For two years after it made its deadly appearance, Sadao Mamiya thought that he had escaped the Japanese akoyagai plague.
In Toba, on the Pacific coast, mortality rates were reported to be as high as 60 per cent. On the island of Shikoku, where the contagion began, victims of the plague were going bankrupt and some were said to have committed suicide. But in remote Wakasa Bay on the Japan Sea coast, business at the Mamiya Pearl Co went on as it always had.
In spring, the virgin akoyagai - pearl oysters - would be brought over from the Pacific coast, to be impregnated a few weeks later with a carefully rounded bead of polished shell. For two years or more, they would nurse their burden, coating it with the silvery secretion known as nacre. Finally in the winter, the mature oysters were hauled up from the underwater baskets in which they were kept, and their treasure removed - hundreds of shining, silvery-grey pearls.
But in 1998 Mr Mamiya's luck ran out. "One day we pulled some of the baskets up and found dead oysters," he said. "The next week, more dead, and more dead the week after that." When the survivors were prised open their cream flesh was stained a tell-tale pink. By the end of the season, half of all Mr Mamiya's oysters, and those of those neighbours, were dead, the latest victims of Japan's pearl plague.
The technique of seeding akoyagai to produce "cultured" pearls was invented in Japan in 1893 by a man named Kokichi Mikimoto; in the years following the war they became one of Japan's few exports and earners of foreign currency. By the 1960s, K Mikimoto & Co was world-famous and Japanese pearls - pink, gold, green, blue and white - were sold in the grandest jewellery emporia of London, Paris and New York.
Even the pressure of cheaper competitors from China, Indonesia and the South Seas was not enough to tarnish their nacreous lustre. But then the oyster plague appeared.
In 1995, as in every year for the past three decades, Japan's oysters yielded 70 tonnes of cultivated pearls. The following year the plague arrived, and they produced half as many. By 1998, the harvest had fallen to 25 tonnes, although last year it rose slightly. Recently, after a string of smaller failures, one of the country's biggest pearl, companies, Yamakatsu, went bankrupt. "That was a very, very symbolic moment," said Shigeru Akamatsu, a former marine scientist who works as sales manager at Mikimoto. "It was as if one of the supporting legs fell off the industry."
All of this has been a great disaster for Japanese pearls. No one knows with certainty, but three things seem to be killing the akoyagai. One is a sinister phenomenon known as the "red tide", caused by the sudden appearance of billions of plankton which stain the water red and kill marine creatures trapped within it. The second is the large quantities of the chemical formalin hosed into the water as an anti-parasite agent by cultivators of the famous fugu or Japanese puffer fish. But the greatest killer is the so-called "reddening" disease, about which so little is known about it that it doesn't have an official name.
A handful of marine biologists have ascertained that it is a virus, and that it seems to have made its first appearance on the island of Shikoku. It is here that most Japanese cultivators purchase the virgin oysters that they will later seed - this accounts for the contagion's rapid spread across Japan. "If living tissue is infected there is an incubation period before the symptoms appear, like any other virus," said Mr Akamatsu.
High sea temperatures make the oysters more vulnerable, but also encourage nacre production and thus nurture big pearls.
Cultivators like Mr Mamiya have a ready explanation for the virus - foreign oysters, imported from China, where a similar disease is known to flourish. But the story may not be so simple. For decades, despite notional quotas and occasional inspections, Japanese pearl farmers have filled their shallow waters with more than they are capable of supporting. With too many oysters in a restricted space, the creatures' immunity has decreased; industrial pollution has worsened the problem. By definition, an oyster with a pearl inside it is a sick oyster - the secretion of a nacre is an allergic reaction to a foreign body. "We stimulate nacre production by giving the akoyagai stress - by frequently moving them, for instance," said Mr Akamatsu. "But too much stress and they become ill. This is the paradox of pearl cultivation."
Big companies such as Mikimoto, which hatch their own oysters and do not depend on the Shikoku suppliers, claim that their trade in the highest quality pearls has suffered little. Smaller operators such as Mr Akamatsu are experimenting with hybrid varieties of foreign oysters mated with the hardiest Japanese varieties, those which have survived the plague. But many more are struggling to keep their heads above water.
"It's impossible to cure the virus. There is even a chance it could mutate," said Mr Akamatsu. "We have to accept the disease, eliminate overproduction and understand more about the eco-system."