On the taxonomic relationship between Theodoxus pallasi andT. astrachanicus (Gastropoda: Neritidae) from the Ponto-Caspianregion

Observations on the morphological variability of Theodoxus pallasi Lindholm, 1924 and T. astrachanicus Starobogatov in Starobogatov, Filchakov, Antonova & Pirogov, 1994 from the Caspian Sea and the Sea of Azov populations reveal their close similarity. Statistical analyses of the morphometrics of shells, opercula, and radulae show very similar ranges of their measured parameters in all populations involved. No clear morphological distinction between individuals of these 2 nominal species was discovered. Data presented here support the hypothesis that these taxa are conspecific and should be considered as local morphotypes of a single species. Based on the principal of priority, the species should be named T. pallasi.