Sex-specific morphological variation associated with environmental heterogeneity in a generalist fish, Zacco platypus

Stream fishes often exhibit morphological variation driven by environmental heterogeneity. Although population-level morphological variation associated with environmental gradients has been documented, less is known about how environments and sex jointly shape morphological patterns. Here, we studied body shape variation in Zacco platypus, a habitat generalist distributed across diverse environments in South Korea. Using landmark-based geometric morphometrics, we quantified body shape in six stream populations collected during the spawning season in 2024. A total of 14 environmental variables, including water quality and habitat variables, were used to investigate the shape-environment relationships. Our results showed that body shape differed consistently by flow conditions, with more streamlined bodies in lotic habitats and deeper bodies in lentic environments. Notably, the association between body shape and environment was sex-dependent: male body shapes exhibited less inter-population variation and a weaker association with environmental variables than those of females. These sex-specific patterns suggest stronger sexual selection on males during the spawning season, whereas females exhibit greater shape variation in response to local environmental conditions. Our study highlights the combined effects of environment and sex on intraspecific morphological variation in a habitat generalist.