Combined effect of growth promoter roxarsone and copper on the earthworm Eisenia fetida

Roxarsone (ROX) and copper (Cu) are growth promoters in livestock to promote growth and prevent disease. These chemicals and their metabolites are released to the soil through manure application and have a potential adverse effect on soil-dwelling organisms. The objective of this study was to investigate the combined subacute effect of ROX exposure (0, 80, 240, 720 mg kg⁻¹) and Cu exposure (0, 80, 160 mg kg⁻¹) in earthworms (Eisenia fetida). Growth, reproduction, spermatogenesis under light microscope, and heavy metal residue were investigated during 56-day exposure period. Results showed that Cu exposure of 80 or 160 mg kg⁻¹ alleviated the effect of ROX on cocoon production or hatching. The cocoon number exhibited an increase (P < 0.05) at 80 mg kg⁻¹ ROX on day 28, compared with the 0 mg kg⁻¹ ROX, in the presence of 80 mg kg⁻¹ Cu, whereas there was no effect (P > 0.05) in the presence of 160 mg kg⁻¹ Cu. The hatching success at 80 or 240 mg kg⁻¹ ROX exhibited a decrease (P < 0.05) on day 28, in the absence of Cu, whereas no effect (P > 0.05) was observed in the presence of 80 or 160 mg kg⁻¹ Cu. The other reproductive parameters (cocoon weight, juvenile number, and biomass) demonstrated a decrease (P < 0.05) only at 720 mg kg⁻¹ ROX in the presence or absence of Cu. However, with increasing exposure time, the above reproductive parameters were not affected (P > 0.05) in all groups on day 56. On the other hand, sperm deformity (%) increased (P < 0.05) at 240 or 720 mg kg⁻¹ ROX on day 28, in the presence or absence of Cu; however, the microstructural alteration in seminal vesicles occurred only at 720 mg kg⁻¹ ROX, exhibiting disordered distribution and decreased mature sperm bundles. In addition, ROX or Cu residues in earthworms demonstrated an increase with increasing ROX or Cu exposure concentration. Our present results may provide important insight on combined toxicity of chemicals in soils.