Iranian Medicinal Plants Society Journal of Medicinal plants and By-products 2322-1399 15 4 2026 07 01 Studying Protective Effects of Thymol, Curcumin and Menthol on Gill Histopathological Changes in Mercury-Exposed Juvenile Common Carp (Cyprinus carpio) 435 439 135422 10.22034/jmpb.2026.371952.2129 EN Sara Shokrpoor Department of Pathology and Clinical Pathology, Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran Hooman Rahmati-Holasoo Department of Aquatic Animal Health, Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran Mahroo Mohammadi Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran Alireza Nassiri Department of Aquatic Animal Health, Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran Sakineh Khanamani Falahatipour Department of Comparative Bioscience, Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran Journal Article 2026 01 02 Heavy metal pollution, particularly from mercury, poses a significant threat to aquaculture sustainability and food safety. This study investigated the protective efficacy of dietary thymol, curcumin, and menthol against mercuric chloride (HgCl₂)-induced gill histopathology in common carp (<em>Cyprinus carpio</em>). A total of 240 fish (50 ± 2 g) were distributed in a completely randomized design into eight treatments for 8 weeks: Control, Hg (0.44 mg/L HgCl₂ in water), Thymol (100 mg/kg feed), Curcumin (15 g/kg feed), Menthol (2.5 g/kg feed), and three co-exposure groups (Hg+Thymol, Hg+Curcumin, Hg+Menthol). Histopathological examination revealed severe lymphoplasmacytic proliferative branchitis, necrosis, and lamellar destruction in the Hg group. Thymol supplementation demonstrated the most potent protective effect, reducing lesions to mild branchitis and edema. Curcumin provided moderate protection, notably mitigating the severity of the injury. Conversely, menthol failed to alleviate mercury-induced pathology and, when administered alone, induced mild branchitis and aneurysms. The results indicate that the protective effects of these phytochemicals are compound-specific. Thymol is highly effective in preventing HgCl₂-induced gill damage, curcumin offers significant but moderate protection, while menthol is ineffective and potentially irritating at the tested dose. Thymol is recommended as a promising dietary supplement for enhancing carp resilience in mercury-contaminated aquaculture environments. https://jmpb.areeo.ac.ir/article_135422_3ace247ed65a2f839bbde987c85b98be.pdf