Experiments were carried out in diluted seawater media at different salinities (4-29%o) and different concentrations of the chelator NTA (nitrilotriacetic acid) to determine the relationship between the chemical speciation of cadmium and the toxicity of cadmium to Palaemonetes pugio, grass shrimp. After four days of exposure to a given concentration of CdCl2, shrimp mortality decreased with increasing salinity and increasing concentration of NTA. The protective effect of high salinity or NTA was attributable to the complexation of cadmium. Mortality was related to the measured free cadmium ion concentration that, in turn, was determined by the total concentration of cadmium and by the level of complexation by either chloride ion or NTA. Fifty percent mortality occurred at a free cadmium ion concentration of ~4 X 10-7 M.

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We recently demonstrated that zinc, copper, and hemocyanin metabolism in the blue crab varies as a function of the molt cycle. To extend these observations, and better delineate metal metabolism in marine crustaceans, we have conducted experiments to determine if environmental temperature and season of the year affect concentrations of hemocyanin and copper in the hemolymph and copper and zinc in the digestive gland. Overwintering, cold water crabs (6°C) had decreased hemocyanin and copper in the hemolymph and normal zinc and copper in the digestive gland with respect to summer crabs collected at 20-30°C. When these crabs were warmed to 20°C and fed fish for three weeks, they showed increases in the concentrations of copper in the digestive gland, and copper and hemocyanin in the hemolymph. In addition, a change from a zinc to a copper-dominated metallothionein was found in a majority of the warmed crabs, suggesting the involvement of copper metallothionein in the resynthesis of hemocyanin. Based on these observations and previous data (Engel, 1987) a conceptual model of copper and zinc partitioning in the blue crab has been constructed. In this model, metallothionein has an important role in metal regulation both during molting and in the changes related to season of the year. Metallothionein-bound copper and zinc appear to be regulated at the cellular level for the synthesis of metalloproteins, such as hemocyanin (copper) and carbonic anhydrase (zinc), both of which are necessary for normal growth and survival. Finally, we present evidence showing that copper metallothionein can directly transfer its metal to the active site of apohemocyanin. Copper insertion seems to precede the formation of viable oxygen binding sites.