Young French oysters have fallen prey to a killer virus because they have used up too much energy developing their sex organs, scientists believe.
An expert team has been trying to find out why oysters have been decimated in all but one of France's coastal beds, dealing a severe blow to the industry.
A warm winter and wet spring left the young oysters especially vulnerable to Oyster Herpesvirus type 1, they say.
They matured too fast, feeding on abundant plankton, the scientists say.
The destruction of oysters aged 12 to 18 months ranges from 40% to 100% in all the French oyster beds except for one area at Arcachon in the south-west.
An expert from the Ifremer institute, Tristan Renault, told the French news agency AFP that "the animal has been using up a lot of energy developing its genitalia and using a lot less to defend itself".
Samples collected by Ifremer suggest that the Vibrio splendidus bacteria has contributed to the oysters' weakness.
Oyster farms hope to recover some of their stock by getting the uninfected survivors to breed.
France is the biggest oyster producer in Europe and the fourth biggest in the world, after China, Japan and South Korea, AFP reports.