Histological and gene expression study of Liza Abu (Haeckel) tissues exposed to high concentrations of cadmium chloride: An integrated biomarker approach for aquatic toxicity assessment in the Tigris river basin

Background: Cadmium (Cd) is a persistent, highly toxic heavy metal with no biological function, posing severe risks to freshwater ecosystems. Its bioaccumulation and oxidative stress-inducing capacity make it a priority pollutant, especially in poorly regulated regions such as Iraq. Objective: This study aimed to evaluate acute cadmium chloride (CdCl?) toxicity in Liza Abu (Haeckel, 1870), a native Iraqi fish, through an integrated histopathological and molecular biomarker approach. Methods: Adult L. abu were exposed for 96 h to 1.0 mg/L (lower concentration) and 2.5 mg/L (higher concentration) controlled laboratory conditions. Target tissues (gill, liver, kidney, intestine, and muscle) were examined histologically using hematoxylin-eosin staining, and lesion severity was semi-quantitatively scored. Concurrently, hepatic expression of stress-related genes (MT, HSP70, CYP1A, SOD, CAT) was quantified by RT-qPCR, with ?-actin as reference. Data were analyzed using ANOVA, Tukey’s HSD, and Pearson correlation. Results: Exposure induced concentration-dependent behavioral impairment and mortality (60% at 2.5 mg/L). Histopathological analysis revealed severe gill lamellar fusion, hepatic necrosis, renal tubular degeneration, intestinal erosion, and muscle fiber separation. Lesion scores increased significantly across all tissues (p < 0.001). Gene expression profiling showed robust upregulation of MT (25.3-fold) and HSP70 (12.8-fold), alongside moderate induction of CYP1A, SOD, and CAT. Strong correlations (r > 0.85, p < 0.01) were found between histopathological indices and MT/HSP70 expression, validating these genes as predictive biomarkers of tissue damage. Conclusion: The combined structural and molecular evidence confirms that Liza Abu is a sensitive sentinel for Cd pollution in Mesopotamian rivers. Early biomarker responses (MT, HSP70) provide valuable tools for biomonitoring, enabling detection of sublethal toxicity before irreversible organ damage occurs. These findings support the urgent need for regulatory thresholds and biomarker-based monitoring frameworks in Iraq.