The term mussel is used for several families of bivalve mollusks inhabiting lakes, rivers, and creeks, as well as intertidal areas along coastlines worldwide. The freshwater mussels (several allied families, the largest being the Unionidae) and saltwater mussels (family Mytilidae) are not thought to be immediate relatives and are grouped in different subclasses, despite some similarities in appearance. Marine mussels are abundant in the low and mid intertidal zone in temperate seas
of the northern and southern hemispheres. Within their geographic range, some
species prefer salt marshes or quiet bays, while others thrive in pounding surf,
completely covering exposed wave-washed rocks.
