Species Diversity, Diagnostic Characters, and Distribution of Tubenose Gobies of the Genus Proterorhinus (Gobiidae): What are P. semipellucidus and P. nasalis?

As a result of comparative morphological and molecular studies of tubenose gobies from Caspian populations, the species Proterorhinus semipellucidus, previously described by Kessler in 1877, was revalidated and considered as a valid species. This species differs from other species of Proterorhinus distributed in the Caspian Sea basin by a significantly smaller number of branched rays of the dorsal and anal fins, the number of mid-lateral scales, and some body proportions. The range of P. semipellucidus is rather narrower compared to the previously accepted opinion, which assumed its distribution in the basins of the Caspian and Azov Seas and invasive distribution in the Volga River basin and the north-eastern part of the Baltic Sea. According to our data, this species is distributed along the eastern coast of the Caspian Sea and is more common in its northeastern part. However, an individual of P. semipellucidus with mtDNA from P. nasalis was detected in Northern Caspian Sea, the area of secondary contact with P. nasalis, that suggests introgressive hybridization. Contrary to P. semipellucidus, the morphologically closest P. nasalis is distributed in western and partially northern Caspian Sea (including Volga system) and Azov basin (including the Kuban and Don systems) and is composed of two sub-lineages according to COI mtDNA data—(i) Southern and (ii) Northern. An identification key for the Proterorhinus species distributed in the Caspian basin is provided.