Ecophysiological variance within fish populations occupying different trophic niches of contrasting ecoregions.

Understanding how fishes physiologically cope with contrasting environmental conditions is fundamental to linking trophic ecology with ecosystem function. This study compared erythropoietic and osmoregulatory strategies across fishes representing distinct feeding niches inhabiting two ecologically divergent tropical ecoregions-the freshwater hill streams of the Sub-Himalayan Terai-Dooars and the estuarine rivers of the Indian Sundarbans. Flow cytometric analyses revealed significantly higher Na⁺/K⁺-ATPase protein expression and chloride-cell abundance among carnivorous species of the Sundarbans, followed by omnivores and herbivores (Welch's ANOVA; t-tests). Conversely, hill-stream carnivores exhibited elevated erythropoietic activity and reactive oxygen species (ROS) generation, reflecting enhanced oxygen demand under hypoxic and fast-flowing conditions. Integrative modelling using Random Forest classification and quantile regression demonstrated that trophic guild identity, ecoregional attributes, and their interaction significantly influenced cellular-level variation, delineating distinct physiological traits across ecosystems. Collectively, the findings reveal ecoregion-specific physiological trait syndromes that align with trophic strategies, reflecting adaptive responses to local salinity, oxygen, and flow regimes. These results highlight how organismal physiology mechanistically links trophic ecology with environmental constraints across contrasting tropical ecosystems, offering predictive insight into species resilience and adaptive potential under changing climatic regimes.