Abstract
ABSTRACT Muskrats (Ondatra zibethicus) commonly prey upon unionid bivalves, discarding the empty shells in discrete piles or middens. We found large numbers of live unionids and the shell remains of zebra mussels (Dreissena polymorpha) in muskrat middens on the upper Ohio River and upper Mississippi River. Muskrats apparently began preying on zebra mussels that were attached to unionids following an increase in zebra mussel abundance at the study sites. Significantly fewer zebra mussels were attached to live unionids at muskrat middens than to ones collected from reference sites away from middens. We also found a group of stones with numerous zebra mussel byssal threads at an Ohio River midden, suggesting muskrats retrieved and ate the attached zebra mussels. We view this, as well as the fact that muskrats discarded live unionids, as evidence that muskrats were selectively preying on zebra mussels rather than unionids.
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