From WTTW Chicago:

Scientists believe the Quagga mussel first stowed away in the ballast water on transoceanic ships from the Caspian Sea. The mussels made their way into the lakes when that ballast water was purged.

The tiny fingernail-sized mussels closely related to another invasive, known as the Zebra mussel, first appeared in lake waters here in 1988.
The Quagga mussel is now the most pervasive and destructive invasive species ever to enter the Great Lakes. Over the last 15 years, the Quagga population has exploded, eclipsing the Zebra mussel and infecting all five of the Great Lakes. Read more.