Relating the reproductive toxicity of five ingested metals in calanoid copepods with sulfur affinity

The sublethal toxicity of dietary Ag, Cd, Hg, Mn, and Zn to marine copepods, measured as depressed egg production, was evaluated as a function of ambient metal concentrations and metal concentrations in copepod tissues. All metals were toxic following 4-h feedings on metal-contaminated phytoplankton food, but there was a four order of magnitude difference between the inhibitory concentration in copepod tissues of the most toxic (2.4 nmol g^-1 dry wt. for Hg) and least toxic (13.4 μmol g^-1 dry wt. for Mn) metals. Metal concentrations in copepods that elicited a toxic response were about 3 times higher than background concentrations for Ag and 9 times higher for Hg, about 2 times higher for Cd, two orders of magnitude higher for Mn, and only 5% higher for Zn. Copepods exposed to lower concentrations of Zn for longer periods (1 week), resulting in tissue Zn concentrations similar to those in the short-term exposures, were not affected, suggesting that copepods were capable of adjusting to slowly increasing Zn by sequestration of this metal. Toxic metal concentrations in copepod tissues were directly proportional (r²=0.85) to the affinity of the metals for sulfur, as indicated by the solubility products of the metal sulfides. This relationship, together with protein analysis of eggs, published recently, suggests that metals affect egg production by binding to enzymes involved with vitellogenesis.