COPPER SENSITIVITY OF ADULT PACIFIC OYSTERS 305 



RESULTS 



Toxicity Experiments 



The family of mortality curves from our first two experiments 

 with oysters was difficult to interpret because increased copper 

 concentrations in the water did not consistently result in increased 

 mortality rates; 100% mortality of the oysters occurred first in the 

 water containing 500 rather than 900 or 1500 jUg Cu/liter. Since 

 oysters can prevent the entrance of water into their mantle cavities 

 by tight closure of the valves of the shell, they might have reduced 

 their exposure to copper in this way. In experiments 3 and 4, oysters 

 were observed at 4-hr intervals from 8:00 a.m. to 12:00 p.m. so that 

 the exposure of each oyster could be corrected for the difference 

 between the time the animal was first observed with its shell open 

 and the time it was introduced into the water and also by the ratio of 

 the number of times the shell was seen opened to the number it was 

 seen closed (Table 1). Typical mortality curves were generated with 

 adjusted exposure times (Fig. 1), and, in all further measures of 

 toxicity, we used adjusted exposure time. A logit instead of a probit 

 analysis was performed on the time-vs. -mortality data (American 

 Public Health Association, 1975). Differences between the slopes of 

 the lines fitted to the curves and the intercepts of the lines were 

 tested using a t statistic (Snedecor and Cochran, 1967). The slopes 

 were significantly different from the controls (P < 0.01), but the 

 intercepts were not. 



Sensitivity to copper sometimes differed between the two groups 

 of oysters in experiments 3 and 4. Such differences are expected 

 when oysters are collected at different times, the number of animals 

 tested at each concentration is small, and the exposure conditions 

 and concentrations are not exactly identical (Table 1). 



The concentration-vs.-time-to-50%-mortality data from experi- 

 ments 3 and 4 were used to generate a toxicity curve (American 

 Public Health Association, 1975). Acute lethality stopped at con- 

 centrations below about 230 ng Cu/liter (Fig. 2). In this group of 

 oysters, the LC5 value (lethal concentration for 50% of the test 

 population) ±95% confidence interval at 48 hr was 650 ± 100 idg 

 Cu/liter; at 96 hr, 430 ± 60 /ug Cu/liter; and at 168 hr, 30 ± 100 )Ug 

 Cu/liter. The incipient lethal or threshold concentration is defined as 

 the concentration that kills 50% of the population during an 

 exposure sufficiently long for acute lethal action to cease (Sprague, 

 1969). In this group of oysters, the estimated incipient lethal 

 concentration (incipient LC5 ) was 230 ± 100 /ig Cu/liter. 



