Diversity, taxonomy and phylogenetic relationships of the “Niphargus borutzkyi” ingroup (Crustacea: Amphipoda: Niphargidae) in Western Georgia, SW Caucasus

Two new Caucasian species of the genus Niphargus Schiödte, 1849 (Crustacea: Amphipoda: Niphargidae), N. rachalechkhumensis sp. nov. and N. tvishiensis sp. nov., are described from the stygobiotic habitats of the Racha-Lechkhumi and Kvemo Svaneti karst systems in Western Georgia. Both newly described species belong to the southwestern Caucasian “Niphargus borutzkyi” ingroup related to the European “carphaticus” species complex and can be clearly separated from the congeners by morphological features, mostly in uropod III and epimeral plates, and genetically. Identification key for all known species to the “Niphargus borutzkyi” ingroup is provided, as well as their phylogenetic relationships, the estimated time of the origin and the current distribution of the ingroup in the Colchis lowland of the southwestern Caucasus are also discussed.