Distribution-based assessment reveals structural shifts in a protected estuarine gastropod, Clithon retropictus (Mollusca: Gastropoda)

Understanding temporal variation in estuarine populations is essential for conserving species inhabiting environmentally heterogeneous coastal habitats. Clithon retropictus is a protected brackish-water gastropod in South Korea that inhabits algae-covered stones in small tidal rivers and is vulnerable to changes in salinity gradients, substrate structure, and fragmentation of estuarine habitats. We evaluated long-term changes in the abundance structure of this species using national-scale ecological monitoring data collected from 2016 to 2024. Rather than relying solely on mean abundance, we applied a distribution-based framework to occurrence-positive sampling units across three survey phases: phase 1 (2016–2018), phase 2 (2019–2021), and phase 3 (2022–2024). Abundance data were log-transformed, candidate probability distributions were fitted, and structural variation among phases was quantified using distributional parameters, quartile thresholds, skewness, and excess kurtosis. Statistical differences were assessed using the Kruskal-Wallis test and Cliff’s delta. The gamma distribution provided the best overall fit to the observed abundance structure. Abundance structure shifted markedly during phase 2, with a decline in the gamma shape parameter ( α ), increased skewness and kurtosis, and an expanded proportion of low-abundance observations. These results indicate a more right-skewed and heavy-tailed distribution, suggesting increased spatial heterogeneity and temporal restructuring of abundance across estuarine habitats. Effect size analysis revealed moderate differences between phases 1 and 2 ( δ = 0.34) and between phases 2 and 3 ( δ = -0.36), whereas phases 1 and 3 differed little ( δ = -0.01). Mean abundance alone showed only moderate temporal change and did not adequately represent this structural variation. By quantifying changes in the abundance distribution of C. retropictus at a national scale, this study demonstrates that distribution-based approaches can improve the interpretation of long-term monitoring data for protected species. The findings are best relevant as supporting evidence for national-level conservation assessment and future review of the species’ legal protection status, rather than as direct criteria for site- or region-specific habitat management.