Shellfish come in many forms. Some, like hard clams, tend to bury themselves in layers of sand sediment, while others, like scallops, are free moving. Many are edible and have become prized ingredients in the world of high-class gastronomy. The term shellfish also covers prawns, shrimp and lobster.
Clams come in a number of different varieties. The term is often used to cover all stationary bivalve mollusks, but the type of clams most commonly eaten are hard clams. They have small, pale shells. Other kinds of clams include razor clams (which have a long, sharp shell), the soft-shelled clam and the ocean quahog. They are often eaten in stews, such as clam chowder, or included in seafood rice dishes like Spanish paella.
Mussels are one of the most palatable shellfish. They are bivalve molluscs in the Clam family, and tend to cling to rocks in low-water areas using their beard, a series of strong threads. Mussels are popular around the world, especially in North America and Western Europe. Famous mussel dishes include moules mariniere, in which mussels are served in a butter and white wine sauce. Like clams, they are often served in seafood broths or chowders.
Oysters are valuable both as a foodstuff and as producers of pearls. Most varieties of oysters are edible (although pearl-producing ones are not), and they are often eaten raw. Pearls are produced by the oyster covering an invasive parasite with mother-of-pearl; over time, enough mother-of-pearl (or nacre, as it is also known) is present to form a perfect pearl. Today, most pearls are produced by factory farming. Natural pearls are very rare.
Scallops are also bivalve mollusks, but unlike clams, mussels and oysters they can move through the water by a means of propulsion centred around them rapidly opening and closing their shell. They are filter feeders and are edible. Their flesh is quite meaty, and is usually served only lightly sautéed; the flesh in the middle of the scallop is often raw. They are usually served in a cream sauce, or with a salty cut of meat, like pancetta.