Is it an oyster, a clam, or a mussel?

Oysters, clams, and mussels... what's the difference?


These three types of shellfish are all mollusks and are part of the invertebrate phylum Mollusca and class bivalves. Bivalves are the second largest group within the phylum Mollusca and have two shells that are connected by a flexible ligament, which covers and protects their soft insides. There are more than 10,000 different species of bivalves, around 80% of which are found in marine habitats, but many do live in freshwater.

Bivalves breathe and eat by filtering water through their gills, an essential organ for these creatures. Clams can filter around 24 gallons of water daily and oysters can filter nearly 50! However, because of their incredible work as effective filters and cleaners of the ocean's waters, they can actually accumulate toxins. Occasionally, these toxins become poisonous for people, so it is important to keep that in mind when eating seafood. Also, all three of these mollusks produce pearls unique to each type.

Bivalves like the clams, oysters, and mussels have a hard exoskeleton, or shell, made of calcium carbonate. A growing problem for these mollusks is ocean acidification. Ocean acidification occurs when natural and (mostly) anthropogenic sources of carbon pollution get absorbed by the ocean, which gradually becomes more acidic as it dissolves more CO2. This becomes a problem for shellfish, corals, and the like because the increase in pH (increase in acidity) decreases the amount of available carbonate and causes shells to become thinner, making the shellfish more vulnerable, and increasing death rates.


That said, it is quite easy to get the different types of shellfish mixed up, so what exactly differentiates these different mollusks from each other?

In terms of their habitat, clams and mussels are found in either fresh or saltwater, while oysters only live in salty water or brackish habitats, like estuaries. Clams often dig into the sand and sediment underwater, but some smaller species of clams can be found on beach shores. Mussels release byssal threads - thin fibers that help them stick to rocks or other mussels in a cluster - which is how they are often found. Oysters are also quite sedentary and often attach to other oysters in larger oyster reefs.

One of the easiest ways to tell them apart is by their shells. Clams have smooth, oval, equally-sized shells, whose colors range from light brown, tan, to white. Mussels are darker in color: normally black to a dark blue and are slightly more elongated. Oysters, on the other hand, have rough unevenly shaped shells that can be brown, white, or even gray-ish.