Taxonomic diversity and distribution of the genus Proterorhinus (Teleostei: Gobiidae) in the Caucasus biodiversity hotspot with conservation implications

The magnitude of biodiversity in the Caucasus biodiversity hotspot remains largely unknown, because species diversity is usually assessed based on morphology. One ecologically important fish group with cryptic diversity is the tubenose goby members of the genus Proterorhinus, for which recent studies have shown that species assignments are difficult. Using mitochondrial COI data (652 bp) from 59 widespread localities, a phylogenetic and phylogeographical framework is applied to assess the species-level status of different Caucasian populations of Proterorhinus, previously classified as Proterorhinus nasalis. We infer how post-Miocene evolution of the Caspian basin may have shaped the present diversity and distribution of P. nasalis. The results suggest that Proterorhinus diversity in the Caucasus has been underestimated: (i) specimens from Azerbaijan and the Samur River delta on the Caspian shore share haplotypes with Proterorhinus semipellucidus; (ii) specimens from the Black Sea shore at Kodori Bay, the Kintrishi, and Mcnishta rivers of Georgia share haplotypes with two lineages of Proterorhinus semilunaris; and (iii) south Caspian specimens from the Iranian sites studied belong to P. nasalis. A pattern of low haplotype and nucleotide diversity was observed in P. nasalis, suggesting a founder event by a single or a few mitochondrial haplotypes following the isolation between the Black and Caspian seas in the early Pliocene or a demographic bottleneck during salinity and lake-level changes in the Babadjanian, Balakhanian and Tyurkyanian stages. Integrative use of genetic assessments for monitoring Caucasian biodiversity will help to understand the extent of biodiversity and reliably assess threat status for critical species. Considering the species' narrow distribution range, small population size and low genetic variability, P. nasalis may merit an upgrade from Least Concern to an IUCN Red List Threatened Category (i.e. Vulnerable, Endangered or Critically Endangered).