Mussel

Mussel is the common name for members of several families of bivalve molluscs found in both salt water and fresh water, distinguished by shells more elongated and asymmetrical than those of other edible clams, which tend to be rounder or oval. The term most often refers to marine bivalves of the family Mytilidae, most of which live on exposed shores in the intertidal zone, attaching to firm surfaces with strong byssal threads commonly called a beard, though a few species have adapted to hydrothermal vents on the deep ocean floor; their shells are typically much longer than wide, wedge shaped or asymmetrical, dark blue, blackish or brown outside and silvery and iridescent within. The common name also covers many freshwater bivalves, particularly the freshwater pearl mussels of the order Unionida, which live in lakes, ponds, rivers, creeks and canals despite belonging to a different subclass of bivalves. Freshwater zebra mussels and their relatives in the family Dreissenidae are not closely related to either group; although they resemble Mytilus species in shape and attach to hard surfaces the same way, they are classified with the Heterodonta, the group that includes most commercially important clams.