 The word "mussel" is frequently used to mean the bivalves of the marine family Mytilidae, most of which live on exposed shores in the intertidal zone, attached by means of their strong byssal threads ("beard") to a firm substrate.A few species (in the genus "Bathymodiolus") have colonised hydrothermal vents associated with deep ocean ridges.In most marine mussels the shell is longer than it is wide, being wedge-shaped or asymmetrical.