Freshwater mussels (Mollusca: Unionacea) of the Little Wabash River basin, Illinois, with comments on historical changes in the mainstem during the past half-century

A freshwater mussel (Mollusca: Unionoidea) survey of the Little Wabash River basin, Illinois, was conducted to determine the status and distribution of the assemblage. During the summer of 2007, living unionids and valves of dead specimens were collected by hand at 29 sites, 12 of which were previously sampled in 1956 and 1988. A total of 3,879 live individuals of 28 species was found, and an additional 10 species were found as shells only. The four most abundant species found were Quadrula quadrula (Rafinesque, 1820), Tritogonia verrucosa (Rafinesque, 1829), Amblema plicata (Say, 1817), and Megalonaias nervosa (Rafinesque, 1820), which together comprised 75.7% of the live unionids collected. An examination of literature and museum records brings the total number of species recorded from the basin to 48, of which 34 are extant. Regression analyses indicated that the lower half of the mainstem had significantly higher relative abundance (r2= 0.57, P = 0.005) and historical species richness (r2 = 0.73, P = 0.0004) than the upstream half. Regression analyses also showed a reduction in species richness (r2 = 0.98, P = 0.04) but not relative abundance (r2 = 0.44, P = 0.54) in the mainstem from 1956 to 2007. This diverse and abundant, yet declining, assemblage of the Little Wabash River basin is similar to those reported for other basins within the Wabash River drainage.