Defining and recording the loss of species diversity is a daunting task, especially if identities of species under threat are not fully resolved. An example is the Pontocaspian biota. The mostly endemic invertebrate faunas that evolved in the Black Sea – Caspian Sea – Aral Sea region and live under variable salinity conditions are undergoing strong change, yet within several groups species boundaries are not well established. Collection efforts in the past decade have failed to produce living material of various species groups whose taxonomic status is unclear. This lack of data precludes an integrated taxonomic assessment to clarify species identities and estimate species richness of Pontocaspian biota combining morphological, ecological, genetic, and distribution data. In this paper, we present an expert-working list of Pontocaspian and invasive mollusc species associated to Pontocaspian habitats. This list is based on published and unpublished data on morphology, ecology, anatomy, and molecular biology. It allows us to (1) document Pontocaspian mollusc species, (2) make species richness estimates, and (3) identify and discuss taxonomic uncertainties. The endemic Pontocaspian mollusc species richness is estimated between 55 and 99 species, but there are several groups that may harbour cryptic species. Even though the conservation status of most of the species is not assessed or data deficient, our observations point to deterioration for many of the Pontocaspian species.

The Caspiinae comprise small gastropod species inhabiting low-salinity environments of the Pontocaspian region, specifically the Caspian Sea and the limans, estuaries and lower river reaches of the northern Black Sea Basin. There is considerable discrepancy in the literature as to which taxa are attributed to this hydrobiid subfamily, which of them are valid and what rank they have. Moreover, the generic classification is not agreed upon. Here, we aim at elucidating systematic relationships and species identities by a thorough taxonomic and systematic revision of the Caspiinae, including a detailed morphological study along with information on biogeography, ecology, conservation and fossil record. We use historical and recent collections from all around the Pontocaspian region acquired over the past 170 years, including several type series that were only recently discovered as well as selected Pleistocene and Holocene materials. Several of the species are illustrated properly here for the first time, using both macrophotographs and scanning electron microscopy images to facilitate evaluation of morphological details. The genus Andrusovia, which has been classified in a number of different families and even gastropod subclasses before, is here attributed to the Caspiinae. By designating a lectotype for the type species, A. dybowskii, we finally fix the identity of this previously ambiguous species and genus. Our revision yields four genera and eleven accepted species of Caspiinae for the modern Pontocaspian fauna: Andrusovia (1), Caspia (1), Clathrocaspia (7) and Ulskia (2). In addition, two species of uncertain status (1 nomen dubium, 1 taxon inquirendum) are discussed, and two species introduced as Caspiinae are here found not to belong in that group. Andrusovia antecessor sp. nov. and Andrusovia cyrensis sp. nov. are introduced as new (sub)fossil species from the Caspian Sea.

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An illustrated overview of the type series of nominal taxa of the gastropod genus Theodoxus (Neritidae) inhabiting the Ponto-Caspian region is presented. It is made on the basis of a revision of large European malacological collections and includes descriptions of the type specimens of 16 taxa of different rank (species, variety, morph). Apart of these, the “type” specimens of four taxa with unavailable names have been traced in ZIN collection; we illustrate them to show the variability of Ponto-Caspian Theodoxus. In most cases, the photographs of the type specimens are published for the first time. We provide a taxonomic analysis of all studied type series, with comments on the nomenclature, systematic position and taxonomic rank of all nominal taxa of Theodoxus recorded from the Ponto-Caspian area.