250 EISLER 



(Colwell and Nelson, 1975; Colwell et al., 1976); and accumulation 

 after death (Glooschenko, 1969). Two studies, both on adult 

 American oysters, Crassostrea virginica, are worth emphasizing. 

 Cunningham and Tripp (1973) held oysters in seawater containing 

 10 [ig Hg/liter as mercuric acetate. After 45 days the whole-body 

 mercury concentration of exposed animals was 28,000 Mg/kg wet 

 weight. The mercury concentration in control oysters was always less 

 than 20 Mg/kg wet weight. The mercury concentration in exposed 

 oysters dropped to 18,000 Mg/kg by day 60, probably because of 

 spawning. At day 60, oysters were transferred to mercury-free 

 seawater for 160 days. During the first 18 days, levels declined to 

 15,000 /ig Hg/kg, but thereafter no further decline occurred. It was 

 concluded that oysters can concentrate 10 jug Hg/liter by a factor of 

 2800 and that total self-purification was not achieved over a 6-month 

 cleansing period. Kopfler (1974) found that continuous exposure to 

 1 jUg Hg/liter in any of the three mercury compounds tested caused 

 oysters to concentrate mercury rapidly in their tissues far in excess 

 of 0.5 mg/kg wet weight, i.e., at levels potentially hazardous to 

 humans ingesting these oysters, according to the action guideline 

 established by the Food and Drug Administration. Details of 

 Kopfler's study follow: 



Accumulation of mercury compounds by oysters was determined in two 

 experiments, each utilizing three groups of 100 adult oysters. In the first 

 experiment, conducted between and 10 C, mercury levels were main- 

 tained at 50 /ig/1 with flow rate adjusted to one liter per oyster per hour. 

 In the second experiment, mercury levels were reduced to one^<g/l, the 

 water temperature varied between 25 and 35 C, and flow rates maintained 

 at 2 1/oyster/hr because of the increased temperature. Controls were 

 maintained for each study. In the first experiment, the administration of 

 organic mercury compounds was terminated after 19 days because many 

 of the oysters in the groups receiving salts of either methylmercury or 

 phenylmercury were dead or moribund. Oysters classified as moribund 

 exhibited slow, incomplete valve closure when disturbed. When oysters 

 which survived 19 days of exposure to methylmercury and phenylmercury 

 were placed in flowing seawater, about half in each group died within a 

 week with all oysters in both groups dead within 14 days. Oysters exposed 

 to inorganic mercuric chloride exhibited no apparent ill effects over a 

 42-day period of exposure to 50 ;Ug/l of Hg. Mean mercury levels in 

 experimentals after one week was lOOOX over controls. Mercury values in 

 experimental oysters at 7 days, in mg Hg/kg wet wt, ranged between 15 

 and 25; for controls this was 0.02. Copper and zinc levels were depressed 

 in flesh from all three groups of oysters after one week exposure to 

 mercury; in the two groups exposed to organomercury, Cu and Zn 

 declined over the 19-day exposure. Copper and zinc levels in the oysters 

 exposed to mercuric chloride began to increase during the third week and 

 continued to do so until they were essentially the same as control values. 

 In the second experiment methyl- and phenylmercury were concentrated 



