White muscle and mitochondrial traits under different locomotor demands in Amazonian fishes

This study characterized mitochondrial and muscle fiber organization in the white muscle of six Amazonian fish species from the Negro and Solimões rivers, representing distinct locomotor strategies and ecological contexts. Mitochondrial traits (number, area, and volume) and muscle fiber characteristics (nuclei number, fiber size, and sarcomere length) were quantified using light and transmission electron microscopy. Significant interspecific differences were observed, particularly in the nuclear number, with Semaprochilodus taeniurus and Triportheus albus exhibiting the highest values, Pygocentrus nattereri intermediate values, and Oxydoras niger, Pterygoplichthys pardalis, and Serrasalmus gouldingi the lowest. Differences in mitochondrial traits were also detected among species. Multivariate analyses revealed that morphological variation was better explained at the species level than by dietary grouping, with higher classification accuracy when species were analyzed individually. CAP analysis further indicated that trait variation is structured along coordinated multivariate gradients integrating nuclear density, mitochondrial characteristics, and muscle fiber architecture, providing a clear separation of species according to locomotor profiles. These results indicate that muscle and mitochondrial organization are more closely associated with functional demands related to locomotor behavior than with trophic category alone. Species characterized by sustained swimming exhibited greater structural and metabolic investment, whereas benthic and burst-swimming species showed reduced or alternative organizational strategies. Diet appears to play an indirect role by influencing behavioral and locomotor patterns. Overall, our findings highlight the importance of integrating morphological and ecological approaches to understand functional adaptation in fish muscle and identify locomotor demand as the primary axis structuring white muscle and mitochondrial traits in Amazonian fishes.