The zebra mussel (Dreissena polymorpha) is a small freshwater mussel. The species was originally native to the lakes of southern Russia and Ukraine, but has been accidentally introduced to numerous other areas and has become an invasive species in many countries worldwide. Since the 1980s, the species has invaded the Great Lakes, Hudson River, and Lake Travis.
The species was first described in 1769 by German zoologist Peter Simon Pallas in the Ural, Volga, and Dnieper Rivers. Zebra mussels get their name from a striped pattern commonly seen on their shells, though it is not universally present. They are usually about the size of a fingernail, but can grow to a maximum length around 2 in (5.1 cm). Shells are D-shaped, and attached to the substrate with strong byssal fibers, which come out of their umbo on the dorsal (hinged) side.
