Cyprogenia stegaria

Turgeon et al. (1998) recognized two species, Cyprogenia stegaria and C. alberti. Subsequent molecular data suggested cryptic species diversity in the genus (Serb and Barnhart 2008, Grobler et al. 2011). The most recent molecular analysis of Cyprogenia identified three independent evolutionary lineages: C. aberti in the Ozark drainages of Arkansas, Missouri, and Kansas; C. stegaria in the Ohio River Basin; and a third lineage in the Ouachita River drainage in Arkansas (Chong et al. 2016, Williams et al. 2017). Formerly placed in the genera Obovaria and Unio.

This mussel is rare throughout its limited range in the Tennessee and Ohio river systems; four viable populations remain in Kentucky and Tennessee. Steep declines (>80%) have occurred over the past century and particularly in recent decades. Remaining fanshell populations are small and geographically isolated from each other. Water quality degradation, invasive species, and habitat loss threaten most populations

This species was historically considered endemic to the eastern highlands east of the Mississippi River, USA. It was historically widely distributed in the Tennessee, Cumberland, and Ohio River systems (Parmalee and Bogan 1998), although it has become very rare in recent years (USFWS 2019). In the Ohio River drainage, records from the latter part of the 20th century exist for the deep channel of the Ohio River between Cincinnati and Pittsburgh (Johnson 1980); lower Muskingum and Walhonding rivers, Ohio (Stansbery et al. 1982); Salt and Licking rivers, tributaries of the Ohio River (Stansbery, pers. comm. to J. Cordeiro); Green River, Kentucky (USFWS 1991); Kanawha River, West Virginia (Stansbery, pers. comm. to J. Cordeiro); Allegheny River, Pennsylvania (Dennis 1970); and lower Clinch River in Scott County, Virginia (Neves 1991).

The four best, reproducing populations occur in the upper Green and Licking rivers in Kentucky (Cicerello and Schuster 2003), the Clinch River in Tennessee (USFWS 1991, 2019), and Rolling Fork River in Kentucky; the last is relatively small compared to the first three (USFWS 2019). The only Ohio River extant population in Ohio is from Muskingum Island near the confluence of the Ohio and Muskingum rivers (anonymous, 1996; Watters et al. 2009) at the Belleville dam pool (Watters and Flaute 2010) and the Markland dam pool in eastern Indiana (Watters and Flaute 2010), but weathered shells are known from Killbuck Creek (Muskingum River tributary) (Watters et al., 2009). A few other apparently non-reproducing populations are known from Indiana (Tippecanoe River [Cummings and Berlocher 1990, Fisher 2006; as shell only] and a few tributaries of the Wabash River such as lower East Fork White River [Fisher, 2006]), West Virginia (upper Kanawha River [Taylor and Horn 1983]), and Illinois (Wabash River in 1984 [Cummings and Mayer 1997]). It has been collected in Kentucky in the Middle Green and Barren rivers (Cochran and Layzer 1993). Also, a small reproducing population may exist in the Tennessee River below Pickwick Landing Dam (Parmalee and Bogan 1998). In the Alabama and Mobile basin, it occurred historically across the state of Alabama and the Elk River, but is known to be extant only in Wilson Dam tailwaters with questionable viability (Williams et al. 2008). A population has also been located in the Rolling Fork River (tributary of the Salt River, which flows to the Ohio River in Kentucky; USFWS 2019). Adults from the Licking River have also been translocated to other rivers in Ohio, West Virginia, and Kentucky, but it is unknown if these have resulted in recruitment (USFWS 2019).

Smith (1971) ranked the causes of extirpation or declines in fish species as follows: siltation, drainage of bottomland lakes, swamps, and prairie marshes, desiccation during drought, species introductions, pollution, impoundments, and increased water temperatures. All of these factors render habitats unsuitable, cause extirpations, and lead to the isolation of populations thereby increasing their vulnerability to extirpation for many aquatic species (including mussels) throughout North America. The loss of many historical populations was likely due to the impacts of impoundments, navigation projects, pollution, and habitat alterations, such as gravel and sand dredging, that directly affected the species and reduced or eliminated its fish host (USFWS, 1991). The zebra mussel, Dreissena polymorpha, has destroyed mussel populations in the Great Lakes and significantly reduced mussels in many of the large rivers of the eastern North America and has the potential to severely threaten other populations especially if it makes its way into smaller streams. Additionally, the black (Asian) carp (Mylopharyngodon piceus), a known mussel predator, has now been recorded in the lower Ohio River and poses a potential threat if populations expand and move upstream (USFWS 2019). Pollution through point (industrial and residential discharge) and non-point (siltation, herbicide and fertilizer run-off) sources is perhaps the greatest on-going threat to this species and most freshwater mussels. Lowered dissolved oxygen content and elevated ammonia levels (frequently associated with agricultural runoff and sewage discharge) have been shown to be lethal to some species of freshwater naiads, including C. stegaria (Horne and McIntosh 1979, Hernandez et al. 2016). Residential, mineral and industrial development also pose a significant threat. Destruction of habitat through stream channelization and maintenance and the construction of dams although slowed in recent years is still a threat in some areas. Impoundments reduce currents that are necessary for the most basic physiological activities such as feeding, waste removal and reproduction. In addition, reduced water flow typically results in a reduction in water oxygen levels and a settling out of suspended solids (silt, etc.), both of which are detrimental. Dredging of streams has an immediate effect on existing populations by physically removing and destroying individuals. Dredging also affects the long-term recolonization abilities by destroying much of the potential habitat, making the substrates and flow rates uniform throughout the system. Rotenone, a toxin used to kill fish in bodies of water for increased sport fishery quality, has been shown to be lethal to mussels as well (Heard, 1970). Natural predators include raccoons, otter, mink, muskrats, turtles and some birds, which feed heavily upon freshwater mussels (Simpson, 1899; Boepple and Coker, 1912; Evermann and Clark, 1918; Coker et al., 1921; Parmalee, 1967; Snyder and Snyder, 1969). Domestic animals such as hogs can root mussel beds to pieces (Meek and Clark, 1912). Fishes, particularly catfish, Ictalurus spp. and Amieurus spp., and freshwater drum, Aplodinotus grunniens also consume large numbers of unionids.

The distribution and reproductive capacity of this species have been seriously impacted by the construction of impoundments and navigation facilities, dredging for channel maintenance, sand and gravel mining, and water pollution. The four known reproducing populations are threatened by a variety of factors. The Green River has been degraded by runoff from oil and gas exploration and production sites and by alteration of stream flows by an upstream reservoir. Land use practices along the Clinch River have contributed to a decline in water quality and mussel populations. The Clinch River has also experienced some adverse impacts from coal mining, and the river has been subjected to two mussel kills resulting from toxic substance spills from a riverside coal-fired power plant. At least 30 collecting sites on the Clinch River once contained more than 18 different species of freshwater mussels (Ahlstedt, 1984). At one point, the mussel abundance in the Clinch River had decreased from an average of 11.64 mussels per square meter in 1979, to 6 mussels per square meter by 1988 (USFWS, 1991). The fanshell has been known from about 86 river miles of the Clinch River; yet, this species comprised less than 1 percent of the mussels collected at 11 sampling sites in 1979 and 1988 (USFWS, 1990).