Potamilus amphichaenus

This species was formerly placed in the genus Proptera which was widely used in the 1950s and 1960s. A ruling published in the Bulletin of Zoological Nomenclature (ICZN, 1992) recommended retention of the older name Potamilus. Results of phylogenetic and phylogeographic analyses by Smith et al. (2019) support the recognition of three evolutionarily divergent groups within the P. ohiensis species complex: P. amphichaenus, P. ohiensis, and P. streckersoni.

This species is currently a Texas endemic (thought extirpated from Louisiana; Oklahoma report likely in error) restricted to the Trinity, Neches, and Sabine river systems. It is generally considered to be rare, with fewer than 500 individuals, live or dead, reported in the past 25 years and it is known from <25 locations. Numbers encountered do not appear to be drastically different from those encountered historically, but direct comparisons cannot be made. It appears that there has been some range contraction (13%). The threat level is medium to high, with population growth, development, and water management (dewatering, stream flow, siltation, pollution, erosion, scouring) being the primary threats.

The Texas Heelsplitter is restricted to the Sabine, Neches, and Trinity Rivers in Texas (Walters and Ford 2013); current records indicate that is an area of 44,342 km² (GeoCat calculation). Historical reports from the Brazos River are attributable to Potamilus ohiensis (Neck and Howells 1994). The only record for Oklahoma, a sight record of one individual, as Proptera (Leptodea) amphichaema [sic amphichaena], from the Mountain Fork River, McCurtain County, reported on 22 April 1959 (Branson 1973, 1984), is likely in error (probably actually P. purpuratus) and that location is thought to now be inundated by Broken Bow Lake (C.C. Vaughn, in litt., 23 November 2018). The species is considered historical in Louisiana; last reported in 1968 from the Sabine River at a location that is now south of Toledo Bend Reservoir, a lake constructed in 1967.

Burlakova et al. (2011) reported four populations. Dickson (2018) found the species at four sites and determined from occupancy modelling that it is a difficult species to detect (detection probability of 0.11 +/- 0.06). Walters et al (2017) indicated the species occurs at 20 locations. NatureServe data from the 2000s indicate there may be closer to 25 locations.

Although not currently known from the Sabine and Neches Rivers in Louisiana, this species is known from these rivers just over the border in Texas (Vidrine, 1993) and may have historically occurred in Louisiana. In Texas, Neck and Howells (1994), Howells et al. (1996), and Howells (1997) documented 18 sites on the Neches River Living populations are also known from the Trinity River upstream of Lake Livingston (Neck, 1990; Howells, 1997); as well as Steinhagen Reservoir (Neches River drainage) in Tylor Co. in 1996 and the Sabine River in Panola Co., Texas in 1995 (Roe and Lydeard, 1998). In 2006, populations were confirmed in the Trinity River channel in Lake Livingston and upstream in several sandbars between Trinity and Centerville (approx. 30 km) (Don Barclay, pers. comm., October 2006).

Continued human population growth and resultant urban expansion will be a persistent threat. The region is already highly fragmented from agriculture (farming and ranching), and oil, gas, and timber extraction, the latter primarily from pine plantations. With urban development, expansion of roadways will continue, which could lead to construction of more stream crossings. Water usage will also increase as might diversion of water from streams in this species' range to larger metropolitan areas, which will further contribute to dewatering. Additional dam construction has also be posited as a solution to future municipal water supply issues. Current (and future) water management affects stream flow, contributes to scouring, erosion, and siltation. Drawdowns of reservoirs, whether by drought or from intentional management to eradicate invasive aquatic plants that degrade mussel habitat, such as water hyacinth (Eichhornia crassipes) and hydrilla (Hydrilla verticillata), can kill this thin-shelled species by desiccation or by exposing it to mammalian predators (USFWS 2009; Walters and Ford 2013). Climate change will continue and exacerbate many of these issues.

Pollutants in waterways is also of concern for this species. The USFWS (2009:66265) said that "increases in acidity, runoff, effluents from wood pulp and paper mills, human-caused nutrient enrichment, tar and oil, and increased silt loads due to land clearings are show to have damaging effects on mussel habitat. Pollutants of these types have been reported in the upper Trinity River, in Pine Island Bayou (a tributary to the Neches River), and in the lower Neches River…" Threats from urban and agricultural run-off include elevated levels of ammonia, nitrogen, and phosphorus, among other chemicals, resulting from use of fertilizers, herbicides, pesticides, etc. Threats may also include those coming from "emerging contaminants." These potential contaminants, which include pharmaceuticals and hormones, are coming from urban environments as well as from areas where livestock are kept. Elevated fecal coliform, mercury, and selenium have been reported for streams within the region (Texas DEQ).

One saving grace with this species is that its niche breadth appears to be wider than many other threatened unionids; it is found in lotic and lentic waters, is not "restricted by the clay content of the soils," and is able to deal with a "range of volumetric flow rates" (Walters et al 2017:1251), i.e., it is not a habitat specialist.