The zebra mussel, Dreissena polymorpha, is a species of small freshwater mussel, an aquatic bivalve mollusk. This species was originally native to the lakes of southeast Russia . However, it has been accidentally introduced in many other areas, and has become a problematic invasive species in many different countries.
Although zebra mussels superficially resemble marine mussels in the family Mytilidae, and like them, are attached to solid substrates with a byssus, nonetheless, zebra mussels are not at all closely related to the mytilids; they are much more closely related to the Veneridae, the venus clams.
Zebra mussels get their name from a striped pattern which is commonly seen on their shells, though not all shells bear this pattern. They are usually about the size of a fingernail, but can grow to a maximum length of nearly two inches (5 cm). The shape of the shell is also somewhat variable.
The native distribution of the species is the Black and Caspian Sea. Zebra mussels are an invasive species in North America, the British Isles, Spain, and Sweden.