The link was announced to 15,000 scientists in San Diego, California, at a meeting of the American Chemical Society last week. "I am amazed," said George Fisher, a professor of chemistry at Barry University in Miami, who led the research team with his graduate student Raul Mirza and Antimo D'Aniello, of the Laboratory of Neurobiology in Naples. "I have been a scientist for 40 years and my research has never generated interest like this. For centuries, old wives' tales have said that eating raw molluscs - oysters in particular - would stimulate the libido but there has really been no scientific evidence as to why and if this occurs.

"We think this could be the first scientific evidence of some substance. Did Casanova's 50 oysters really make him frisky? Could be." Previous speculation about the powers of oysters has centred on the refuelling powers of their high zinc content. Zinc is found in sperm and men lose between one and three milligrams per ejaculation. Dr Fisher and his team, partly funded by the United States' National Institutes of Health, bought samples of bivalve molluscs, which include mussels and clams, from markets near Dr D'Aniello's Naples laboratory. They then used a process called high-performance liquid chromatography to identify which amino acids were present and in what quantities.

They found two unusual ones - D-aspartic acid (D-Asp) and N-methyl-D-aspartate (NMDA). "They are not the normal amino acids that Mother Nature uses," said Dr Fisher. "You can't just find them in a vitamin shop."

Dr D'Aniello had found in earlier experiments that injecting the amino acids into rats triggered a chain reaction of hormones that ended with the production of testosterone in males and progesterone in females. "Increased levels of those hormones in the blood means that you are more active sexually," he said. "Yes, I do think these molluscs are aphrodisiacs. If the male is having difficulties, they have to eat a lot of mussels or oysters. Spring, when the molluscs breed, is best. There is the highest concentration of these two amino acids then."

The oysters have to be eaten raw to be most effective, because cooking reduces the quantity of D-Asp and NMDA molecules. Casanova, who admitted seducing 122 women in his memoirs, offered his own serving suggestion in Volume Six: "I placed the shell on the edge of her lips and after a good deal of laughing, she sucked in the oyster, which she held between her lips. I instantly recovered it by placing my lips on hers."

Telegraph, London