Mussel is a common term for members of various bivalve mollusk families found in both saltwater and freshwater environments. In comparison to their close relative, the clams, which are frequently more or less spherical or oval, mussels have a shell that is elongated and asymmetric. "Mussel" means the bivalves of the marine family Mytilidae, which dwell on exposed coasts in foreshores or seashores and they hang onto rocks and other stable surfaces by their strong byssal threads or their beard.