Two new species of Stygobromus Cope, 1872 (Amphipoda, Crangonyctidae) amphipods from hypotelminorheic seepage springs in the Washington, D. C., USA area

Shallow groundwater habitats, including hypotelminorheic seeps, support highly specialized and often narrowly endemic fauna, yet biodiversity in these systems remains poorly documented, particularly in urban landscapes. Here, we describe two new stygobiotic amphipod species, Stygobromus scottsruni sp. nov. and Stygobromus paveki sp. nov., from hypotelminorheic habitats in the Washington, D. C. metropolitan region, and report new locality records for S. anacostensis.

Morphological and molecular analyses place both new species within the Stygobromus tenuis species group. Phylogenetic analyses based on mitochondrial (co 1 + 16 S) and nuclear (28 S) loci, together with multiple species delimitation approaches, consistently support the recognition of both taxa as distinct evolutionary lineages. Despite occurring in close geographic proximity to other congeners, both species exhibit strong genetic differentiation and extremely restricted distributions, consistent with fine-scale isolation among shallow groundwater systems.

With the addition of these taxa, described stygobiotic amphipod species richness from the Piedmont and Atlantic Coastal Plain of Maryland, Virginia, and the District of Columbia now totals 16 species, 10 of which are known from in and around the Washington, D. C. metropolitan area, based on currently described taxa. These results highlight hypotelminorheic habitats as important centers of micro-endemism and demonstrate that urban groundwater systems can harbor unexpectedly high levels of cryptic subterranean biodiversity, underscoring the need for targeted surveys and conservation attention to shallow subterranean ecosystems.