Two tropical oligochaetes as potential standard test species: a case study evaluating single and combined copper and manganese toxicity

The lack of toxicity tests using indigenous species in risk assessments of tropical regions has often been discussed. Thus, this study evaluated two tropical oligochaete species (Allonais inaequalis and Dero furcatus) for potential use as test species. To this end, the toxic effects of copper and manganese, both isolated and mixed, were evaluated for both species via acute tests. The test protocol developed by the Organisation for Economic Co-operation and Development (OECD) for temperate sediment-dwelling benthic oligochaetes could be successfully applied for both test species. The 96-h LC₅₀ values (mean ± standard deviation) for D. furcatus were 31 ± 3 µg L⁻¹ and 18 ± 0.061 mg L⁻¹ for copper and manganese, respectively. The corresponding values for A. inaequalis were 26 ± 1 µg L⁻¹ and 28 ± 0.063 mg L⁻¹, respectively. Compared to the toxicity values for temperate arthropods, D. furcatus and A. inaequalis had comparable or greater sensitivity to the evaluated metals. The mixture toxicity tests indicated a dose ratio-dependent deviation of the concentration addition (CA) model, with antagonism occurring especially if the effects in the mixture were provoked by copper. Overall, these findings indicated that these taxa are sensitive candidates for use in tropical ecotoxicity testing. Future studies can elucidate the mechanisms of chemical mixture toxicity.