The freshwater snail Galba cubensis (Pfeiffer, 1839) has a large distribution in the Americas. Despite being an intermediate host of Fasciola hepatica—the trematode causing fasciolosis in livestock and humans—its population genetics have never been studied. We isolated and characterized 15 microsatellite loci in G. cubensis to evaluate its genetic diversity, population-genetic structure and mating system. We tested the microsatellite loci in 359 individuals from 13 populations of G. cubensis from Cuba, Guadeloupe, Martinique, Puerto Rico, Venezuela, Colombia and Ecuador. We also tested cross-amplification in three closely related species: G. truncatula, G. viator and G. neotropica. We found that G. cubensis has a similar population structure to other selfing lymnaeids that live in temporary habitats: low genetic diversity, large departure from Hardy-Weinberg equilibrium, marked population structure and high selfing rate. We found that seven and six loci amplified in G. truncatula and G. viator, respectively, and that all 15 loci amplified in G. neotropica. This last finding suggests a close relatedness between G. cubensis and G. neotropica, probably being conspecific and synonymous. This new set of microsatellite markers will be a useful tool to study the genetic diversity of this snail species across a large geographical range and, consequently, to understand the emergence and re-emergence of fasciolosis in the Americas.

We examined the extent to which supply‐side, niche, and competition theories and concepts help explain a trematode community in which one species comprises 87% of the trematode individuals, and the remaining 15 species each have <3%. We collected and dissected the common and wide‐ranging snail host Heleobia australis over four seasons from three distinct habitats from the intertidal area of the Bahía Blanca estuary, Argentina. Inside a snail, trematodes interact with each other with outcomes that depend on facilitation, competition, and preemption, suggesting that dominant species should be common. The abundant trematode species, Microphallus simillimus, is a weak competitor, but has life‐history traits and strategies associated with higher colonization ability that could increase its probability of invading the host first, allowing it to preempt the rare species. Rather than segregate by habitat, trematode species aggregated in pans during the summer where dominant trematode species often excluded subordinate ones. Despite losses to competition, and a lack of niche partitioning, M. simillimus ruled this species‐rich trematode guild through strong recruitment and (potentially) preemption. Therefore, extremely skewed species abundance distributions, like this one, can derive from extremely skewed colonization abilities.

Cryptic species can present a significant challenge to the application of systematic and biogeographic principles, especially if they are invasive or transmit parasites or pathogens. Detecting cryptic species requires a pluralistic approach in which molecular markers facilitate the detection of coherent taxonomic units that can then be analyzed using various traits (e.g., internal morphology) and crosses. In asexual or self-fertilizing species, the latter criteria are of limited use. We studied a group of cryptic freshwater snails (genus Galba ) from the family Lymnaeidae that have invaded almost all continents, reproducing mainly by self-fertilization and transmitting liver flukes to humans and livestock. We aim to clarify the systematics, distribution and phylogenetic relationships of these species with an integrative approach that includes morphology (shell and reproductive anatomy), molecular markers, wide-scale sampling across America, and data retrieved from GenBank (to include Old World samples). Our phylogenetic analysis suggests that the genus Galba originated ca. 22 Myr ago and today comprises six clusters of species. Four of these clusters ( G . truncatula , G . cubensis / viator , G . humilis and G . schirazensis ) are morphologically cryptic and constitute species or species complexes with wide variation in their genetic diversity, geographic distribution and invasiveness. The other two clusters constitute a single species or a species complex ( Galba cousini / meridensis ) that demonstrate more geographically restricted distributions and exhibit an alternative morphology more phylogenetically derived than the cryptic one. Further genetic studies are required to clarify the status of both G. cousini / meridensis and G . cubensis/viator . We emphasize that no Galba species should be identified without molecular markers and that additional sampling is required, especially in North America, Eurasia and Africa to clarify remaining questions in systematics and biogeography. We also discuss several hypotheses that can explain crypsis in Galba , such as convergence and morphological stasis, and hypothesize a role for stabilizing selection in amphibious (rather than strictly freshwater) habitats.

Misaligned sequences derail evolutionary inference. Datasets from GenBank require verification of positional homology and orientation before alignment and phylogenetic analysis. Liver-fluke case studies reveal how overlooked errors skew results, underscoring the need for rigorous checks in parasitology and all molecular research.