What’s the difference between clams, mussels, and oysters?

Although they are frequent neighbors on market ice and easily confused because of the similarity in their appearance, these three shellfish are as distinct from one another as cats, civets, and genets. They all belong to the same molluscan class—Bivalvia—but differ when it comes to where they live, what they look like up close, and how they interact with their environment. Before diving into the details of what sets each animal apart, let us first look at the traits they share.

Similarities

Clams, mussels, and oysters are soft-bodied aquatic animals protected by a shell made of two halves joined at a hinge by a ligament. This design gives their class its name, Bivalvia—from Latin bi and valvae, meaning “two valves.” They obtain their nourishment through filter feeding: As water moves through their gills, it is filtered for microscopic organisms that these mollusks then consume as food. Most individuals in each group begin life as a free-swimming trochophore larva. Once the larval form finds a suitable surface, it settles and begins to secrete its shell-forming substances in preparation for adult life. It is at this point that the differences between the organisms start to emerge.

Dissimilarities

Adult clams burrow into sand or mud in salty or freshwater habitats using their muscular foot, which helps them remain concealed from predators. While buried, they extend two siphon tubes above the sediment for respiration and feeding. Adult oysters, by contrast, attach themselves to hard surfaces for life, fusing their shells to the surface and growing in irregular clumps. They live only in marine and brackish waters of coastal regions, where their shells accumulate after death to form reefs.

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Mussels are found in both freshwater and saltwater environments. They anchor themselves to a surface or one another using strong, self-secreted, proteinaceous filaments called byssus or byssal threads. Freshwater mussels prefer the flowing waters of streams and rivers, where they may gather in thousands to form mussel beds at the bottom of the streams. In coastal regions, mussels are often found hanging from rocks in high-energy surf zones where waves crash. Their byssal threads also help them hold on to human-made structures such as piers, ships, and ropes.

The three groups of animals also differ in their ability to move. Whereas adult clams can move about using their muscular foot, adult oysters are sessile and remain fixed to the substratum. Adult mussels are largely sedentary, capable only of limited repositioning about their points of attachment.

Telling the three bivalves apart

What About Scallops?

Known for their fan-shaped shells, scallops are also a group of marine bivalves. Besides the distinctive shape of their shells, which may have a smooth or ridged surface, scallops differ from clams, mussels, and oysters in being the only bivalves that swim freely as adults. They move by jet propulsion, rapidly clapping their shells together using a powerful adductor muscle to expel water and dart through the ocean. Cooked scallops are tender and have a mild, sweet flavor.

Clams, mussels, and oysters can be visually differentiated by their distinct shell structures. Clams have the most symmetrical of shells, which are neatly rounded or oval in shape, with each half approximately equal in size. Although often brightly colored on the inside, their shells are usually white, tan, or brown on the outside. They typically have a smooth and shiny surface—the result of constant abrasion against the sand that they burrow in.

Mussel shells can be smooth or ribbed with concentric ridges. They are oblong or triangular and narrower than clam shells. They are often colored blue, black, or dark brown on the outside, with interiors made of the shining nacre, or mother-of-pearl, layer. Oyster shells—colored brown, gray, or white externally—have the roughest texture of the three bivalves. The outer surface of the shells has bumps, ridges, and wavy lines, and they are irregularly shaped. Internally, oyster shells are also made of nacre.

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All three bivalves are consumed as low-fat, high-protein seafood. Within each group, there are edible and inedible species. The methods of consumption and the taste profiles of the edible species vary. Clams are consumed raw or cooked and have a bolder, saltier flavor than mussels. They are also chewier. Mussels are almost always cooked, and they have a sweeter taste than clams. Oysters are commonly eaten raw; their tender and buttery texture combined with their sweet flavor makes them a popular choice.