Size-, sex- and site-dependent morphological variation in the stone flounder (Kareius bicoloratus) from Korean coastal waters.

Kareius bicoloratus is a commercially important flatfish in Korean coastal fisheries and an ecologically relevant benthic species in nearshore ecosystems. To evaluate how multiple biological and environmental factors jointly shape flatfish morphology, we analysed 81 morphometric indices and two-dimensional geometric morphometrics (GMM) in wild K. bicoloratus collected from two Korean coastal regions, incorporating size, sex and sampling site. Classical morphometrics based on a three-way ANOVA framework identified body size and sex as the primary drivers of shape variation, particularly influencing surface area-, depth- and angular traits, whereas geographic effects were more limited and context dependent. GMM analyses supported these patterns. Procrustes ANOVA detected significant shape effects of size, sex and site, and CVA revealed clear multivariate separation among groups, with Mahalanobis distances of 3.28-5.49 among sex groups and Procrustes distances of 0.032-0.049. Geographic divergence was also evident in GMM space, with Procrustes distances of 0.033-0.036 between sites. Allometry analyses indicated heterogeneous allometric trajectories for size and sex but not for site. Mitochondrial DNA analyses revealed moderate genetic differentiation between regions, and Mantel tests detected a weak but significant association between genetic and morphological distances. These findings provide a robust morphological baseline for K. bicoloratus.