Freshwater mussels are members of one of the largest groups of animals on the planet – the phylum Mollusca. Freshwater mussels belong to a major subgroup of molluscs, the bivalves (Bivalvia), which inhabit marine and freshwater ecosystems throughout the world. They are distinguished from other molluscs by the absence of a radula (a rasping “tongue” used by snails to graze periphyton) and the presence of a two-part (bivalve) calcareous shell enclosing their soft anatomy (Figure 1). Bivalves are laterally compressed (narrower across than from top to bottom). Freshwater mussels are sessile filter feeders that burrow into the substratum via a large muscular foot that can also be used for rudimentary locomotion; generally, most bivalves spend their entire lives in a small area (Pearse and others 1987).
Globally, there are approximately 1,000 described species of freshwater mussels divided among five families, with the greatest diversity occurring in the United States. Mussel taxonomists in the United States recognize 341 species and subspecies in the families Unionidae (293), Margaritiferidae (7), Sphaeridae (38), Corbiculidae (1) and Dreissenidae (2). The latter two families are not native to North America (Neves and others 1997). Most are found east of the Mississippi River, with a few endemic species occurring in watersheds along the Pacific Coast. The Southeast is the most speciose region of the country (186 species) with Alabama possessing the most mussel species (175), followed by Tennessee (131), Kentucky (103), and Georgia (99) ( Hackney and others 1992, Williams and others 1993, Neves and others 1997).
Mussels are long-lived with individuals of most species living more than 10 years and some reported to live more than 80 years (Kuznik 1993, Neves and others 1997). Mussels are sexually dimorphic and must live as an ectoparasite for at least part of their life cycle. The union of male and female gametes produces a special embryo called a glochidium; glochidia are housed in chambers formed by the gill lamellae of the female (called a marsupium). After a period of a couple of weeks to a few months depending on species, the glochidia are expelled and must come in contact with a host organism. The hosts for a number of mussel species are fish, although some use other hosts such as salamanders and turtles (e.g. Utterbackia imbecillis). The duration of the parasitic phase varies, but typically ranges from two weeks to a few months. Then, the metamorphosed juvenile mussels fall to the substratum and begin their benthic, infaunal life.
Unionids (Family Unionidae) inhabit all types of inland waters (lotic and lentic). Immobility, longevity, and requirements for fish hosts have all made freshwater mussels highly susceptible to impoundments, dredging, pollution, and excessive sedimentation (Harris and others 1997). Twenty-one species are thought to be extinct and more than 100 species are federally classified as endangered or threatened under the Endangered Species Act. Over 70 species are of special concern and 70 species are considered stable.
The Upper Mississippi River (UMR) begins at its source near Lake Itasca in the North Woods of Minnesota and flows through the states of Minnesota, Wisconsin, Illinois, Iowa, and Missouri. Historically 51 species of freshwater mussels are known from the system, although only 44 have been collected during recent collection efforts. Four mussel species in the UMR are protected under the Endangered Species Act of 1973 and two others are listed as candidate species. These species are the Spectaclecase (Cumberlandia monodonta, Candidate) , Higgins’ Eye Pearly mussel (Lampsilis higginsii, Endangered), Scaleshell (Leptodea leptodon, Endangered), Sheepnose (Plethobasus cyphyus, Candidate), Fat Pocketbook (Potamilus capax, Endangered), and Winged Mapleleaf (Quadrula fragosa, Endangered).