RESUMO
The acute toxicity of Mn and Cu against Tilapia guineensis and Tympanotonus fuscatus and the sub-lethal effects of the metals on weight changes were investigated in laboratory experiments employing standard bioassay techniques. Based on 96 hr LC50 values, copper was more toxic than manganese to either test animal by several orders of magnitude. Against either test metal, T. guineensis was remarkably more susceptible than T. fuscatus. At sub-lethal concentrations, particularly the two highest concentrations tested (0.016 and 0.03 mg Cu L(-1), 23.0 and 46.0 mg Mn I(-1)), both metals adversely affected the test species by bringing about reduced weight gains or weight loss in exposed animals compared to untreated controls at the end of the test periods. The implications of these findings for ecosystem viability and management are discussed.