, low salinity, extreme light and various toxins. However, large-scale bleaching episodes are usually associated with unusually high sea temperatures. This relationship has led to the suggestions that coral reefs are showing early signs of stress due to global warming caused by green house gas emissions. Bleached corals are still alive and can recover fully if the stressful conditions are not too severe or prolonged.

Rapid population growth and migration to coastal areas where coral reef ecosystems occur exacerbate the problem. The resulting coastal congestion leads to increasing competition for limited resources, to increased coastal pollution, and to problems related to coastal construction.

Technology also allows humans to exploit the reef with mechanical dredges, hydraulic suction, dynamiting, and large-scale poisoning. More specifically, the major causes of coral reef ecosystem decline include over exploitation of reef resources, excessive domestic and agricultural pollution and poor land use practices that increase sedimentation.

CONSEQUENCES
Degradation of coral reef ecosystems would have significant impact on world food sources and long-term negative economic impacts on fishery and tourist industries, and a devastating effect on millions of people around the world for which coral reefs represent the primary source of livelihood.

In Pakistan, the environmental conditions are unfavourable for coral growth, and corals are not well developed. There are a few isolated patches of coral growth on hard substrates but the high sedimentation and very turbid conditions limit more extensive growth. Isolated, small patches of living coral colonies are found on hard substrates between Hub River outfal and Churna Island. Live corals have been recorded at several locations along the coast, to a depth of 20 m. These Coral require clean water and are sensitive to pollution. Land based pollution, sewage, industrial effluents, sedimentation and dredging appear to be the main problems for reefs in Pakistan’s coastal waters, but corals are also collected by local fishermen, to be used in traditional Islamic medicine. Near Churna Island, destructive fishing methods contribute to the degradation of the marine environment.

The species may adapt to gradual raise in temperature but due to the release of hot water by nearby thermal power plant the specie is under the thermal stress and there is risk of extinction. It is required that country should Develop capacity to locate and identify possible reef areas. The development of area where coral are found should be carried keeping protection of this unique ecosystem in mind.

The writer abdulwaheed27@hotmail.com is an environmental scientist

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