The Collaborative International Dictionary of English v.0.48:

Mussel \Mus"sel\, n. [See Muscle, 3.]
   1. (Zool.) Any one of many species of marine bivalve shells
      of the genus Mytilus, and related genera, of the family
      Mytidae. The common mussel (Mytilus edulis; see
      Illust. under Byssus), and the larger, or horse, mussel
      (Modiola modiolus), inhabiting the shores both of Europe
      and America, are edible. The former is extensively used as
      food in Europe.
      [1913 Webster]

   2. (Zool.) Any one of numerous species of Unio, and related
      fresh-water genera; -- called also river mussel. See
      Naiad, and Unio.
      [1913 Webster]

   Mussel digger (Zool.), the grayback whale. See Gray
      whale, under Gray.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mussel
    n 1: black marine bivalves usually steamed in wine
    2: marine or freshwater bivalve mollusk that lives attached to
       rocks etc.