Theodoxus fluviatilis’ re-establishment in the River Rhine: a native relict or a cryptic invader?

The endangered freshwater snail Theodoxus fluviatilis is a widely distributed European member of the gastropod family Neritidae. This taxon was abundantly found in the River Rhine until the end of the 20th century, and was considered to be extinct there since the late 1990s. Since 2006, a new, but morphological different form of T. fluviatilis has been recorded in the Upper Rhine region. Our aim was to identify the source of the recent populations by analysing individuals from five sites throughout the current known distribution along the River Rhine. Therefore, we sequenced the mitochondrial cytochrome c oxidase subunit I (COI) gene, and compared the data with those from individuals collected in the early 1990s and 40 already known haplotypes from a pan-European study. Our results show that all studied recent Rhine individuals harbour only one COI haplotype that corresponds to an already known haplotype described from the River Danube and the Ukraine region near the Black Sea. This suggests that a re-colonisation of the River Rhine by T. fluviatilis from the River Danube is the most likely scenario of the re-establishment of the species. This route of invasion is known for other freshwater taxa that originate from the Ponto-Caspian region. Even though the current Rhine populations belong to T. fluviatilis their invasion may have consequences for the native Central European populations. Therefore, we recommend considering the current Rhine population as ‘cryptic invader’.