oral reefs are sites of unrivalled biodiversity. But, like rainforests, they are in need of some TLC. They make up less than 1 per cent of ocean environment yet they support 25 per cent of marine species. When the conditions are right, a coral reef provides a foundation for a complex ecosystem that can grow to support in excess of one million species, encompassing everything from seaweed, grasses, anemones, worms and shrimp to fish, seahorses, turtles, octopus, eels and sharks.
Despite the fact that they bring both pleasure and profit, these dazzling formations are under threat: from global warming, over-fishing, sewage run-off, voracious starfish, hurricanes and even clumsy divers. The Nature Conservancy, an organisation dedicated to preserving the diversity of life on Earth, predicts that "if the present rate of destruction continues, 70 per cent of the world's coral reefs will be destroyed within our lifetime".
Animal, mineral or vegetable?
Coral reefs thrive in tropical waters between 23-25C and are usually found at depths of 25m or less where sunlight can easily penetrate. They cannot survive in turbulent seas yet need enough wave action to deliver freshly oxygenated water and food and remove sediment. The water needs to be clear and the salinity just right for reefs to flourish. With such sensitivity to environmental conditions it is no surprise that they face a constant battle to survive.
To the untutored eye, a coral reef looks like a piece of rock. But at night the truth is revealed. Hard or reef-building coral such as brain or lettuce coral is made up of millions of tiny but multi-talented carnivores called coral polyps. Mostly mouth and tentacles, they belong to the cnidaria family, also occupied by jellyfish and anemones. When the sun sets they come out of hiding to feed, using their stinging tentacles to paralyse passing zooplankton (microscopic animals).
These creatures start life as larvae the size of a pinhead. They drift in open seas until they find a hard surface to attach to. They then get to work, absorbing carbon dioxide and calcium from the surrounding sea water and secreting a protective limestone shell called a corallite. When the polyp dies, another attaches to the cup left behind. Many of the world's established coral reefs are between 5,000 to 10,000 years old.The largest reefs often have the most sluggish growth rates, measuring no more than 2.5cm a year. Even fast-growing branching coral expands at just 10cm a year, so it's no surprise that ship anchors or divers with wayward fins can cause long lasting damage.
When it comes to procreation, the coral polyp swings both ways; it can perform asexual as well as sexual reproduction. To start a colony they can divide to produce a new but genetically identical polyp. For sexual reproduction, individual polyps release both egg and sperm through the mouth into the water; these combine and the fertilised eggs develop into larvae.
This sometimes gives rise to the phenomenon of synchronous spawning, where on one night of the year, all the coral in one area will release so much egg and sperm into the surrounding water it becomes cloudy. Marine biologists are still pinpointing the exact trigger mechanisms but water temperature, tides and lunar cycles all play a part.
While coral polyps do most of the work, the reef structure is also fortified by cementing algae, microscopic invertebrates called bryozoans and the shells of molluscs.
There are three main types of reef: the atoll, the barrier and the fringe.
Fringe and barrier reefs both run parallel to an island or continental coastline, but barrier reefs are separated from the coast by a lagoon or deep-water channel. The largest and perhaps most famous reef in the world is the Great Barrier Reef in Australia. Running for 2,100km (1,260 miles) along the north-east coast of Australia, the structure consists of 2,900 separate reefs and contains 400 species of coral. Along with the Great Wall of China, it can be seen by astronauts admiring the view from their space capsules.
Atolls are found mostly in the Pacific and Indian oceans and they start out as fringing reefs around islands. Over millions of years, as the landmass sinks into the ocean, the coral continues to grow outwards. Eventually the landmass will be completely submerged leaving a lagoon fringed by coral.
The low-lying Maldives in the Indian Ocean are a nation of atolls.
Mutually assured survival One of the most puzzling aspects of coral reefs is their ability to create an abundance of life in sterile areas. The answer lies in the existence of a species of algae with a tongue-twisting name, zooxanthellae. Like many species on the reef, symbiosis is the key to its success. Without nutrients from these single-celled organisms, most coral reefs could not survive in the nutrient-poor waters in which they grow.
