Class Bivalvia: Order Mytiloida.
Elongate bivalved molluscs of sea coasts and estuaries. Somewhat similar in shape to freshwater mussels of family Hydriidae, but generally smaller.
Mytilidae is cosmopolitan family which includes marine or estuarine species of commercial importance. No Australian species is found far from the sea, but two species ( Modiolus pulex (= confusus ) and Fluviolanatus amarus ) occur in upper estuarine localities in virtually fresh water.
Bivalves accumulate toxic chemicals in the tissues. Overseas, Modiolus capax has been utilised as a biomonitor of arsenic and selenium in Mexico.