Making Mussels

At least 65 species of native freshwater mussels are found in Missouri waters. These humble shellfish once paved the bottom of rivers in incredible numbers, filtering the water and providing habitat and food for other animals.

Mussel populations, however, have crashed in a relatively short time, raising alarms about the health of Missouri's streams and rivers. Of nearly 300 North American species, 38 are presumed to be extinct, and 77 others are critically imperiled. In Missouri, these unique creatures are among our most endangered freshwater wildlife.

It's not surprising that mussels are in trouble. Mussels can be harmed by just about any of the many problems that affect our rivers. Adults live for decades on the bottom of the river. They can't move far to escape. They become victims if their part of the stream dries up in a drought, gets drowned by a reservoir, or is cut off by a channelization project. Mussels can also be uprooted by streambed erosion or buried by silt. They are at least as sensitive to water pollution as fish are, and they may be even more affected by pollutants in the bottom sediments in which they live.

As vulnerable as adult mussels are, their reproduction seems to be most at risk. Our native mussels cannot reproduce without the help of native fish. Female mussels produce thousands of tiny larvae called glochidia. Each is smaller than the head of a pin. These larval mussels must attach to the gills or skin of particular kinds of fish, where they remain attached for a few days or weeks. They grow very little during this time, but they complete their development and get a free ride to new habitat before leaving the fish.

Each species of mussel needs particular species of native fish to reproduce. Non-native fish, such as trout and carp, won't do. Biologists suspect that reduced populations of native fish are one of the reasons mussel populations have declined.

Concern for native mussels is not new. A century ago, mussels were harvested in huge numbers in Missouri and other parts of the Midwest. Their shells were the basis of a multimillion-dollar, button-manufacturing industry. Two prominent University of Missouri biologists, George Lefevre and Winterton Curtis, studied the biology of mussels from about 1906 to 1914 and concluded that overharvest was decimating mussel populations.

Recognizing that natural reproduction was insufficient to maintain populations, Lefevre and Curtis devised methods