Facts About Scallops the Misunderstood Mollusk

Scallop, showing adductor muscle
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It's easy to recognize a scallop when it's sitting on your plate at a restaurant, but do you know what kind of creature it is? Found in saltwater environments like the Atlantic Ocean, scallops exist the world over. Unlike their relative the oyster, scallops are free-swimming mollusks that live inside a hinged shell. What most people recognize as a "scallop" is actually the creature's adductor muscle, which it uses to open and close its shell in order to propel itself through the water. But there is even more to know about this fascinating shellfish.

They're Mollusks

The eyes of a Bay scallop
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Scallops are in the phylum Mollusca, a group of animals that also includes snails, sea slugs, octopuses, squid, clams, mussels, and oysters. Scallops are one of a group of mollusks known as bivalves. These animals have two hinged shells that are formed of calcium carbonate. Bivalves such as scallops are threatened by ocean acidification, which affects the ability of these organisms to build strong shells.

They Live All Over

Great Mediterranean scallop
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Scallops are found in saltwater environments worldwide, ranging from the intertidal zone to the deep sea. Most prefer beds of seagrass amid shallow sandy bottoms, although some attach themselves to rocks or other substrates.

In the United States, two kinds of scallops are sold as food. Atlantic sea scallops, the larger kind, are harvested wild from the Canadian border to the mid-Atlantic and are found in shallow open waters. Smaller bay scallops are found in estuaries and bays from New Jersey to Florida.

There are large scallop populations in the Sea of Japan, off the Pacific coast from Peru to Chile, and near Ireland and New Zealand. The majority of farmed scallops are from China.

They Can Swim

Swimming scallop
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Unlike other bivalves such as mussels and clams, most scallops are free-swimming. They swim by clapping their shells quickly using their highly developed adductor muscle, forcing a jet of water past the shell hinge, propelling the scallop forward. They're surprisingly speedy.

They're Iconic

Open scallop shells
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Scallop shells are easily recognized and have been a symbol since ancient times. The fan-shaped shells have deep ridges, and two angular protrusions called auricles, one on either side of the shell's hinge. Scallop shells range in color from drab and gray to vivid and multihued.

Scallop shells are an emblem of St. James, who was a fisherman in Galilea before becoming an apostle. James is said to be buried at Santiago de Compostela in Spain, which became a shrine and pilgrimage site. Scallop shells mark the road to Santiago, and pilgrims often wear or carry scallop shells. The scallop shell is also the corporate symbol for the petrochemical giant Royal Dutch Shell.

They Can See

Close up of Spiny Pink Scallop
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Scallops have anywhere from 50 to 100 eyes that line their mantle. These eyes may be a brilliant blue color, and they allow the scallop to detect light, dark, and motion. Compared with other mollusks, the eyes of a scallop are pretty unique. They use their retinas to focus light, a job the cornea does in human eyes.

They Get Pretty Big

Open male an female scallops
NOAA Teacher at Sea Program

Atlantic sea scallops can have very large shells, up to 9 inches in length. Bay scallops are smaller, growing to about 4 inches. In Atlantic sea scallops (shown here), one can determine gender. The females' reproductive organs are red while the males' are white.

They're Muscles (Sort of)

Cumbrian scallops with wild coastal cumbrian sea herbs. A dish created by chef Simon Rogan at his Cumbrian restaurant Lenclume.
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Scallops swim by opening and closing their shells using their powerful adductor muscle. This muscle is the round, fleshy "scallop" that anyone who eats seafood will instantly recognize. The adductor muscle varies in color from white to beige. The Atlantic sea scallop's adductor muscle may be as big as 2 inches in diameter.

They're Filter Feeders

Rock scallop
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Scallops eat by filtering small organisms such as krill, algae, and larvae from the water they inhabit. As water enters the scallop, mucus traps plankton in the water, and then cilia move the food into the scallop's mouth. 

They Reproduce by Spawning

Pacific pink scallop
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Many scallops are hermaphrodites, which means that they have both male and female sex organs. Others are only male or female. Scallops reproduce by spawning, which is when organisms release eggs and sperm into the water. Once an egg is fertilized, the young scallop is planktonic before settling to the sea floor, attaching to an object with byssal threads. Most scallop species lose this byssus as they grow and become free-swimming.​