Gateways to the underworld: molecular phylogenetic analyses identify patterns of groundwater fauna and two new species of hypogean Hyalella Smith, 1874 (Amphipoda, Hyalellidae) from the northern Mojave Desert

In North America, the genus Hyalella (Crustacea: Amphipoda) is primarily epigean, with only two described species found in groundwater. In contrast, Stygobromus is an amphipod genus occupying subterranean habitats throughout most of the Nearctic. Using both molecular and morphological traits, we describe two new species of stygobitic amphipods, Hyalella plutonia sp. nov. and Hyalella keepuikantun sp. nov. from groundwater habitats in Nye County, Nevada, USA. These species form a monophyletic group that also contains the similar H. cretae from Ash Meadows National Wildlife Refuge. Members of this “ H. cretae ” complex are similar in terms of overall morphology but can be distinguished through examination of taxonomically relevant characters within the genus such as the third uropod, gnathopods, and pereopod 7. The description of these species highlights the unexpected diversity of stygobitic Hyalella within the northern Mojave Desert, where four such species comprise two independent lineages. Stygobromus is absent from these locations. This diversity of Hyalella is hypothesized to be due in part to the evolutionary history the genus and the ecological/climatic conditions of the region itself, which may exclude other more wide-ranging Nearctic stygobites like Stygobromus .