Invasion of the giant oysters

Professor Karsten Reise does not normally use the sort of emotive language found in Hollywood horror movies. He is a serious and highly informed marine biologist in his late fifties who has been researching the environment around Germany's North Sea island of Sylt for well over a decade.

But on a grey and sea mist-bound afternoon, the heavily bearded professor walked over to a shelf in his office at Sylt's Alfred-Wegener Polar and Marine Research Institute overlooking the island's tidal sand flats and removed what looked like some ghastly freak of nature.

The remains of the creature in question was a giant barnacle-covered shell almost three times the size of an adult male's hand, yet shaped in the unmistakable form of an oyster. "This is the monster I found on the sands out there about four years ago," Dr Reise said, waving his arm in the direction of the sea. "Since then, the oyster phenomenon has turned into a proper invasion," he added.

Giant oyster specimens are not the only factors that seem to have induced uncomfortable feelings of Metro-Goldwyn-Meyer-style panic on Germany's northernmost island. Even the academic literature on the subject speaks unreservedly about an apparently unstoppable "invasion" by an "alien species" of the island's protected tidal sand flats.

Sylt's alien is the Crassostrea gigas, or Pacific oyster, which currently accounts for about 90 per cent of all oysters consumed globally.

The Pacific oyster was introduced to European coasts in the 1970s from Japan and British Columbia following the virtual collapse of the Continent's native oyster industry. The Pacific oyster was not introduced to Sylt, which boasted a formidable pre-war oyster industry, until 1986, and then only as a product that would be carefully farmed in an environment controlled partly by man.

"The Sylt oyster producers were granted a licence to fish farm, but back then all of the experts were convinced that the waters around the island were far too cold in winter to enable the oysters to survive without human intervention," Dr Reise said. But in the meantime, the experts have been proved hugely, even alarmingly, wrong.

The scale of Sylt's oyster invasion is massive. The Alfred Wegener Institute first noticed that something unusual was happening on the island's protected tidal sand flats back in 1995 –nine years after the Pacific oyster was introduced at a lone oyster farm on Sylt's northernmost tip. Handfuls of wild Pacific oysters were found that autumn nestling on an indigenous blue mussel bed that was exposed at low tide.

At that stage, the island's marine biologists were not unduly surprised. A few wild, or feral, oysters were expected as a natural byproduct of the local oyster farm. In 1995 the feral Pacific oyster population was about one oyster per square metre of tidal sand flat on Sylt. By 2004 the figure had leapt to nearly 500 per square metre. By 2007 the island's feral Pacific oyster count jumped to a staggering 2,000 per square metre. "What we are now experiencing is exponential growth of the wild oyster population," says Dr Reise. "We don't yet know where the process will end."

At low tide out on the wind and wave-beaten expanses of sandy mud and seaweed, evidence of the island's oyster invasion is everywhere. Pairs of oystercatchers and the occasional Brent goose spring into flight from sands that are covered for almost as far as the eye can see with a carpet of feral oysters that crunch underfoot. Unlike those consumed in posh restaurants, these are stuck together in vast clumps made up of scores of small oysters, with the odd native blue mussel and winkle nestling in between. "Six or seven years ago there was nothing here but sand," Dr Reise said.

Marine biologists are unanimous in their explanation for the causes of the Pacific oyster invasion. Sylt, which is on the same latitude as Newcastle, used to experience temperatures that were low enough to freeze the shallow salt waters surrounding it during winter. But the last harsh winter on Sylt was at the beginning of 1996. Since then, the island has experienced nothing but mild winters. "Cold winters can bring the spread of wild oysters to a halt, but warm winters enable the oyster larvae to flourish," said Dr Reise. "Their increase is a direct result of global warming."

During the warmer-than-average summer experienced in northern Europe during 2006, the sea temperature was greatly increased and the warm water appears to have encouraged an explosion in Pacific oyster larvae. In its wild form, the oyster has managed to spread as far north as Bergen in Norway.

The experts are not sure whether the current oyster explosion will have serious environmental consequences. During the early stages of the invasion, oysters were found almost exclusively on Sylt's indigenous blue mussel beds, raising fears that the rising oyster population would kill off the mussels.

Seabirds that normally rely on mussels as a food source have also been affected. Eider ducks, which used to visit Sylt on a regular basis in winter, have all but disappeared. The oystercatcher, a wading bird that lives all year on the island, also relies on mussels to survive, despite its name.

Even tourism is affected. Visitors who used to enjoy strolling across the sands in their bare feet now risk injury on the razor-sharp shells that cover the Pacific oyster beds.

Sylt's lone oyster farm, Dittmeyer's Oyster Company, has been farming its "Sylter Royal" Pacific oysters in one of the island's bays since 1986. Mr Dittmeyer insists that the sudden invasion of wild oysters on Sylt simply shows the island is catching up with much of the rest of northern Europe, where the Pacific oyster dominates. "It is happening everywhere," he said.

Mr Dittmeyer started his oyster farm on Sylt with Pacific oysters imported from Colchester. It was a case of history repeating itself. The Danish King Knut the Great did exactly the same during the 11th century, when he imported shiploads of oysters from England to what is now Germany's north-west coast.

As in much of the rest of Europe, the inhabitants of Sylt were able to harvest oysters from the seabed for the next 900 years, until industrial fishing and disease rendered them virtually extinct. Marine biologists such as Dr Reise say that today's explosion of the wild Pacific oyster population is just one of global warming's effects. Nobody has been able to predict its consequences. One of the factors that makes the spread of wild oysters almost impossible to control is the fact that the creatures have no natural predator – yet.

In his office, Dr Reise has another shell that sits next to his monster Pacific oyster. It is a black and grey speckled conch about the size of a large fist that houses a large sea slug or snail called the Rapana, which is found in Japanese waters. The creature is capable of boring through the shell of a Pacific oyster. "It was introduced in the Black Sea and has since spread to the Mediterranean," the professor said. "It could conceivably be introduced in the North Sea."