Combined Effects of Microplastics and Mercury on Growth, Hematology, Tissue Morphology, GH/IGF Axis, and Antioxidant–Immune Responses in Nile Tilapia

Microplastics (MPs) and heavy metal mercury (Hg) have drawn global surveillance as major contaminants due to their toxic effects on aquatic organisms. The individual effects of both contaminants have been extensively characterized; however, their coexposure effects remain insufficiently explored. As aquatic organisms are increasingly exposed to multiple pollutants simultaneously in natural environments, this investigation explored the combined effects of polyamide MP (PA‐MP) and Hg on Nile tilapia ( Oreochromis niloticus ) fingerlings, focusing on survival, growth, hematological balance, tissue structure, GH/IGF axis regulation, and immune–antioxidant responses. Over a 42‐day trial, 240 Nile tilapia fingerlings were allocated into four triplicated treatments: control (no PA‐MP or Hg), PA‐MP (10 mg/L), Hg (0.03 mg/L), and PA‐MP + Hg (10 mg/L + 0.03 mg/L), with 20 fingerlings per tank. The coexposure group showed increased MP accumulation and mortality, suppressed growth indicators, and substantial shifts in blood physiology, including elevated glucose (126.83 ± 2.40 mg/dL) and lowered hemoglobin (9.45 ± 0.85 g/dL), along with higher cellular and nuclear abnormalities. The histoarchitectural assessment identified severe structural deformities in the gills, intestine, liver, and kidney of coexposed fish compared to control and individual contaminants. At the transcriptional level, the expression of growth hormone‐secreting gene ( gh ) in the pituitary and insulin‐like growth factors ( igf-1 and igf-2 ) in the liver showed a significant downregulation under coexposure treatment. Moreover, coexposure to PA‐MP and Hg induced hepatic oxidative damage by affecting antioxidant defense, as evidenced by altered activity of superoxide dismutase ( sod ) and catalase ( cat ), and simultaneously modulated immune responses by significantly upregulating interferon‐γ ( ifn-γ ) and tumor necrosis factor‐α ( tnf-α ) while downregulating interleukin‐1β ( il-1β ), indicating an oxidative–inflammatory response. These outcomes collectively underscore that MP and Hg coexposure aggravates both systemic and molecular impairments in Nile tilapia, resulting in weakened molecular responses, impaired physiological functions, and decreased survivability.