Sub-lethal effects of mercury, copper and cadmium, in dual combination on the filtration rates of the common inter-tidal bivalve Donax incarnatus Gmelin were investigated. The results indicated that when dual combinations were present in the medium, conspicuous variations in filtration performance occurred, mostly in the median concentrations tested. The presence of heavy metals in combination at lower and unrealistic higher concentrations elicited comparable filtration performance in the test species Simple additivity observed in majority of cases indicated at least two reactions as 1) independent action of the metal components involved, where the most toxic one influences filtration performance or 2 a joint action of the two in the combination. These results showed an, hitherto, unrecorded reaction by a bivalve to the presence of heavy metals and may be an indication of positive and negative feed backs, both ultimately leading to breakdown. The study also indicated that filtration rate is not a suitable index of stress at least in the case of clams.

Heavy metal toxity-bivalves