A gleam in the dark: phylogenetic species delimitation in the confusing spring-snail genus Bythinella Moquin-Tandon, 1856 (Gastropoda: Rissooidea: Amnicolidae).

We re-assess the value of morphological specific descriptors within the spring-snail genus Bythinella by sequencing mitochondrial COI and nuclear ITS1 gene fragments. Taxonomic coverage represents 16 nominal species sampled among 35 populations from France. Application of monophyly and cohesive haplotype networks as criteria to delineate species allow us to identify 10 mitochondrial species-level lineages, all but one of which are recovered by ITS1. COI species thresholds that are estimated from newly delimited species (ca. 1.5%) agree with values found among other hydrobioids. Our results strongly suggest that classical morphological descriptors may not constitute valid specific criteria within Bythinella. Our analyses support a complex scenario of invasions of subterranean habitats, as illustrated by the syntopy of several mitochondrial lineages or the conflicting evolutionary histories between COI and ITS1 in caves. In addition, morphological convergence related to subterranean ecological constraints that affect shell shape and size among the hypogean springsnails studied is suspected.