COPPER SENSITIVITY OF ADULT PACIFIC OYSTERS 311 



three-fourths that of the inflow. The actual copper concentrations to 

 which the animals were exposed in Fujiya's experiments were 

 probably lower than he reported. 



The greatest mortality at 96 hr in our experiments and in those 

 of Okazaki (1976) occurred at intermediate rather than at high 

 copper concentrations. Our observations indicate that this response 

 resulted from the oysters' remaining open for shorter periods in the 

 water containing high concentrations of copper. In an early 

 experiment one oyster maintained at 1200 /ig Cu/liter was closed 

 during each observation period for 14 days. When sacrificed at the 

 end of the experiment, the oyster appeared normal, and its heart was 

 beating regularly. We observed during preliminary experiments that 

 oysters will stop circulating seawater when copper is added to it at 

 levels >1000 Mg/hter. Establier and Pascual (1974) noted increased 

 incidence of shell closure in C. angulata at high copper concentra- 

 tions. This response of oysters may serve as protection from episodic 

 releases of copper. Moreover, it suggests that oysters may not be the 

 best biological indicators of copper pollution even though they have 

 been used as such in the past (Huggett, Bender, and Stone, 1973). 

 They may accumulate copper only when the concentration in the 

 water is at low and intermediate levels. Since accumulation may not 

 be continuous, the copper content of oysters may not reflect the 

 total amount of copper in their environment. 



Mussels, like oysters, can detect copper in their environment. 

 Mytilus edulis avoids the detrimental consequences of exposure to 

 copper by closing its shell valves (Davenport, 1977). Davenport 

 suggested that mussels and other animals possessing similar closure 

 mechanisms are of doubtful use as a biological pollutant-monitoring 

 system. 



The toxic response in some 6inimals is attributed to the 

 concentration of ionic copper in the water (Sunda and Guillard, 

 1976). In our experiments the ionic fraction (as operationally 

 defined) was greater than 75% of the total in all test systems. If 

 oysters are sensitive to only the ionic form, however, the LC50 

 values for the toxic copper form should be lower than that reported. 



The impact of past releases on oyster populations can be 

 approximated by comparing the concentrations of copper in past 

 releases to concentrations known to elicit a toxic response in oysters. 

 Two important sources of copper in marine systems are municipal 

 wastes and power-plant effluents. A survey of 108 municipal waste 

 discharges along the Atlantic coast reported concentrations of 20 to 

 5900 Mg Cu/hter; 50% of the effluents contained more than 100 jug 

 Cu/liter (Mytelka et al., 1973). These represent the total copper 



