Phylogenetic relationships and description of a new stygobite species of Bythinelia (Mollusca, Gastropoda, Caenogastropoda, Amnicolidae) from southern France

A population of the aquatic gastropod Bythinella that does not conform to any described hypogean or epigean species of this genus was discovered in a cave stream in southern France (Folatiere cave). Members of this population lacked body pigments and eyespots, which suggests adaptation to a Subterranean environment. Mitochondrial COI sequences revealed two lineages within this population, separated by a genetic divergence higher than the inter-specific threshold commonly admitted for hydrobioid species. In contrast, a phylogeny based on the nuclear ITS1 gene clustered the sequenced hypogean specimens in a single clade. Moreover Multivariate analyses performed on shell parameters did not enable discrimination between hypogean specimens attributed to each of these two mitochondrial lineages. The high COI variability within the same population and conflict between mtDNA and nDNA trees could be explained by a scenario of introgression by hybridization. Consequently, based on nuclear gene phylogeny, shell and anatomical characters, and ecological evidences, we hypothesize that the easily recognizable hypogean morph of the Folatiere cave call be attributed to a distinct and new species - here described and named B. navacellensis n. sp. Our extensive sampling suggests that B. navacellensis n. sp. is distributed within a small delimited and cohesive area located around southern Larzac, covering c. 300 km(2). Considering its distribution, restricted to a few subterranean aquifers only, B. navacellensis n. sp. call be categorised as a Vulnerable species, VU D2.