Burrowing shellfish is a term used to describe clams, which are mollusks that live in the sand or mud of the ocean floor. Clams have two shells that are connected by a hinge and they use their muscular foot to dig into the sand or mud. Clams are filter feeders, meaning they filter water through their gills to capture food particles. Clams are an important part of the marine ecosystem and are harvested for food.