Recurrent founder-event speciation across the Mediterranean likely shaped the species diversity and geographic distribution of the freshwater snail genus Mercuria Boeters, 1971 (Caenogastropoda: Hydrobiidae)

Abstract

Dispersal is known to play an important role in shaping the diversity and geographic range of freshwater gastropods. Here, we used phylogenetic methods to test for the influence of dispersal and other biogeographic processes (such as vicariance) on the speciation and distribution patterns of Mercuria Boeters, 1971, a snail genus widely distributed in the western Palaearctic. The 25 extant species traditionally thought to comprise the genus, which were described mainly on the basis of morphology, have been recorded from lowland waters in both the Mediterranean and Atlantic river basins of Europe and North Africa. Using molecular phylogenies based on three gene fragments (COI, 16S rRNA and 28S rRNA) from 209 individuals, four molecular species delimitation methods and a shell characterization, we identified 14 putative species in our dataset, nine of which correspond to species classified by traditional taxonomy. Furthermore, biogeographical modelling favoured a scenario in which recurrent founder-event speciation since the late Miocene is the most probable process explaining the species diversity and distribution of the Mediterranean clades, whereas episodes of postglacial northward colonization from Iberian refugia by the species M.tachoensis may explain the current presence of the genus in Atlantic lowlands. The dispersal events inferred for Mercuria, probably promoted by multiple factors such as the changing connectivity of drainage basins driven by climate change or better access for avian dispersal vectors in lowlands, may explain the rare case among hydrobiids of a species-rich genus containing individual species with a large distribution area.