The toxic effect of Hg, Cu and Cd applied in concert, on the life of Perna
indica was investigated. The results indicate that Cu was the most toxic am
ong the three metals tested, when present in combination. The combined acti
on of the three metals was either less than additive or simple additive. Sh
ifting of toxicity in the triad combinations was found to be controlled eit
her by Hg or the Cu concentration. The toxicity indices were found inadequa
te to assess the combined toxicity of triad metal mixture.