Evaluation of Toxic Impact of Mercuric Chloride on Some Behavioural and Neurological Indices of Clarias gariepinus (Burchell, 1822)

Abstract

Mercury chloride (HgCl₂) is a harmful pollutant that accumulates in fish tissues, and this study examines its effects on the behaviour and neurological health of Clarias gariepinus. Juvenile fishes (120) procured from the same brood were used in this study. The fish were acclimatised for 14 days before exposure to HgCl₂ concentrations of 0.2, 0.4, 0.6, 0.8, 1.0, and 1.2 mg/L in a 96-hour acute toxicity test. Mortality and behavioural responses were recorded, and the LC₅₀ value from Probit regression informed the selection of sublethal concentrations of 0.067, 0.130, 0.200, and 0.267 mg/L for a 28-day sublethal test. After exposure, fish were anaesthetised and dissected, with brain samples analysed for biochemical markers such as acetylcholinesterase, glutamate dehydrogenase, malondialdehyde, glutathione, succinate dehydrogenase, superoxide dismutase, and nitric oxide. Data were analysed using one-way ANOVA and LSD post-hoc tests (p<0.05). Histological analysis assessed neurodegeneration. Behavioural symptoms included erratic swimming, balance loss, and increased mucus production, worsening with higher mercury concentrations and prolonged exposure. Mortality rates rose with mercury levels, reaching 100% at 1.2 mg/L within 48 hours. The LC₅₀ for HgCl₂ was 0.267 mg/L. Biochemical results showed increased activities of acetylcholinesterase, glutamate dehydrogenase, malondialdehyde, and nitric oxide and decreased levels of glutathione, succinate dehydrogenase, and superoxide dismutase in exposed fish. Histological examination revealed no brain changes in controls but significant neurodegeneration in mercury-exposed fish, especially at 0.200 and 0.267 mg/L. These findings highlight HgCl₂’s severe neurotoxicity to Clarias gariepinus, affecting brain function and inducing oxidative stress.