Deciphering the Impact of Eriochrome Black T on Tubular Air Sac of Heteropneustes fossilis and Dendritic Organs of Clarias batrachus

Azo dyes, like Eriochrome Black T (EBT), enter freshwater ecosystems through textile effluent, posing potential risks to fish, which are key bioindicators and significant aquaculture resources. Despite the widespread use of azo dyes, toxicological data about their effects on air-breathing organs (ABOs) is scarce. This study examines the impacts of EBT on two catfish species, Heteropneustes fossilis and Clarias batrachus, selected for their distinctive ABO morphologies, which facilitate interspecific comparison to EBT intoxication. To achieve this goal, acclimatized fishes (both species) were exposed to three sublethal concentrations of the median LC5096h value of EBT (1, 10, and 20 mg L-1) for 96 h. Both species showed significantly higher accumulation of EBT, with the ABO of C. batrachus showing more significant accumulation. EBT exposure caused higher levels of lipid peroxidation and a substantial increase in the activities of antioxidant enzymes (such as SOD, GPx, and GR), while reducing catalase (CAT) activity. Histological examination showed structural damage and signs of respiratory distress in the ABOs of both species. Histochemical analysis also showed changes in number, percent area occupancy, and size of mucous cells, highlighting the deteriorating impact of EBT on fishes. Moreover, a careful assessment of the results demonstrated that C. batrachus manifested greater sensitivity to all concentrations of EBT in comparison with H. fossilis. These findings illustrate the susceptibility of fish respiratory organs to dye exposure and highlight the necessity of monitoring azo dye contamination for protecting aquaculture species and freshwater ecosystems.