A phylogeographic assessment redefines the distributions of Truncilla species (Bivalvia: Unionidae) in Texas

ABSTRACT Truncilla macrodon¸ the Texas Fawnsfoot, is currently considered to be restricted to the Brazos and Colorado river drainages in central Texas, USA. Subfossil specimens resembling T. macrodon from shell middens in the Trinity river drainage have raised questions about its presumptive range. Extant Truncilla populations occupying the Trinity river drainage are believed to be Truncilla donaciformis, the Fawnsfoot, but the species identity has not been assessed using molecular techniques. In this study, we set out to use phylogeographic and morphometric approaches to resolve the distribution of Truncilla spp. in Texas, with an emphasis on assessing species identity of Truncilla in the Trinity river drainage. Our molecular analyses indicated individuals from the Trinity river drainage are T. macrodon rather than T. donaciformis, and morphometric analyses using ratios of shell length, width and height supports this finding and showed differentiation among T. macrodon, Truncilla cognata and T. donaciformis. These combined findings suggest the range of T. macrodon is larger than previously recognized and includes the Colorado, Brazos and Trinity river drainages. The status of Truncilla spp. in the San Jacinto river basin remains unclear due to lack of information on historical or extant populations, but we recommend conservationists consider T. macrodon as part of the mussel fauna since the drainage shares an embayment with the Trinity river drainage. Molecular data also indicated the Trinity population of T. macrodon is distinct from, and more genetically diverse than, the Brazos and Colorado populations. This difference suggests the Trinity River population should be treated as a separate management unit from the Brazos and Colorado River populations to facilitate effective conservation and recovery planning. Our study adds to the growing body of literature highlighting the utility of molecular data in conservation and management of freshwater mussels.