Zooxanthellae live in the cells of coral polyps and as well as providing more than 90 per cent of the polyps' energy needs, they release oxygen and absorb carbon dioxide and nitrogen waste. The algae photosynthesise and so need sunlight, which is why reef-building coral is not found in the dark depths. Zooxanthellae also give coral its colouring - pure coral is white and without the red, green or brown algae, the reefs would be as white as the cliffs of Dover.
A reef offers protection and a regular food source for both permanent residents and visiting species. A wide range of species, including shrimp, lobster, flatworms, sponges and turtles, find the nooks and crannies of a reef the perfect place to hide. Eels use reefs to ambush their prey at night, plankton-eating fish stay close to the reef, and juvenile pelagic fish such as tuna and barracuda will also use it as a home until they are large enough to survive out in the big blue.
Up to 50 per cent of fish on a reef are herbivores and some, such as parrot, angel and butterfly fish, survive by eating the algae in or on the coral. Others including starfish and urchins live on coral polyps. The cleaner wrasse eats parasites and dying tissue of larger fish. Some fish, turtles, octopus and even some snail species are happy to chomp on the invertebrates such as crabs that hide at the bottom of the reef. And at the top of the food chain are the large predators that prey on smaller fish.
Life on the reef flourishes because every species finds a niche.
Reef grief It doesn't take much to knock the reef environment off kilter. When sea temperatures rise just a few degrees above 25C, coral polyps expel the zooxanthellae and this leads to coral bleaching. Not only do the corals lose their colour, they lose their main food source. Reefs can cope with short-term fluctuations in temperature and bleaching is a regular part of the lifecycle that can be reversed. But over time, if water temperatures don't cool, the corals will slowly starve to death.
Coral bleaching has been observed in reefs in all parts of the world and since the 1980s the problem has become more widespread. Huge swathes of the Great Barrier Reef in Australia and reefs in the South Pacific, Seychelles and Maldives suffered from bleaching epidemics in 1998. That year, above-average sea temperatures were caused by El Nino, a cyclical ocean current in the Pacific which alters weather patterns. The Australian Institute of Marine Science reported that 16 per cent of the world's reefs were seriously damaged in just nine months.
The National Oceanic and Atmospheric Administration in the US is currently running Coral Reef Watch, an international observation programme using satellites to monitor sea temperatures. At the end of last summer it warned of a major coral bleaching outbreak in the Caribbean. On-the-spot monitoring has shown that 85-95 per cent of reefs in Puerto Rico were affected and reefs around the coasts of Grenada, the US Virgin Islands, Panama and Costa Rica also show significant damage.
Over-fishing is another factor that damages the delicate balance of the reefs. The high fish densities that reefs are renowned for have attracted fishing boats, resulting in over-fishing. Too-few fish can leave the reef exposed to algae and coral-eating organisms that smother the reef or leave it insufficient time to recover. Damage is also caused by fishing methods themselves. Dynamite is used in some parts of the South Pacific and South East Asia. Where live fish are required for restaurants or aquariums, cyanide spraying is the preferred method of capture. The fish are temporarily incapacitated but the reefs left behind are destroyed. This is a particular problem in the Philippines and Indonesia.
Sewage outflows, coastal dredging and soil erosion run-off from agriculture, logging and construction also causes devastation. The bacteria in sewage and fertiliser deposits from farming provide extra nutrients that boost algae growth. Combined with excess sediments, these reduce water clarity, making it difficult for zooxanthellae to photosynthesise.
Coral also faces threats from natural sources. Hurricanes churning up the oceans can break reefs up and leave them covered in sediment. Government officials in Mexico reported that large sections of coral reef off the coast of Cozumel could take as long as 100 years to recover after the pounding they received from hurricane Wilma in October.
The 2004 Asian tsunami and earthquake left acres of coral damaged beyond repair in some parts of Indonesia. Many reefs were broken up while other shallow reefs were left permanently exposed as landmasses tilted. In many areas, reefs were covered in debris carried by the retreating wave.
