Theodoxus transversalis (C. Pfeiffer, 1828) (Gastropoda: Neritidae) in the Western Balkans: an endangered freshwater snail bouncing back?

Abstract

Populations of Theodoxus transversalis, striped nerite, have declined in recent decades, arguably due to negative anthropogenic impacts on their habitats. The species is classified as endangered by the International Union for Conservation of Nature, and there are generally few reports of its recent occurrence. This is especially true for the Western Balkans and Serbia, where data on the distribution of T. transversalis are more than a decade old. Herein, we present the latest data based on an extensive field study of this species conducted in 2023 and 2024. The Velika Morava-Južna Morava-Nišava catchment and ecoregion (ER) 5 were confirmed as the most important refugium for this rare snail in the investigated region. Of particular interest are the findings of an abundant population from the Drina River (where the species has not been reported for 15 years), from the lower Serbian section of the Danube (reports from the Danube are 30 years old), and the discovery of a new population from the Zapadna Morava River 100 km upstream from the nearest known population. Our results show considerable progress in the restoration of the former range of T. transversalis in the Western Balkans, although stable populations still appear to be localized. There is also a possibility that this species is spreading into new areas, which is a possibility for the Zapadna Morava River catchment. While it appears that T. transversalis is more resilient to negative anthropogenic impacts than previously described in the available literature, a more detailed ecological and genetic study should shed more light on the presence and prospects of the species in the region.