Systematics and geographical distribution of Galba species, a group of cryptic and worldwide freshwater snails

Cryptic species are a major problem in systematics and biogeography, especially if they are invasive or transmit parasites or pathogens. Detecting cryptic species requires a pluralistic approach in which molecular markersallow to detect coherent taxonomical units that can then be analyzed using various markers (e.g., internal morphology) and crosses. In asexual or selfing species, the latter criteria is of limited use. We studied a group of cryptic freshwater snailspecies from the family Lymnaeidaethat invade almost all continents, reproduce mainly by selfing and transmit liver flukesto humans and livestock. We aim to clarify the systematics, distribution and phylogenetic relationships of Galbaspecies based on a sound approach that included morphology, molecular markers, wide-scale sampling (the whole America) and data retrievedfrom GenBank (therefore including Old Worldsamples). We suggest that the genus Galbacomprises five species or species complex. Galba cousiniis the only species that can be identified using shell morphology and internal anatomy (a derived trait) and has the most restricted distribution within the group. The other four species--G. truncatula, G. cubensis/viator, G. humilisand G. schirazensis--are all cryptic and constitute clades with wide variation in their genetic diversity, geographic distribution and invasiveness. Further genetic studies are required to clarify the status of the G. cubensis/viator (a single or two species). We highlight that no Galbaspecies should be identified without molecular markersand that more sampling is required, especially in North America, Eurasia and Africa to clarify their systematics. We also discuss various hypotheses that can explain crypticity in Galba, such as morphological stasis or stabilizing selection.