mussel
1. any of various marine bivalves of the genus Mytilus and related genera, esp M. edulis (edible mussel), having a dark slightly elongated shell and living attached to rocks, etc.,
2. any of various freshwater bivalves of the genera Anodonta, Unio, etc., attached to rocks, sand, etc. having a flattened oval shell (a source of mother-of-pearl). The zebra mussel, Dreissena polymorpha, can be a serious nuisance in water mains
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005