Phenotypic Differentiation Among Helostoma temminckii (Actinopterygii: Helostomatidae) Populations from East Kalimantan, Indonesia, Using Truss Morphometry

Helostoma temminckii is a freshwater fish species with high ecological flexibility; however, information regarding population variation in Indonesia remains limited. This study aims to analyze morphological variation among H. temminckii populations in the waters of East Kalimantan using a truss morphometric approach. Specimens (n =73) were collected between 2024 and 2025 from three locations: Mesangat River (n = 14), Muarapahu River (n = 30), and Semayang Lake (n = 29). Notably, the Mesangat River population had a smaller sample size and exhibited a broader body size range, which presents a limitation due to potential ontogenetic shape bias. A total of 22 morphometric characters derived from a truss network system based on 10 anatomical landmarks were analyzed using analysis of variance, discriminant function analysis (DFA), and UPGMA cluster analysis. Significant morphometric differentiation was observed among populations (p < 0.05). The main variations occurred in the anterior head and caudal peduncle region, which are hypothesized to be associated with localized habitat conditions, feeding strategies and swimming performance. Based on discriminant function analysis, the classification accuracy was 75.34, which decreased to 54.79 after cross-validation. Although these morphological differences suggest phenotypic plasticity responses to environmental factors and hydrological connectivity, this stock structure is inferred solely from phenotypic evidence and lacks genetic confirmation. Truss morphometrics provides a preliminary baseline for identifying phenotypic differentiation and potential stock structure of H. temminckii. © 2026, Egyptian Society for the Development of Fisheries and Human Health. All rights reserved.