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Clams, Mussels, Oysters, Scallops: A Guide to Bivalve Mollusks

Written by MasterClass

Last updated: Jun 23, 2020 • 6 min read

Bivalve mollusks are one of the world's most popular seafood options, harvested at twice the rate of crustaceans. These filter feeders taste very much like the oceans, lakes, and rivers where they live, and they make their way into a variety of classic dishes including Spanish paella, Maine-style fried clams, and Italian linguine alle vongole.



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What Are Clams?

Clams are bivalve mollusks with two adductor muscles that hold their shells together and a burrowing foot that allows them to dig. They live most of their lives buried under sand or mud in the shallow waters of oceans or rivers. They range in size from less than a millimeter to more than four feet in diameter.

5 Common Types of Clams

There are about 150 edible types of clams, but only a handful of them are commonly available commercially. Some of the most popular clam varieties include:

  1. Northern quahogs (Mercenaria mercenaria), also known as Atlantic hard-shell clams, are saltwater clams found in intertidal zones on the Atlantic coast from the Gulf of St. Lawrence to the Gulf of Mexico. They have grayish-white shells and are sold by different names depending on their size. Littleneck clams are the smallest northern quahog you can buy, at one and a half inches wide. Cherrystones are about two and a half inches wide. Chowder clams are the largest northern quahog—three inches and wider.
  2. Pacific geoducks (Panopea generosa) live on the Pacific coast from southern Alaska to Baja California. With shells seven to nine inches long and outer siphons up to four feet long, they are the largest burrowing clam. (A single Pacific geoduck can weigh up to eight pounds.) They have a white oblong shell. The siphon is often served raw as sushi or ceviche, while the belly has a stronger flavor and is typically used in soups or stir-fried.
  3. Soft-shell clams (Mya arenaria) are iconic in New England but live on both sides of the North Atlantic and on the Pacific coast. Soft-shell clams have long siphons and grayish-white, oval-shaped shells that are usually one and a half to three inches long—but can grow up to six inches long. Their thin, brittle shells gape open, which can make them sandy, so you need to purge them before cooking.
  4. Razor clams (Solenidae family) have brittle shells that gape open. Atlantic jackknife clams are long and straight, while Pacific razor clams are more round in shape.
  5. Surf clams (Spisula solida) are large (up to six inches long) and perfect for clam chowder. They account for almost one third of the clam harvest in the United States, where they are mainly found in deep water off the coast of New Jersey.
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What Are Mussels?

Mussels are bivalve mollusks of the Mytilidae family. Mussel shells are dark blue-black in color, hinged, and oval in shape. Though the majority of mussels eaten today are farmed, you can easily find them in the wild along exposed shores of cold water in intertidal zones around the world, clinging to dock pilings and rocky outcrops in the water via strong byssal threads. Green mussels live in both Asia and New Zealand. Freshwater mussels, though similar in appearance to the conventional navy variety, are a slightly different species, and generally aren’t eaten.

What Are Oysters?

True oysters are bivalve mollusks of the Ostreidae family. (Oysters that produce pearls belong to the Aviculidae family.) Edible oysters live in the warm and temperate waters along the coasts of every ocean and have been cultivated as food for 2,000 years. The two valves, or hinged oyster shells, are slightly different in shape—one is convex, and the other is flatter, though both shells are bumpy. Common oyster varieties include:

  1. European oysters (Ostrea edulis) are a small species found on the Atlantic coast of England, Ireland, France, Belgium, and the Netherlands.
  2. Portuguese oysters (Crassostrea gigas), also known as giant Pacific oysters, are a larger variety (up to 10 inches) found in Europe and Asia.
  3. American oysters (Crassostrea virginica) can reach up to seven inches in diameter and live on the Atlantic coast from New Brunswick, Canada, to the Gulf of Mexico.


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What Are Scallops?

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Scallops are molluscs like mussels, oysters, and clams. They’re unique in that they’re the only free-swimming bivalve: Scallops use their large central muscle (called the adductor) to clasp their shells shut, shooting water out one end and propelling themselves forward in the other direction. This big muscle is the part of the scallop that we eat. This muscle cooks quickly and is naturally tender, with a mild, sweet ocean flavor. A brief sear on the stovetop is the most common way of preparing scallops, which are high in free amino acids and sugars that easily form a deep golden brown crust. The scallop family includes about 400 species ranging in size from a few millimeters to three feet in diameter, but most of the scallops we eat are one of two types:

  1. Sea scallops belong to the species Pecten and Placopecten and are dredged from deep waters year-round. They’re larger than bay scallops, and great for grilling and searing.
  2. Bay scallops, also known as calico scallops, of the species Argopecten are dredged or hand-gathered closer to shore during a specific season. They’re small and delicate, ideal for sauteing or ceviche.

Clams, Mussels, Oysters, and Scallops: How Are They Different?

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Clams, mussels, oysters, and scallops are all part of the class Bivalvia in the phylum Mollusca, which means that they share many of the same characteristics. Although clams, mussels, oysters, and scallops are all part of the same taxonomic class, they belong to different subclasses, orders, and families. Heterodonta is a subclass of bivalves that burrow, including clams. Pteriomorphia is a subclass of bivalves that attach to rocks, including oysters (order Ostreoida, family Ostreidae), scallops (order Ostreoida, family Pectinidae) and mussels (order Mytiloida). There are a few key differences between these four types of bivalve mollusks:

  • Habitat: Bivalve mussels have two shells that connect at a hinge. They usually live burrowed just under the surface of oceans and rivers, although some species are deeper-burrowing, such as razor clams. In general, cams prefer soft, sandy shores and have smooth, rounded shells. Oysters live on rocky coasts and have bumpy shells. Mussels are often grown on ropes that they cling to via byssal threads. Scallops, like oysters, generally have one convex shell and one flatter shell; the convex shell is the side of the scallop that burrows under the sand.
  • Preservation: Because scallop shells are relatively loose, they are typically shucked immediately after harvest, which means that quality starts to deteriorate before scallops hit the market. To preserve their freshness, scallops are often frozen directly on the boat, or soaked in a solution of polyphosphates, which helps preserve their moisture and turns the scallops glossy white (these are called wet-packed scallops). In contrast, clams, mussels, and oysters are usually sold live in their shells, to be released by steaming or shucking.
  • Preparation: Protected by their shells, all bivalve mollusks have soft, squishy bodies that range from delicate to chewy when cooked. Most bivalve mollusks consume plant particles, algae, and bacteria through a cilia filtration system, which imbues them with the flavor of the waters in which they live. Although bivalve mollusks are relatively similar in flavor, different cooking preparations show off their unique qualities. Oysters are typically eaten raw or smoked on the half shell. Clams and mussels are often cooked in the same way: steamed or deep-fried. Scallops are usually pan-seared.

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