Phenotypic Plasticity and Adaptive Significance of Morphological Variation in Clarias gariepinus Across Environmental Gradients in North Central Nigeria

Phenotypic plasticity allows individual organisms to express alternative phenotypes in response to environmental variation, and distinguishing plastic from genetic components of morphological variation is fundamental to understanding adaptation in freshwater fish facing rapid environmental change. This research applies a quantitative genetics framework with common garden experiments to partition genetic and plastic components of morphological variation in Clarias gariepinus across environmental gradients in river, reservoir, and floodplain habitats of North Central Nigeria (Benue, Kwara, and Niger States). Wild populations from nine habitat types spanning a gradient of flow velocity, turbidity, and resource availability were sampled. Geometric morphometric analysis using 28 landmark coordinates characterised body shape variation. A half-sibling split-rearing design across two controlled environments enabled estimation of heritability and genotype-by-environment interaction components for six key morphological traits. Reaction norm analysis quantified phenotypic plasticity. Population structure was characterised using 12 microsatellite loci to distinguish plastic responses from genetic divergence. Geometric morphometric analysis revealed significant body shape differentiation between flow regimes, particularly in head depth, caudal peduncle depth, and fin insertion positions. Common garden experiments demonstrated significant genotype-by-environment interactions for pectoral fin morphology and head shape, indicating that both plasticity and local genetic adaptation contribute to observed variation. Heritability estimates for the most hydrodynamically relevant traits (0.31 to 0.52) were moderate, indicating evolutionary potential. Reaction norm shapes across populations revealed population-level differences in plasticity magnitude, suggesting co-evolution of plasticity with genetic adaptation. Keywords: phenotypic plasticity, geometric morphometrics, Clarias gariepinus, local adaptation, North Central Nigeria.