Stygobite phylogenetics as a tool for determining aquifer evolution

Abstract: The use of aquifer-dwelling organisms (stygobites) for learning about past and present subterranean hydrologic connections was evaluated in the Edwards (Balcones Fault Zone), Trinity, and EdwardsTrinity (Plateau) aquifers of Texas and adjacent areas in north Mexico, an area with complex karst groundwater flow and sociopolitical problems stemming from overuse and contamination. A priori predictions of subterranean hydrogeologic history were made based on a literature review, and these predictions were compared to phylogenies of two aquifer dwelling isopods created based on mitochondrial gene sequences (16S ribosomal RNA and cytochrome c oxidase subunit I). Using likelihood and parsimony-based comparisons, Cirolanides (Isopoda: Cirolanidae) was found to have a phylogenetic history congruent with a priori predictions of subterranean hydrogeologic history in its terminal nodes. Conversely, basal branches of the phylogenetic tree had placement that was not predicted by this history, a phenomenon that may be indicative of a lack of understanding of subterranean hydrogeology of the area. Lirceolus (Isopoda: Asellidae) had a phylogenetic history congruent with an alternative hypothesis of water flow, namely the patterns of surface drainages. This difference of patterns for two species that both live in the aquifer is probably related to their ecology and evolutionary history, with Cirolanides having invaded the cave habitat as a single marine population and Lirceolus invading the cave habitat as a freshwater migrant with possible pre-existing genetic structure determined by surface drainages. This study pioneers testing of a priori biogeographic hypotheses using phylogenies of aquifer organisms and the creation of hydrogeologic histories in a karst setting, and supports the use of these methods to aid in understanding biogeography and aquifer evolution.