Missouri's Freshwater Mussels

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Published on: Aug. 2, 1999

Last revision: Nov. 3, 2010

of glochidia!

The glochidia attach to the fish's gills, (which usually doesn't harm the fish) become encysted, develop into juveniles and then drop to the stream bottom. If they land in suitable habitat, they will grow into adults and repeat the cycle. If they land in poor habitat, they will die. Similarly, if glochidia fail to encounter a specific host fish, they will not develop vital organs and slough from the fish and die.

Mussels threatened

Freshwater mussels are declining at an alarming rate throughout North America. They are sensitive to disturbance, because they are relatively immobile organisms, sometimes staying in a single spot for their entire lives. They have a complex life cycle that is easily disrupted, causing reproductive failure. Habitat alteration and loss, illegal and overharvesting, and competition from introduced species are the primary reasons for their decline.

Many Missouri watersheds have been destabilized and water quality has been degraded from poor land-use practices and urbanization. Freshwater mussels cannot tolerate a shifting, unstable stream bottom. They need stable habitat consisting of rocks, sand, cobbles and boulders for securing themselves in an otherwise turbulent environment.

Excessive silt and gravel loads go hand-in-hand with excessive land disturbance. Heavy silt loads interfere with the filtering and feeding activities of adults and smother young juvenile mussels. Mussels can close their shells to avoid temporary slugs of pollutants coming downstream, but eventually they have to open up to breathe and feed, so long-term water quality problems in a watershed will eventually kill them.

Mussel invaders

Two non-native freshwater mussel species--Asian clam (Corbicula fluminea) and the zebra mussel (Dreissena polymorpha) --have found their way into Missouri.The Asian clam was introduced into the western U.S. in the 1930s and quickly spread eastward. Since 1968 it has spread rapidly throughout Missouri and is most abundant in streams south of the Missouri River. Asian clams are believed to compete with native mussels for food and habitat.

In the mid-1980s Zebra mussels hitched a ride in the ballast water of freighter ships traveling from Europe to the Great Lakes. It has rapidly spread throughout the Mississippi River basin and westward to Oklahoma. In Missouri, the zebra mussel is thus far restricted to the Mississippi River, but its expansion into inland lakes and streams is anticipated. In April biologists confirmed a sighting of a zebra mussel in the Missouri River at Sioux City, Iowa.

Asian clam and zebra mussel larvae don't require a fish host to reach a

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