 The external colour of the shell is often dark blue, blackish, or brown, while the interior is silvery and somewhat nacreous.The common name "mussel" is also used for many freshwater bivalves, including the freshwater pearl mussels.Freshwater mussel species inhabit lakes, ponds, rivers, creeks, canals, and they are classified in a different subclass of bivalves, despite some very superficial similarities in appearance.