Range reduction of Oblong Rocksnail,Leptoxis compacta, shapes riverscape genetic patterns

Abstract

Many freshwater gastropod species face extinction, including 79% of species in the family Pleuroceridae. The Oblong Rocksnail, Leptoxis compacta, is a narrow range endemic pleurocerid from the Cahaba River basin in central Alabama that has seen rapid range contraction in the last 100 years. Such a decline is expected to negatively affect genetic diversity in the species. However, precise patterns of genetic variation and gene flow across the restricted range of L. compacta are unknown. This lack of information limits our understanding of human impacts on the Cahaba River system and Pleuroceridae. Here, we show that L. compacta has likely seen a species-wide decline in genetic diversity, but remaining populations have relatively high genetic diversity. We also report a contemporary range extension compared to the last published survey. Our findings indicate that historical range contraction has resulted in the absence of common genetic patterns seen in many riverine taxa like isolation by distance as the small distribution of L. compacta allows for relatively unrestricted gene flow across its remaining range despite limited dispersal abilities. Two collection sites had higher genetic diversity than others, and broodstock sites for future captive propagation and reintroduction efforts should utilize sites identified here as having the highest genetic diversity. Broadly, our results support the hypothesis that range contraction will result in the reduction of species-wide genetic diversity, and common riverscape genetic patterns cannot be assumed to be present in species facing extinction risk.

Assessing Genomic Diversity, Connectivity, and Riverscape Genetics Hypotheses in the Endangered Rough Hornsnail,Pleurocera Foremani, Following Habitat Disruption

“…Observed spatial genetic diversity in P. foremani was similar to riverscape genetic patterns seen in previously studied pleurocerids ( Whelan et al 2019 ; Wright et al 2020 ). This was surprising because the 2 previous population genomic studies on pleurocerids focused on species from one of the least modified major rivers in the southeastern United States.…”

“…For example, L. ampla populations in tributaries of the Cahaba River in central Alabama displayed lower levels of genetic diversity, on average, than mainstem populations ( Whelan et al 2019 ). Furthermore, Wright et al (2020) found that Leptoxis compacta , a species restricted to the mainstem Cahaba River, had relatively high levels of genetic diversity when compared with tributary populations of L. ampla , and our results show similarity with spatial genetic diversity in P. foremani . Thus, low genetic diversity of P. foremani in Yellowleaf Creek compared with Lake Mitchell and the Coosa River at Wetumpka is likely the result of natural processes.…”

“…Furthermore, determining genetic responses to anthropogenic activities is central to conservation genetics ( Khan et al 2016 ; Allendorf 2017 ). Despite well-documented declines of freshwater fauna in the southeastern United States ( Williams et al 1993 ; Lydeard and Mayden 1995 ; Neves et al 1997 ; Johnson et al 2013 ), little or no data exist about riverscape genetics of most species (but see Fluker et al 2014 ; Whelan et al 2019 ; Barnett et al 2020 ; Wright et al 2020 ). This hinders broad understanding of how genetic patterns in a global biodiversity hotspot are influenced by human activity.…”

“…Endangered Species Act and 33 extinct species ( Johnson et al 2013 ). Despite the high imperilment rate of pleurocerids, only 2 pleurocerid species have been the focus of population genomic studies ( Whelan et al 2019 ; Wright et al 2020 ), and both studies focused on species in the Cahaba River, one of the few major rivers in the southeastern United States that has not been impounded ( Ward et al 2005 ). Whelan et al (2019) showed that riverscape genomic patterns in the pleurocerid Leptoxis ampla did not follow patterns predicted by the Mighty Headwaters Hypothesis, but more studies are needed to determine whether patterns seen in L. ampla are universal to pleurocerids.…”

“… Whelan et al (2019) showed that riverscape genomic patterns in the pleurocerid Leptoxis ampla did not follow patterns predicted by the Mighty Headwaters Hypothesis, but more studies are needed to determine whether patterns seen in L. ampla are universal to pleurocerids. Furthermore, Wright et al (2020) focused on a range-restricted pleurocerid, L. compacta , only found at mainstem sites in the Cahaba River. Additionally, we have no information on how pleurocerid genomics are influenced by impoundments.…”