Even though both mussels and clams are mollusks, they're distinctly different other than just having different colored shells. Just like mussels, white-shelled clams are also bivalve mollusks, but they only live in freshwater, not saltwater and freshwater like their counterparts. Also, clams don't attach themselves to a substrate like an oyster or mussel. Clams can grow much larger than mussels, and they've got a heart and organs as well as a mouth and a nervous system. Most shellfish lovers like the taste of clams raw, but few will eat mussels with nothing more than an acidic marinade - they're almost always cooked. The fresh, umami fish-like taste of a clam is far bolder than a mussel, and clams are a little chewier when cooked.