Clams are wider and shorter than mussels, often with a "leg" protruding from the shell, while mussels appear dark blue or green, are long and teardrop-shaped and live in clusters attached to a hard surface such as a rock. Both mussels and clams fall under the scientific class Bivalvia.
A clam's "leg" is not actually a leg, but is a muscular organ used to move through sand or other substrate and to burrow in order to escape predators. Saltwater mussels live in the intertidal zones of seas and oceans. Mussels and clams are consumed as seafood and are often harvested recreationally for consumption. Some harvesting requires a license or is only allowed during certain seasons. It is important to check on pollution levels of an area before harvesting shellfish, as molluscs can absorb toxins from their environment that are dangerous for humans. While the terms "clam" and "mussel" are colloquial words used to distinguish two types of mollusc and are often used by seafood enthusiasts, these names do not fit within scientific classification. The terms "mussel" and "clam" can be confusing because most species referred to as "mussels" belong to the family Mytilidae. However, some freshwater species commonly called "mussels" fall into different families. "Clams" fall into a variety of families within the class Bivalvia.