Morpho-molecular studies suggest a new species from a transboundary river of Turkiye: Gammarus arasensis sp. nov. (Amphipoda: Gammaridae)

A new amphipod species is described from the Aras River, a significant water body in the Caucasus Region. The description is based on comprehensive genetic and morphological analyses. Newly described species has the following character combination: antenna 1 bears 17–20 flagellar segments; the antennal gland cone reaches the distal tip of the third peduncular segment of antenna 2; pereopod 4 has short posterior setae; coxal plates 1–4 have a few long setae on their inner surfaces, and uropod 3 has an endopod/exopod ratio of approximately 0.64.

Gammarus arasensis sp. nov. is morphologically similar to Gammarus kischineffensis. Molecular analyses (COI and 28S) revealed genetic distances of 16.23% and 1.44% from G. kischineffensis, respectively, which exceed the widely accepted species-level thresholds. Phylogenetic analyses (ML, BI) supported the distinct lineage status of the new species, although ML support for relatedness to G. kischineffensis was low.

Species delimitation methods (ASAP, PTP) confirmed that Gammarus arasensis sp. nov. is a distinct MOTU. Detailed illustrations and descriptions of the holotype (male) and allotype (female) are provided. The species’ diagnostic characteristics are discussed, highlighting its distinctions from closely related congeners.