First insight into cryptic diversity of a Caucasian subterranean amphipod of the genus Niphargus (Crustacea: Amphipoda: Niphargidae)

Abstract The Caucasus region harbors a rich subterranean amphipod fauna. However, it was rarely studied using molecular tools. Recent field explorations unveiled a number of populations that morphologically correspond to the little known amphipod Niphargus alasonius described from Georgia. The populations are distributed along the Kura River drainage, spanning a range that exceeds 300 km. This unusually broad distribution implies that N. alasonius is a good disperser, exploiting the riverine interstitial as a distributional corridor, or alternatively, that it is a complex of cryptic species. We tested these two hypotheses using a molecular genetic approach. Multilocus phylogenetic analysis suggested that N. alasonius is a monophyletic taxon, closely related to Crimean taxa. Species delimitations and molecular dating imply that N. alasonius comprises at least five closely related cryptic species that diverged during the Late Miocene/Early Pliocene. Since these putative species are distributed along the entire drainage system, we hypothesize that the Kura River played a crucial role in shaping biogeographic patterns of this species. To promote further research of subterranean amphipods in the region, we provide a checklist of Caucasian Niphargus, and define shared morphological traits of the N. alasonius species complex.