Wonders of the coral reef

'Coral Reefs: Secret Cities of the Sea' at the Natural History Museum

Some 120 years ago, a splendid old coral adorned the Shark Bay reef off the coast of Western Australia. It had been growing for more than a century, starting as a simple cup and developing into a mass of ruffled ridges about the size of a washing machine as its individual plates fused together.

The Turbinaria coral hosted a vivid variety of marine organisms, from multicoloured fish darting around its folds to oysters and sea urchins embedded deep within them. That life ended with an act of scientific vandalism that would be unthinkable today; the Victorian naturalist William Saville-Kent had the 300kg coral wrenched off the reef and shipped to the Natural History Museum in London.

Later this month, after spending many years encased in a crate and then undergoing extensive cleaning and conservation, the Turbinaria goes on show as the centrepiece of London's biggest-ever coral exhibition. The museum is exhibiting 200 items from its collections, plus a living mini-reef with a dazzling display of marine creatures - "a small but perfectly formed coral reef", as the museum's director, Michael Dixon, puts it.

The show illustrates the way coral reefs pack in biodiversity more densely even than tropical forests. Although "rainforest of the oceans" is a common analogy for the diversity of coral reefs, the museum has gone for a different one: "secret cities of the sea". This portrays the reef as "a complex metropolis where the lives of many hang in a fragile balance, competing for space, food and sunlight".

"The city metaphor demonstrates the complexity of the roles played by the reef's inhabitants and the interplay between them," says Ken Johnson, the museum's coral researcher, who has been working on the exhibition for the past two years. "We have builders, gardeners and caretakers, waste managers and cleaners, social and housing services and security and communications specialists."

The exhibition is the first to open since the museum launched its new five-year strategy in January, including a commitment to use its resources to look at "the challenges the world is facing today". Dixon says: "Coral reefs are an excellent ecosystem through which to look at the impact of long-term changes such as climate change and acidification of seawater on the health of the oceans."

Global warming and rising levels of carbon dioxide, which make the oceans more acidic and thus more liable to dissolve coral skeletons, are still in their early stages compared to what many scientists predict for later in the 21st century. So the worst is still to come for the world's more remote and better-protected reefs, many of which are in a good state.

However, local change has already wreaked havoc on reefs lying off highly populated coastlines - particularly through overfishing and pollution, which overthrow the local ecological balance. Looking at the world's main reef regions, Johnson says the Caribbean has suffered most, losing 80 per cent of its coral cover over the past 40 years. Visitors to the exhibition will see historic photos of the Discovery Bay reef off the north coast of Jamaica, which was thriving in the 1960s but is now a heartbreaking grey desert, littered with fragments of dead coral.

Death quickly drains the vivid colours from a living reef. Many corals owe their colour to algae, single-celled microorganisms with which they enjoy a symbiotic relationship. The algae use sunlight that filters through the water to make food by photosynthesis both for themselves and for coral polyps, the tiny animals within coral whose mineral skeletons make up the reef. In return, the polyps supply the algae with other nutrients.

Once the polyps die and the algae disappear, grey or white skeletons of calcium carbonate are left behind. In a few species, such as the red coral loved by the jewellery trade, colour-fast pigment pervades the skeleton, but these are very much the exception. The brilliantly coloured fish that live among coral also lose their hues when they are preserved as museum specimens.

So, except for the living mini-reef and some stunning underwater reef images from the Catlin Seaview Survey - a survey the London-based insurer has conducted since September 2012 to record the health of the world's coral habitats - the Natural History Museum's show consists largely of exhibits in shades of grey and brown. But that should not put off visitors. The absence of bright colour lets viewers concentrate on the astonishing shapes and textures.

The largest specimen is the giant Turbinaria cup coral, more than a cubic metre in volume, still containing some of the creatures that lived between its folds. "We've used eight different treatments to clean it," says conservator Lorraine Cornish. "This would have grown about five millimetres per year and taken 100 to 200 years to reach its present size."

