marine or freshwater bivalve mollusk that lives attached to rocks etc.
source: WordNet 3.0
Any one of many species of marine bivalve shells of the genus Mytilus, and related genera, of the family Mytidæ. 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.