Cultured pearls are the costume jewellery equivalent of artificial insemination, created by introducing a tiny piece of shell into a young oyster. More probably, however, when you crack open an oyster's craggy shell (not easy) you'll find its jelly-like flesh nestling inside. For epicures, this is the real prize. Natives with rounder, smoother shells and the more familiar rock oysters are expensive - selling for about pound;1 each - belying their long history as working-class fare.
In 1836, Charles Dickens noted: "It's a very remarkable circumstance that poverty and oysters always seem to go together." A common accompaniment to ale, half a billion of them were consumed in London in 1850. But by the end of the century oyster production had declined to a tenth of that figure. Even though spawning oystersproduce 800,000 tiny eggs (called spats), very few survive, and even a royal commission could not save them from overfishing. France harvests 2 billion oysters every year, three-quarters of which are eaten at Christmas.
Traditional decrees that you should eat oysters only when there is an "r" in the month, because in the summer they spawn and are liable to higher levels of contamination. But new farming methods have made them an all-year delicacy. Oysters grow best in tidal estuaries where they get a steady flow of seawater through their gills. At Whitstable in Kent, one of the country's best-known remaining fisheries, they are protected by European law from imitations but are not immune to pollution; their sale was banned for a while this year after an outbreak of algae in the resort's shellfish.
The aphrodisiac effect often ascribed to oysters could be down to the fact they are one of the richest food sources of zinc, which is known to aid fertility. Others claim their saucy reputation is due to the sensual - some say stomach-turning - sensation of salty flesh slipping down your throat. But maybe they are just talking molluscs.