The invasive European green crab (Carcinus maenas) poses a significant ecological threat due to its rapid global spread and disruptive impact on coastal ecosystems. In the southwestern Atlantic, the northernmost population was recently recorded in the San Matías Gulf, offering a unique opportunity to study its demographic and life-history traits at the leading edge of its range. We assessed the sex ratio, population density, size distribution, and size at which 50% of females were ovigerous (SM₅₀) in the intertidal zone. Our findings revealed a male-biased sex ratio (1.50 males per female) and low population density (0.42 crabs m⁻²), suggesting early-stage establishment. A size-structure analysis showed a well-structured population with clear cohort separation in both sexes and evidence of two recruitment events. The carapace width at which 50% of females were ovigerous was estimated at 61.48 mm. Although egg-bearing females exhibited a well-marked seasonality, with the highest prevalence from May to July, their presence nearly year-round suggests an extended reproductive period and adaptability to local conditions, which may be advantageous in recently established populations, facilitating invasion success. These traits, combined with the species’ ongoing range expansion, highlight the invasive potential of C. maenas and its possible ecological impacts on the San Matías Gulf.

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 G ₂ 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.

The invasive European green crab ( Carcinus maenas ) poses a significant ecological threat due to its rapid global spread and disruptive impact on coastal ecosystems. In the Southwestern Atlantic, the northernmost population was recently recorded in the San Matías Gulf, offering a unique opportunity to study its demographic and life-history traits at the leading edge of its range. We assessed sex ratio, population density, size distribution, and the size at which 50% of females were ovigerous (SM ₅₀ ). Our findings revealed a male-biased sex ratio (1.79 males per female) and low population density (0.42 crabs m ^-2 ), suggesting early-stage establishment. Size-structure analysis showed a well-structured population with clear cohort separation in both sexes and evidence of two recruitment events. The size at which 50% of females were ovigerous was estimated at 61.48 mm. Although egg-bearing females exhibited a well marked seasonality, with highest prevalence from May to July, their presence nearly year-round suggests an extended reproductive period and adaptability to local conditions, which may be advantageous in recently established populations, facilitating invasion success. These traits, combined with the species’ ongoing range expansion, highlight the invasive potential of C. maenas and its possible ecological impacts on the San Matías Gulf. Simple Summary: Carcinus maenas is an invasive crab species that threaten coastal ecosystems around the world by competing with native species and damaging habitats. Recently, a population of this species was discovered in the San Matías Gulf in Argentina, marking its northernmost record in the Southwestern Atlantic. We studied its demographic and life-history traits and found a male-biased sex ratio and relatively low population density. Additionally, egg-bearing females were present nearly year-round, with an increase in their abundance from May to July, suggesting that this population is adapting well to local conditions and maintaining an extended reproductive period. Together, these findings point to an early stage of invasion with the potential for further expansion, posing risks to native species and north Patagonian ecosystems.