Determination of types of interactions exhibited by binary mixtures of heavy metals tested against the hermit crab,Clibanarius africanus

Single and joint action toxicity of heavy metal compounds (ZnCO₃, CuSO₄ and CdSO₄) were carried out against the hermit crab, Clibanarius africanus in laboratory bioassays. On the basis of 96?h LC₅₀ values, CuSO₄ (96?h LC₅₀?=?0.03?mM) was found to be the most toxic followed by CdSO₄ and ZnCO₃ (96?h LC₅₀?=?0.24?mM) in decreasing order of toxicity. The evaluation of the metal compounds in mixtures of ZnCO₃–CuSO₄, ZnCO₃–CdSO₄ and CdSO₄–CuSO₄ in pre-defined molar ratios revealed that interactions between Zn–Cu and Zn–Cd test mixtures were in conformity with the models of antagonism (reduction in toxicity of constituent metals). The synergistic ratios (S.R.s) and relationship between observed/predicted 96?h LC₅₀ values (relative toxic unit, RTU) were consistently below 1, indicating antagonistic interactions. Analysis using isobolograms also indicated the interactions between the mixture components to be antagonistic. However, with regards to CdSO₄–CuSO₄ test mixtures, the interactions between these metals were mainly in conformity with the model of synergism. The S.R. and RTU values were largely above 1, indicating synergistic interactions. The importance of the joint action evaluations for the management and protection of aquatic ecosystems, particularly in the setting up of water quality criteria was discussed.