what is a mussel?
Full Definition of MUSSEL. 1. : a marine bivalve mollusk (especially genus Mytilus) usually having a dark elongated shell. 2.: a freshwater bivalve mollusk (as of Unio, Anodonta, or related genera) that is especially abundant in rivers of the central United States and has a shell with a lustrous nacreous lining.ull Definition of MUSSEL. 1. : a marine bivalve mollusk (especially genus Mytilus) usually having a dark elongated shell. 2. : a freshwater bivalve mollusk (as of Unio, Anodonta, or related genera) that is especially abundant in rivers of the central United States and has a shell with a lustrous nacreous lining.