Dromus dromas

This species is placed in a monotypic genus closely related to the genus Elliptio.

Formerly widespread throughout the Cumberland and Tennessee River systems in Tennessee, Alabama, Kentucky, and Virginia, this mussel has become extremely rare throughout its present range, with only two viable populations (Clinch and Powell rivers) remaining, and populations even in these have declined since 2011. The species is threatened by water quality degradation (pollution, siltation), habitat loss due to channel dredging (for navigation) and impoundment of rivers and tributaries (for hydroelectric power and recreation), and invasive species

This species is known from the Cumberland and Tennessee river systems in Tennessee and Virginia, USA; historical occurrences extended to Kentucky and Alabama. It is now restricted to the Powell and Clinch rivers near the Tennessee-Virginia border (Parmalee and Bogan 1998, Jones et al. 2004, USFWS 2020). It was once common throughout the Tennessee River system and was known from the middle Cumberland River in Smith County, Tennessee, and the Tennessee River in Meigs County, Tennessee. However, populations in the mainstems of both the Tennessee and Cumberland rivers appear to be extirpated (USFWS 2020). In Alabama, it historically occurred in the Tennessee River downstream of Muscle Shoals but has not been reported in the state since the 1930s, so is thus presumed to be extirpated; however, reintroduction efforts have been undertaken (Mirarchi et al. 2004).

Reproducing populations occur only in the upper Clinch (158 river km) and Powell rivers (79 river km) in Tennessee and Virginia above Norris Reservoir (Jones et al. 2004; USFWS 1984, 2020). Based on shells recovered from aboriginal sites, Jones et al. (2004) reported that this was once one of the most abundant mussels in the Tennessee River. A 1980 survey by Virginia Tech University and TVA identified nine sites in Virginia. By 2010 it was believed that as few as three reproducing populations existed. By 1984, the population in the Tennessee River was considered extremely rare since only three live specimens had been found in the Chickamauga Reservoir below Watts Bar Dam (USFWS 1984); this population is now likely extirpated. Results of attempts at introduction into Alabama and Kentucky are still being evaluated (USFWS 2020).

Impoundments, siltation, and pollution leading to water quality and habitat deterioration, inadequate sewage treatment, coal mining, oil and gas drilling, and poor land-use practices all comprise threats to this species. USFWS (1984, 2020) cited alteration and destruction of stream habitat due to impoundment of the Tennessee and Cumberland rivers and their tributaries for flood control, navigation, hydroelectric power production, and recreation as the single greatest factor contributing to this species' decline. A second factor that has severely affected this species is siltation. This is especially evident with this mussel, as it requires clean, flowing water over stable, silt-free rubble, gravel, and sand shoals to prevent smothering. Decades of coal production in the Appalachian region also increased silt runoff. A third factor, although on a much broader scale, is the impact caused by various pollutants. Evidence of pollution and associated mussel disappearance in these areas dates back to Ortmann (1918). Disease possibly related to several viruses is threatening the Clinch River population, with resulting declines from 2016 to at least 2019 (USFWS 2020).