The exotic zebra mussel (Dreissena polymorpha) was found attached
to native freshwater mussels (Unionidae) in La Grange, Peoria, and
Alton Pools of the Illinois River and Pools 4, 13, and 26 of the
Mississippi River. From 1% to 27% of native bivalves collected
within these pools in 1992 had one or more zebra mussels attached,
with the highest densities of zebra mussels found in the Illinois
River. At one site in the Alton Pool of the Illinois River, 27%
of native mussels had a mean of 2.2 zebra mussels attached.
Densities were lower in the Mississippi River sites, ranging from
1% to 12%, with the highest density occurring below the confluence
of the Illinois and Mississippi Rivers. Of the 25 native species
collected, nine had zebra mussels attached. In the Alton Pool of
the Illinois River, Amblema plicata longer than 82 mm had more
zebra mussels upon them than A. plicata smaller than 82 mm. The
large numbers of young-of-the-year zebra mussels found in the
Illinois and Mississippi Rivers indicate D. polymorpha is
reproducing and possibly posing a threat to unionid populations in
both rivers.