Asian Journal of Advanced Research and Reports

Earthworms are sensitive indicators of soil contamination, but direct comparisons of the toxicity of cobalt chloride and ammonium molybdate in Eisenia fetida remain limited. This study evaluated the effects of cobalt chloride hexahydrate (CoCl₂·6H₂O) and ammonium molybdate tetrahydrate ((NH₄)₆Mo₇O₂₄·4H₂O), individually and in combination, on survival, body weight and behavioural responses. Adult earthworms were exposed to cobalt chloride at 100, 200, 400 and 800 ppm or ammonium molybdate at 200, 400, 800 and 1,600 ppm over seven days. Three independent trials were conducted, with ten worms per treatment group. Mortality and body weight were recorded daily, while separate two-choice tests assessed hydrotaxis, phototaxis and chemotaxis. Cobalt chloride caused 20–40% mortality at 400 ppm and 50–60% at 800 ppm, yielding a mean estimated LC₅₀ of 766.67 ppm. Ammonium molybdate caused no mortality at 200–400 ppm, but mortality reached 50% at 1,600 ppm (LC₅₀ = 1,600 ppm). Progressive body-weight loss was observed in ammonium molybdate treatments. Earthworms retained positive hydrotaxis, negative phototaxis and negative chemotaxis across the tested concentrations. Combined exposure to 100 ppm ammonium molybdate and 50 ppm cobalt chloride resulted in 20–30% mortality, body-weight loss and reduced movement. Under the conditions investigated, cobalt chloride exhibited greater acute lethality, whereas ammonium molybdate produced marked body-weight reductions. The findings demonstrate the value of evaluating lethal and sublethal endpoints together when comparing soil contaminants.