Heritable shell differentiation among populations of the sole lymnaeid snail across freshwater habitats of southern Patagonia

Environmental heterogeneity across freshwater systems often promotes phenotypic variation among populations. Yet, the respective contributions of environmentally-induced and heritable variation to population differences are rarely known. We investigated the geographic distribution and morphological differentiation, and heritability of shell traits among populations of the freshwater lymnaeid snail Pectinidens diaphanus in Patagonia. Extensive field surveys across 193 freshwater sites revealed that P. diaphanus is the sole lymnaeid inhabiting southern Patagonia and occupies a broader range of lentic and lotic habitats than previously documented. While reproductive anatomical structures were conserved across populations, shell shape differed markedly among populations from contrasting habitat types, with population origin explaining nearly 50% of total shape variation. Snails from hydrologically unstable habitats (ponds and streams) exhibited more elongated shells and relatively smaller apertures than lake snails, a pattern consistent with functional responses to hydroperiod variability and desiccation risk. To further investigate whether this differentiation was heritable, we conducted a common-garden experiment across two generations. Shell shape differences between permanent- (lake) and temporary- (pond) habitat-derived populations persisted into the G2 generation reared under standardized laboratory conditions, indicating that the observed variation is not solely a response to local environmental conditions but includes a heritable component. Together, our findings demonstrate that P. diaphanus constitutes the sole lymnaeid across southern Patagonia, occupying a broader range than previously documented, and that populations show heritable shell differentiation potentially associated with contrasting freshwater habitats. By integrating large-scale biogeographic surveys with morphometric and experimental approaches, this study provides new insight into how habitat variation may contribute to ecological and evolutionary differentiation in freshwater gastropods.