Some specimens, such as the Turbinaria, beautiful Bermudan brain corals and a delicate series of Gorgonia sea fans, were obvious choices because of their spectacular appearance. "But on the whole it was very hard to select exhibits from the collection of more than 100,000 [coral] specimens dating back to the 18th century," says Johnson.

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Historical significance was one criterion. Pride of place goes to corals collected by Charles Darwin on the voyage of the Beagle in the 1830s, given to the British Museum in 1841 and transferred to the Natural History Museum when it opened in 1881. They will go on show, alongside Darwin's handwritten explanatory notes. Each object links to a different part of Darwin's drawing of the Cocos (Keeling) Islands in the Indian Ocean where he collected the corals - using a long pole to leap along the reef edge, like a pole vaulter, to avoid getting his feet wet. The first monograph that Darwin published (in 1842) outlined his ideas of the way ocean floor geology affects where and how reefs grow.

As Darwin's original maps and prints in the exhibition show, he proposed three different classes of reef: fringing reefs, growing out from the coast; barrier reefs, further away and separated by deep water from land; and atolls, reefs ringing a lagoon, which grew on subsiding volcanoes. This theory, highly influential in Victorian geology, was finally verified in the mid-20th century.

Far more ancient are the exhibition's fossil corals. The earliest, from Dudley in the West Midlands, are rugose and tabulate corals dating back 440 million years - distant cousins of living species. From then to now there is a fascinating story, told through fossils, of corals waxing and waning as climatic conditions changed.

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But the Coral Triangle, extending out from the Philippines and Indonesia into the tropical west Pacific, seems to have hosted the greatest reef biodiversity for the past 10 million years or so. Two-thirds of today's 800 reef-building coral species live in the region.

The exhibition's mini-reef includes a representative sample of marine life from the west Pacific with 26 coral species, plus dozens of fish, crustaceans, urchins, snails and other invertebrates. It was put together with the help of experts from the Horniman Museum aquarium in southeast London, a world leader in growing living corals. The tank was set up in October and all its inhabitants installed by the end of January. "The community will settle down further and fill out during the exhibition," says Jamie Craggs, Horniman aquarium curator. "We think some of the coral branches will put on three to four inches of vertical growth by September."

Craggs compares running a coral aquarium to expert gardening. "Corals are a bit like plants," he says. "You can slice a piece off, glue it to a new rock and - given the right lighting conditions and water chemistry - it will make a new colony."

But the 100 or so fish are the stars of the mini-reef. Although visitors may recognise a few, and most will be known to aquarium enthusiasts, the whole community shines with fluorescent colours and patterns. For sheer colour the unworldly magenta of the strawberry pygmy basslet stands out. For shape and pattern the long-nosed butterfly fish wins. "We are trying to represent as many facets of the reef ecosystem as we can," says Craggs. "I'd say we are 80 per cent of the way there. Some species could not be included because they are too big or too voracious."

For scenes from real reefs, visitors can turn to the imagery provided by the Catlin Seaview Survey, which the insurer uses to understand how the risks in the world's coral habitats are changing. "Coral reefs are important because they provide coastal protection worth billions of dollars," explains Chip Cunliffe, the Catlin programme manager.

The Bermuda government estimates that property damage from waves generated by storms would increase from $500m to $2bn over 10 years if the island's coral reefs disappeared, while healthy reefs are worth $1.1bn a year to the island's economy.

Much gloom surrounds the state of the world's reefs, with worse threatened as corals dissolve in growing acidity and bleach in the heat to come. But the message of the Natural History Museum show is not meant to be depressing, according to Johnson. "There are reasons to be optimistic, as more and more people become aware of the issues and the need for mitigation," he says. "Studies show that if you look after the local impacts, you can keep the reefs healthy in the face of global impacts."

Clive Cookson is the FT's science editor. 'Coral Reefs: Secret Cities of the Sea' runs at the Natural History Museum from March 27 to September 13; nhm.ac.uk

Slideshow photographs: Felicity McCabe

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