Illinois Natural History Survey - University of Illinois

The Freshwater Mussels of Illinois (The Mississippi, Illinois, Wabash, and Ohio Rivers


Freshwater mussels (also commonly called clams or naiads) may be the most endangered group of animals in North America. Of the approximately 300 species found in North America, 70% are endangered, threatened, or in need of conservation. Because of their habitat and biology (river dwelling, long-lived, sedentary, filter feeders, dependent on fishes for reproduction), mussels are extremely susceptible to the cumulative effects of siltation and other forms of pollution and therefore are excellent indicators of water quality.

INHS biologists have conducted periodic surveys for freshwater mussels and by comparing the number of species found today with that of past studies, we have documented substantial reductions in diversity over the past 100 years. Recent surveys have indicated that many mussels that were once widespread and common in Illinois and throughout North America have been drastically reduced in number--16 species have been extirpated from the state of which 7 are globally extinct. Large rivers have been particularly hard hit because nearly all have been radically altered for navigation or otherwise modified to where they no long function as they did in the past. In addition, exotic species like the zebra mussel, thus far concentrated only in large rivers, have greatly impacted native mussels.

The Mississippi River historically supported 50 species of mussels. However, the construction of locks and dams has drastically altered the natural hydrology of the river and now it functions more as a series of lakes rather than a free-flowing river. That coupled with siltation from agriculture and poor land management has reduced the number of species to 33, a 34% decrease in species richness.

The Illinois River drains about two-thirds of the state and historically supported an abundant mussel fauna, which in turn supported a booming pearl button industry. Overharvest, the installation of locks and dams, chemical pollution, and siltation have all taken their toll on mussels. Of the 47 species once found in the river, only about 26 remain (a 45% decrease). However, parts of the Illinois River have undergone recovery in recent years and 18 species have repopulated the upper Illinois where they formerly had been extirpated.

The Wabash River is the longest free-flowing river in the eastern United States, and it is one of the few large rivers in the country that remains unimpounded and unchannelized throughout most of its length. The Wabash River mainstem in Illinois historically supported 67 species of mussels. Unfortunately, recent surveys indicate that the current number of species is 30 (a 55% decrease). Some of its larger tributaries (i.e., Vermilion, Embarras, Little Wabash) harbor the only populations left in Illinois of a few mussel species now missing from the mainstem.

Like the Mississippi and Illinois rivers, the Ohio River has been dammed and channelized for navigation. Many of the riverine species that once called it home have disappeared. Only 30 of the 51 species historically present in the mainstem Ohio remain today (a 41% decrease).

The genus Epioblasma has been particularly hard hit in the Wabash and Ohio rivers. Of the nine species in the genus once found in Illinois, only one (the snuffbox) remains. Dams, channelization, and siltation have destroyed the habitat of this group (riffle areas in large rivers).

Our surveys and others conducted throughout the eastern U.S. indicate that we have lost many mussel species and that many others are on the verge of extinction. The decline of freshwater mussels is probably due to a combination of factors, but siltation seems to be the primary cause in midwestern streams. Stronger soil conservation measures in lands bordering streams are needed to prevent surface runoff and to help curtail erosion. Keeping an intact vegetated border or riparian zone along streams is perhaps the most important thing we could do to prevent further losses of aquatic biodiversity.

Kevin S. Cummings and Christine A. Mayer, Center for Biodiversity

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