Crown of thorn starfish are a particular problem on the Great Barrier Reef and there have been regular infestations since the 1960s. The bright red starfish grow to 25-35cm and are covered in long sharp spines. The young of the species eat plankton but when they mature they feast on live coral.
Where there is an infestation the coral has no time to repair.
If the worst predictions come true, we will lose more than just a diver's paradise if the world's reefs succumb to these many hazards. A report released by the UNEnvironment Programme in January estimates that coral reefs are worth up to $600,000 per square kilometre. For a start, there's the food supply. As much as 10 per cent of the world's fish catch originates from reefs. Healthy reefs also pull in tourists and sustain local diving-related industries. They also provide natural barriers for coastlines threatened by high tides and storm surge caused by hurricanes.
The report noted that communities with reefs sustained less damage during the Asian tsunami, as they acted as a natural barrier to incoming waves.
Coral reefs are also ripe for pharmaceutical discoveries. Bioprospectors - scientists who scour the earth for compounds from the natural world that will provide a source for new chemicals and medicinal cures - are excited about the possibilities offered by the largely under-explored reef environment. Chemicals from reef sponges have been developed into antiviral drugs to treat HIV and herpes. Australian researchers have created a factor-50 sun block from a species of starfish, and the venom of the cone snail has been developed into a non-addictive painkiller by a US company.
Coral reefs may sustain and protect 25 per cent of all marine animals but we should never forget that they sustain and protect us too. The challenge remains to find ways of exploiting these dazzling ecosystems without destroying them.
* Not all coral organisms are reef-building, and there are hard and soft varieties encompassing thousands of different species. Small coral reefs also exist in deep cold water but they rely on the capture of small fish for sustenance. Soft corals such as sea fans and whips look like underwater plants.
* Coral reefs are found within 30o north or south of the equator.
* 109 countries have coral reefs off their coastline.
* Fossil records show that coral has been around for 400 million years.
* Reef-building coral has existed for 25 million years.
* There are 6,000 known species of coral.
* The solitary fungia coral polyp can grow up to 25cm.
For many years, unlucky mariners created man-made reefs with shipwrecks.
Today, the creation of artificial reefs is no accident, with big piles of man-made rubbish providing shelter, food and spawning areas. There are records of Japanese fishermen creating man-made reefs as far back as the 18th-century, using weighted bamboo to attract fish. Today, everything from tyres, cars, ships, military tanks, airplanes, oil and gas rigs, concrete blocks, decommissioned bridges and smashed up toilets have been used to create new habitats for marine life. It is a practice that is often driven by economics rather than ecological imperatives - and it has sometimes created underwater dumping grounds rather than sustainable marine habitats.
An artificial reef often begins life with a bang, as explosives are used to sink decommissioned ships or oil rigs to the bottom of the sea. Within weeks, plants and encrusting animals appear - including coral if conditions are right - covering the whole structure within a year. What was once a redundant hulk of metal has now become a reliable source of food and, once small fish have estabalished a home there, the reef becomes a hunting ground for large pelagic fish. The transformation is complete.
Reefs are often created to enhance commercial and sport fishing opportunities as well as to support or create local diving industries. Since the 1950s, the Japanese have spent billions creating reefs on an industrial scale to support commercial fishing. To date, the country has more than 6,000 artificial reefs.
More recently, a few artificial reefs have become final resting places, giving a fresh twist to the phrase "sleeping with the fishes". A US company called Eternal Reefs, www.eternalreefs.com, has been mixing the ashes of the deceased with concrete reef structures to create a "permanent environmental living legacy".
From an ecological perspective, concrete blocks have proved the most suitable material for reef building. Unlike ships, which slowly crumble, they can last for hundreds of years and can be designed for optimum biodiversity. They are more flexible than oil rigs, which often have to be buried too deep to be suitable.
Scotland's first artificial reef, composed of 42,000 tonnes of concrete block, was built in Loch Linnhe near Oban in 2001. It led to a increase in the number of marine animals such as cod and lobster. It is hoped that "marine ranches" can be created around the Scottish coast producing enough stock to boost struggling fishing communities.