Evaluation of sublethal toxicity of zinc and chromium in Eudrilus eugeniae using biochemical and reproductive parameters.

Zinc (Zn) and chromium (Cr) act as essential nutrients; however, they can be toxic at higher concentrations. In this study, earthworm species Eudrilus eugeniae was studied for its sensitivity to sublethal doses of Cr (8 ppm) and Zn (350 ppm) in terms of alterations occurred in oxidative stress indices and reproductive parameters in the testis. A significant (p < 0.05) decrease in the rate of food intake was observed on 7, 14 and 21 days of Cr toxicity; while increased rate was evident upon 7 days of Zn exposure. Changes evident in the rate of cocoon production (-39.54 and +38.63 %), hatchability (-77.85 and +30.0 %) highlight higher toxic potential of Cr than Zn, respectively. Moderate to severe vacuolization in spermatic follicles and higher incidence of tailless (+52.6 % in Cr and 20.8 % in Zn), and head bent (+18.8 % in Cr and 0 % in Zn) sperms were evident in Cr(VI) exposed worms emphasizing the higher vulnerability of E. eugeniae to Cr toxicity. A significant (p < 0.05) increase observed in catalase activity (+13.29 % in Zn and +20.88 % in Cr) and glutathione (+52.09 % in Zn and -7.70 % in Cr) suggests a higher compensatory antioxidant response in Zn-exposed worms than Cr. Variations observed in the activities of superoxide dismutase (-9.40 % in Zn and +24.0 % in Cr) and glutathione-s-transferase (-39.39 % in Zn and +1.29 % in Cr) emphasize the metal specific antioxidant responses in testis. Therefore, it can be implied from results that excessive free radical production and inadequate antioxidant defenses have lead to morphological alterations in sperms which sequentially reduced the reproductive rate.