Pleurobema riddellii

This taxon also includes Quadrula (Fusconaia) friersonii (Howells et al., 1996; Neck, 1984). Though there has been taxonomic uncertainty between F. lananensis and F. askewi, P. riddellii is morphologically and genetically distinct from Fusconaia species (Burlakova et al. 2012, Howells et al. 2012).

This rank reflects historical and current rarity, limited number of reports of living or recently dead specimens in recent years, known extirpations from some previously inhabited areas, and continuing modification and degradation of the aquatic ecosystems where it occurs. Although this species may rate a higher ranking, its rarity in conjunction with distribution in a number or river systems in two states, potential problems with field identifications suggests caution regarding its actual status. Many areas where it could occur have not been examined in recent years in great detail without success.

Historically, this species occurred as far west as the San Jacinto and Trinity rivers, Texas, eastward through the Neches and Sabine systems into the Red River and Bayou Pierre of north-central Louisiana (Howells et al., 1996; 1997; Vidrine, 1993). Currently, it occurs from drainages in eastern Texas (Howells et al., 1996) into Louisiana (Vidrine, 2008) and in the Red River tributaries in Arkansas. In Louisiana, it has been found in Vernon, Rapides, Allen, Natchitoches and Red River Parishes. In Texas, it is known from San Jacinto, Trinity, Neches, Sabine Rivers in Texas River Basin.

Until recently within Texas, only two living and two recently dead shells have been documented in the past ten years (Howells, 1997; Howells et al., 1997). Between 1981 and 1988, two live and one weathered shell were collected in the Little Cypress River in the Cypress Bayou system in northeast Texas (Mather and Bergmann, 1994). During surveys of the Village Creek drainage of Hardin, Tyler, and Polk Cos. in southeast Texas in 2001-2002, this species was found in 5 sites (of 22 surveyed) (11 specimens) (Bordelon and Harrel, 2004). Status in Louisiana in recent years is unknown although Vidrine (1993) reports this species from Bayou Dorcheat, Cane River and Kisatchie Bayou, Bayou Pierre in the arm of the Red River (in 1974), Calcasieu River, Sabine River, and Neches River but no recent specimens have been found with the exception of a potential occurrence in Rapides Co., Louisiana. As of 2012, Ford et al. (2012) cited 38 records (many can be combined into single occurrences); most of which are in the Neches River basin.

Within Texas, mussel populations in the eastern pinywoods areas were often more environmentally more secure than others elsewhere in the state; however, current timber cutting, gravel and sand removal, and general human modification of the area is now having dramatic negative impacts on unionids within the region. Similar impacts are presumably occurring in Louisiana as well. Loss of habitat as a result of siltation, and impoundments coupled with the addition of pollutants to streams and rivers may also contribute to the decline of this species.