An edible bivalve mollusc with a dark blue or brown shell formed of two hinged parts (valves) that is longer than it is wide. Mussels are filter feeders, subsisting on plankton that they ingest through a siphon, and they move about by means of a muscular foot. There are several marine and freshwater families (class Bivalvia), marine mussels living in large groups on rocks to which they anchor themselves by strong threads (byssus).