Combined toxicity of copper–nickel co-exposure on growth, digestive function, antioxidant status, and transporter- and inflammation-related gene expression responses in Oreochromis niloticus , with potential mitigation by watermelon rind ( Citrullus

Abstract Although copper (Cu) is an essential trace element for fish, excessive concentrations can induce toxic effects. In contrast, nickel (Ni) is generally regarded as a non-essential heavy metal with potential toxicity. Collectively, exposure to concentrations exceeding safe thresholds for either metal poses a significant risk to fish health. Therefore, the combined toxicity of Cu–Ni co-exposure in Oreochromis niloticus was assessed with or without supplementation with watermelon rind (WMR). Fingerlings (37.30 ± 0.20 g) were randomly divided into four treatments: control, WMR (40 g/kg), Cu + Ni (3.35 and 3.60 mg/L, respectively), and WMR + Cu + Ni for 45 days. The findings demonstrated a significant decrease in growth parameters (weight gain and specific growth rate) and an increase in the feed conversion ratio and fish mortality due to Cu–Ni co-exposure. A decline in growth hormone levels and digestive enzyme activities (amylase, lipase, and trypsin) was accompanied by a rise in leptin levels in the Cu + Ni group. In addition, the histomorphology of intestinal tissues was altered, as evidenced by necrosis and epithelial desquamation, as well as reduced intestinal mucosal fold width and height. Additionally, decreased glutathione peroxidase, superoxide dismutase, and total antioxidant capacity, together with elevated malondialdehyde levels, were induced by Cu–Ni co-exposure. Furthermore, the combined exposure caused a downregulation of nutrient transporters ( slc15a1a, slc15a2 , and slc2a1b ) and an upregulation of inflammatory genes ( il1b and rela ). Interestingly, the WMR diet enhanced antioxidant defences, improved digestive capacity, modulated gene expression, and ameliorated intestinal histology in the Cu + Ni-exposed group, resulting in increased growth and a reduction in mortality.