962 



7. MERCURIALS 



finch and Mott, 1948). Although the cyprids are not killed so readily by 

 Hg+"'", their settlement on the substratum is reduced appreciably by 0.000037 

 mM and completely by 0.00018 mM. The lack of an open gut in the cyprids 

 is suggested as a possible explanation for the reduced sensitivity. Support 

 for the law that nothing is simple or predictable in the response of organisms 

 to metal ions, although no further support is needed, is the fact that the 

 cyprids are less sensitive to Hg++ in 50% diluted sea water relative to normal 

 sea water, in contrast to the amphipod discussed above. Another interesting 

 and complex phenomenon was discovered by Barnes and Stanbury (1948) 

 in studying the effects of Hg++ and Cu++ on the harpacticid copepod Nitocra 

 spinipes. Hg++ is over 1000 times more toxic than Cu++, but when these 

 metal ions are present together at certain concentrations the lethal effect 

 is greater than would be expected on the basis of their actions alone (Ta- 

 ble 7-24). An isobologram for 50% lethality provides a curve characteristic 



Table 7-24 

 Lethal Effects of Mercury, Copper, and Their Combinations on Nitocra spinipes'^ 



" From Barnes and Stanbury (1948.) 



of very definite synergism (see Fig. 1-10-8), i.e., the curve is extremely con- 

 cave upward. It was postulated that lowered vitality due to one metal ion 

 may not allow the animal to deal effectively with the other metal ion. For 

 example, Hg++ might impair the excretory system so that Cu++ would be 

 retained, since it is known that certain crustaceans and mollusks excrete 

 Cu++. The synergistic effects indicate that Hg++ and Cu++ may act by dif- 



