were lower than the vahies g iven for fish. The 

 96-hour TLm for bhiegills, Lepomis macrochirus, to 

 Co-Ral and Di-Syston, reported by Henderson, 

 Pickering, and Tarzwell (1960), was also higher 

 than that for the least tolerant stages of clams. 



NEED FOR FURTHER STUDY 



The examples cited indicate the need for evalu- 

 ating all aspects of to.xicity on rapidly growing and 

 changing animals at each stage of their life cycle. 

 The high tolerance of bivalve larvae to some of 

 these pesticides also suggests that compounds can 



be chosen to control pest species without serious 

 damage to commercial shellfish. Within tlie series 

 of insecticides, for example, Davis (1960) showed 

 that DDT was much more to.xic to oyster larvae 

 than lindane. Similarly, within the series of herbi- 

 cides tested, Amitrol was "safe" at 100 p.p.m., 

 whereas MCPA caused a significant reduction in 

 the rate of growth of oyster larvae at all concen- 

 trations above 0.25 p.p.m. We believe, as Thimann 

 (1964) suggested, that emphasis should be placed 

 on developing "substances whose action is selective 

 and on those which decompose quickly." 



Table 1. — Percentage of eggs of American oyster and hard clam that developed normally, percentage of larvae that survived, 

 and percentage increase in mean length, in the presence of different concentrations of chemicals ' 



See footnotes at end of tabli' 



398 



U.S. FISH AND WILDLIFE SERVICE 



