A holistic perspective on species delimitation outperforms all methods based on single data types in freshwater gastropods (Caenogastropoda: Hydrobiidae: Pseudamnicola)

Freshwater gastropods are one of the most species-rich and severely threatened animal groups in continental aquatic ecosystems. Unfortunately, understanding their species diversity, which is key to conservation, is often hampered by their small size, simple morphology, and restricted distribution. This is particularly the case for spring snails of the genus Pseudamnicola (family Hydrobiidae), a species-rich taxon across the Mediterranean region. Contrasting species diversities derived from molecular phylogenies and traditional taxonomy call for a holistic approach, in which molecular markers are used to identify putative taxonomic units that can then be validated morphologically. We integrated DNA sequence and morphometric data from 92 populations into a species delimitation framework to assess the taxonomic status of 25 nominal and 14 informally recognized species of Pseudamnicola across its geographical range. Although seven discovery methods delimited a varying number of putative species, taking an integrative approach generally improved species support over single datasets. The statistically favoured integrative scheme corroborated 30 of the 39 initial species as separate entities, whereas nine require taxonomic re-evaluation. Six species are described here formally. Pseudamnicola exemplifies the importance of considering multiple lines of evidence to evaluate species diversity in taxonomically complex groups, with newly developed approaches providing the needed toolbox.