Pleurobema oviforme

This species exhibits a high degree of ecophenotypic variation, with headwater forms more compressed than those from large rivers. The relationship between Pleurobema oviforme and Pleurobema clava is a matter of debate as P. oviforme replaces P. clava in headwaters of the Tennessee River, and it has been suggested that they are conspecific (Williams et al. 2008).

Although still relatively widespread, the number of occurrences is low and reflects substantial fragmentation and local extirpation from a once even more widespread and continuous distribution. Many relictual populations appear to be small and likely declining, with questionable viability. Threats to all riverine mussels are multiple and serious.

This mussel is endemic to the Cumberland and Tennessee river systems, two major tributaries of the Ohio River, which itself flows to the Mississippi River, USA. In the Cumberland in Kentucky and Tennessee, it occurs only downstream of Cumberland Falls. In the Tennessee drainage, it is known from headwaters in southwestern Virginia, western North Carolina, and eastern Tennessee downstream to Muscle Shoals, northwestern Alabama. A disjunct population inhabits the Duck River in central Tennessee. The only extant Alabama population exists in the Paint Rock River system (Williams et al. 2008), a relatively small (ca. 100 km) tributary of the Tennessee River in northern Alabama, with some tributaries also draining a portion of south-central Tennessee. Many former populations of this species are now extinct.

Occurrences are now greatly fragmented and restricted compared to the more widespread and continuous historical condition. All extant occurrences are potentially in jeopardy. The large river morph appears to be surviving in low numbers at only a very few sites below dams, and shell measurements in Ahlstedt (1988) suggest that these probably are relict, non-reproducing individuals. Current populations are scattered disjunctly through tributaries in the Cumberlandian region of the Tennessee River, often occurring in limited sections of headwater streams. In Alabama, the species is restricted to the Paint Rock River system, Jackson County (Ahlstedt 1996), with a few likely nonviable occurrences in other Tennessee River tributaries in the northern part of the state (Mirarchi et al. 2004, Williams et al. 2008). Parmalee and Bogan (1998) reported extant populations in Tennessee in non-impounded stretches of the Clinch and Powell rivers in upper east Tennessee, the Tellico River, Elk River, Hiwassee River, Duck River (disjunct population), Little Pigeon River, Big South Fork Cumberland River, and Stone River. Hubbs (2002) found a living specimen in the Elk River (RM 105.4), Tennessee. Johnson et al. (2005) reported the species from Citico Creek and the Hiwassee River, both inside and adjacent to the Cherokee National Forest in, respectively, Monroe and Polk counties, Tennessee. In North Carolina, Bogan (2002) cited occurrences in the Hiwassee, Little Tennessee, and French Broad rivers in Cherokee and Transylvania counties (LeGrand et al. 2006). In Kentucky, populations exist in the lower and upper Cumberland River below Cumberland Falls (Cicerello and Schuster 2003). Virginia populations include Copper Creek (Upper Clinch River drainage; Fraley and Ahlstedt 2000, Jones et al. 2001, Hanlon et al. 2009), upper North Fork Holston River, Smyth and Bland counties (Jones and Neves 2007), and upper South Fork Holston River (Stansbery and Clench 1978).

This species is impacted by channel alteration, inundation from reservoirs (e.g., dams on Columbia River, Duck River, Little Tennessee River), siltation from agriculture and clear-cutting, chemical and organic pollution, urban development, and toxic mine run-off. Commercial clamming may affect relic populations in some reservoirs of the Tennessee River.