The shoulder is the section of the shell margin just below the ends of the hinge. The shoulder shape is an important characteristic in the straight-hinged stage of dreissenids. In Dreissenapolymorpha (zebra mussels) the shoulders have an angular shape, but in Dreissenabugensis (quagga mussels) they have a rounded shape.
Larval Identification Information
Zebra Mussel Identification