Evaluation of the joint-action toxicity of binary mixtures of heavy metals against the mangrove periwinkle Tympanotonus fuscatus var radula (L.).

The joint-action toxicity of binary mixtures of heavy metal compounds prepared in predefined ratios of 4:1, 3:2, 1:1, 2:3, and 1:4 (wt/wt) of Zn:Cu, Zn:Cd, and Cd:Cu, respectively, and tested against the mangrove periwinkle Tympanotonus fuscatus were carried out in laboratory bioassays. The interactions between binary mixtures showed significant departures from the action of the individual constituent metals when acting singly. On the basis of the models such as isobole representations, synergistic ratios (SRs), and concentration-addition models used for the joint-action evaluations, the interactions between the constituent metals in the various test proportions of the mixtures were largely in conformity with the models of antagonism. Furthermore, in most of the test combinations of the binary mixtures, the Zn compound was found to consistently reduce the toxic effect of Cu and Cd compounds when tested jointly against T. fuscatus. The possibility of utilizing the observed antagonistic interactions between Zn(2+) and Cd(2+) in the management of Cd-contaminated waterbodies was discussed and the synergistic ratio model was singled out as a better candidate model for joint-action toxicity evaluations, which are aimed at establishing environmental safe limits of pollutants.