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In this study, we provide a first estimation of the molecular diversity of the freshwater fishes of Georgia. In addition to field collections, we integrated DNA barcode data obtained from recent works and public databases (BOLD and NCBI GenBank). Currently, the DNA barcode reference library for freshwater fishes of Georgia comprises 352 DNA barcodes for 50 species, 36 genera and 15 families (52% of total Georgian freshwater fish diversity), from which 162 DNA barcodes belonging to 41 species were newly generated as part of this study. A total of 22 species are reported from the Caspian Sea basin and 31 from the Black Sea basin. Amongst the studied taxa, seven species were found with large interspecific divergences (> 2%) while 11 species were found to share DNA barcodes within our dataset. In the course of the study, we found the first evidence of the existence of Gymnocephalus cernua (Linnaeus, 1758) and also confirm the second occurrence of invasive Rhinogobius lindbergi (Berg, 1933) in Georgia. Based on the evaluation of currently-available barcode data for Georgian fishes, we highlighted major gaps and research needs to further progress DNA-based biodiversity studies in Georgia. Though this study lays a solid base for DNA, based biodiversity assessment and monitoring approaches, further efforts within the recently started CaBOL (Caucasus Barcode Of Life) project are needed to obtain reference data for the species still lacking DNA barcodes.

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Oxynoemacheilus phasicus, new species, is described from the Rioni and Enguri River drainages in Georgia. It is distinguished from other Oxynoemacheilus species in the O. brandtii group by a mottled or marbled flank pattern in adults, a slightly emarginate caudal fin, and a deep caudal peduncle. Molecular data suggest that the new species is characterized by a minimum K2P distance of 7.5% from O. brandtii from the Kura drainage in the mtDNA COI barcode region. Oxynoemacheilus brandtii and O. elsae are re-diagnosed. A very slender Oxynoemacheilus from the Aras drainage clusters as sister to O. elsae in our molecular analysis and not with O. brandtii from the Kura River. However, it is identified as O. brandtii as it is indistinguishable from this species in morphological characters.

Nile tilapia, Oreochromis niloticus (Linnaeus, 1758) is a fish from the family Cichlidae, native to Sub-Saharan Africa. Nile tilapia is one of the most cultured freshwater fish species worldwide and because of its commercial value and well-developed aquaculture technologies, it has been introduced to many countries. Nile tilapia also has become invasive in areas of suitable climate when escaped from aquaculture facilities to the wild. Georgia is the country in the Ponto-Caspian region situated on the southern slopes of the Great Caucasus eastwards from the Black Sea coast. Due to its very variable landscape, the climate of Georgia fluctuates from cold mountainous to humid subtropical type. Here we present the finding of Oreochromis niloticus in freshwaters of eastern Georgia for the first time, with the discussion of risk assessment with the climate suitability for potential establishment.