Faxonius quadruncus

Based on Crandall and De Grave (2017), the representatives of Orconectes form at least two distinct groups. The nominal group (the "cave Orconectes") form a monophyletic group that is more closely related to members of Cambarus, while the remaining "Orconectes" are more closely related to Barbicambarus, Creaserinus, and other species of Cambarus (Crandall and Fitzpatrick 1996, Fetzner 1996). As the type species of Orconectes, Orconectes inermis Cope, 1872, belongs to the cave-dwelling group, the genus is herein restricted to just those taxa. The surface-dwelling taxa now excluded from Orconectes sensu stricto are herein placed in the resurrected genus Faxonius Ortmann, 1905a, the oldest available name previously considered to be a synonym of Orconectes Cope, 1872.

This is a restricted range species that has experienced a significant range reduction since 1984, primarily due to the invasive crayfish Faxonius hylas. Furthermore, habitat quality in the upper St. Francis River has declined due to long term effects of past heavy metal pollution from mining.

This species is endemic to the Upper St. Francis watershed in Iron, Madison, St. Francois, and Wayne Counties in southeastern Missouri, USA (Pflieger 1996, USFWS 2018). It is found mainly in the upper St. Francis River tributaries on the upper end of the Upper St. Francis River watershed (Riggert et al. 1999, Westhoff 2011). Current extent of occurrence is 1,994 sq. km.

Occurrences are based on the number of records maintained by the Missouri Natural Heritage Program. USFWS (2018) considers it to be a single subpopulation that is defined by gene flow where individuals are able to interbreed and occur within the same stream reach of occupied habitat.

The main threat to this species is an invasive species, the Woodland Crayfish (Faxonius hylas), which was first documented in the Upper St. Francis River watershed in 1984 and is now known to occur in 11 streams in the watershed (DiStefano and Westhoff 2011, Westhoff et al. 2011). The invasion resulted in reduced abundance of the native F. quadruncus in at least 3 streams (Stouts Creek, Orr Hollow Creek, Marble Creek). In addition to F. hylas, another invasive species, the Belted Crayfish (Faxonius harrisoni) was discovered in 1987 in the St. Francis River and has since been documented at multiple other locations in the Upper St. Francis River watershed; its impact on F. quadruncus is unknown (USFWS 2018).

It is not clear why the invasive Woodland Crayfish is able to displace F. quadruncus. Although some studies suggested that the invasive species has life history traits that allow it to outcompete the native species (DiStefano et al., 2002; DiStefano, 2008), subsequent studies found no behavioral dominance in the laboratory (e.g., Rahm et al. 2005) or in fecundity (Mabery et al. 2017). The leading hypothesis is that the mechanism causing the displacement is reproductive interference in the form of hybridization; Fetzner et al. (2016) provides genetic evidence that both native species (F. peruncus and F. quadruncus) readily hybridize with the Woodland Crayfish.

Lead mining is the other known threat that is likely impacting the F. quadruncus (USFWS 2018). Although lead mining ceased in the 1970s, waste from mining operations is still present in the landscape (Missouri Natural Resource Trustee Council 2014). The relocation of mine waste throughout the area as topsoil, fill material, and aggregate for roads, railroads, concrete, and asphalt has further expanded the area of contamination, as has the use of lead mining tailings for agricultural purposes due to its lime content. All these uses have contributed to contamination of streams in portions of the St. Francis River watershed leading the EPA to designate portions of Little St. Francis as impaired for not meeting water quality standards for lead (EPA 2016, USFWS 2018). Heavy metals and mining-related tailings are known to adversely affect riffle-dwelling crayfish. Metal concentrations in crayfish at sites downstream of mining activities were significantly higher than those at reference sites, resulting in lower crayfish densities and lower likely lower survival rates based on crayfish caged at the downstream sites (Allert et al. 2008, 2013; USFWS 2018).