Oyster Basics

The oyster is a bivalve mollusk with a thick, rough, irregular grayish or brownish shell. The top shell is larger and flatter than the lower one, which is concave and houses the body of the oyster. Oysters are classified as either flat or cupped—the flat varieties are more common in Europe.

Oysters thrive in both tropical and temperate seas, but those found in temperate waters mature less quickly, often taking as long as 4–7 years to grow to a length of 3"—a size that tropical oysters sometimes attain in only 2 years. The slow-growing oysters found in temperate waters are renowned for their flavor.

Eaten since prehistoric times, oysters are relatively easy to breed and have been grown in oyster beds for more than 2,000 years. The oyster was a particular favorite of the Romans and Gauls, who were the first to cultivate it on a large scale. In ancient Greece, oysters were used as ballots as well as food: electors voted by scratching their choices onto the surface of the shells. Today, the leading oyster-producing countries are the United States, Japan, Korea, and France, but the oyster is also farmed in numerous other countries, including South Africa, Australia, New Zealand, and Canada.

The ample, shiny flesh of oysters can be grayish brown, pearl-gray, or beige, and may be even slightly green if the oyster has fed on seaweed. Its consistency changes during the reproductive season (from May to August in the Northern Hemisphere), becoming milky and soft. For this reason, it’s often said that oysters are edible only during months that contain the letter “r” (from September to April). However, they aren’t actually inedible during the summer months, just less appetizing and more perishable. At any rate, this restriction is no longer always applicable—at least for cultivated oysters—because American researchers have invented a new genetic procedure that makes oysters sterile, effectively ending the reproductive cycle and eliminating the period during which they are less flavorful.

Like all mollusks, oysters are particularly vulnerable to pollution or toxic algae, and oysters harvested from contaminated waters can cause moderate to severe food poisoning. Don’t eat oysters you’ve caught yourself unless you’re sure that the water in which you found them is clean. Symptoms of illness from oysters include tingling in the lips, neck, or face, as well as headaches, dizziness, general weakness, and breathing difficulties.

In North America, oysters are often named for the areas they inhabit. Bluepoint and Cape Cod oysters are among the most popular varieties in the United States, while Caraquet (named for Caraquet Bay in New Brunswick) and Malpeque (cultivated on Prince Edward Island) oysters are renowned in eastern Canada. Likewise, the most sought-after Australian variety is known as the Sydney rock oyster.

Oysters are also prized for their magnificent pearls, which, unlike those of other mollusks, are large enough to be fashioned into jewelry. A pearl forms when a grain of sand or a piece of the shell becomes lodged inside the oyster, which then secretes a nacreous liquid in order to protect itself from this foreign body. Cultured pearls are produced by inserting a tiny pearl inside the shell, thus prompting the oyster to secrete layers of calcium carbonate. The most beautiful pearls are produced by oysters in the Pinctada genus, which live in warm-water seas.