1) Genus dreissena 2) Mollusk genus 3) Zebra mussels 
Dreissena is a genus of small freshwater mussels in the family Dreissenidae. The exact taxonomy of the species within the genus Dreissena is, however, uncertain. As Kantor et al commented, `...the Russian specialists on freshwater molluscs appeared to be `splitters” comparing with their western colleagues. This sometimes leads to extreme diff......
• (n.) A genus of bivalve shells of which one species (D. polymorpha) is often so abundant as to be very troublesome in the fresh waters of Europe.
Dreis'se·na noun [ New Latin Named after Dreyssen , a Belgian physician.] (Zoology) A genus of bivalve shells of which one species ( D. polymorpha ) is often so abundant as to be very troublesome in the fresh waters of Europe. 
Dreissena is a genus of lamellibranchiate molluscs allied to the mussels, One species Dreissena polymorpha is a native of the streams which flow into the Caspian, but was accidentally introduced into most rivers and estuaries of Europe, including those of Britain, where it is now abundant.
[n] - zebra mussels
genus Dreissena noun zebra mussels