Occurrence of the quagga mussel (Dreissena bugensis) and the zebra mussel (Dreissena polymorpha) in the upper Mississippi River system

The quagga mussel (Dreissena bugensis) was first found in the Ohio River and the upper Mississippi River in the mid-1990s. It has since gone unreported in the Mississippi River system possibly due in part to its phenotypic variability and close morphological resemblance to the more commonly occuring zebra mussel (Dreissena polymorpha). Sampling of the upper Mississippi River system during 2004-2006 revealed that the quagga mussel occurred at several localities outside its previously reported distribution in the Ohio River and upper Mississippi River. Few zebra and no quagga mussels were found in the Missouri River. Quagga mussels were not abundant in our survey, comprising less than 1% of identifiable Dreissena specimens.