series that corresponds to the break in acetone 

 concentrations. 



To obtain the six concentrations of these 

 acetone-soluble compounds, from 0.25 to 10 p.jj.m., 

 we used two stock solutions. The concentrations 

 of 0.25, 0.5, and 1 p.jj.ni. of toxicants were obtained 

 bj- increasing volumes of the less concentrated 

 stock solution so that the concentrations of 

 acetone were 25, 50, and 100 p. p.m. Tlie con- 

 renlrations of 2.5, 5, and 10 ]).]>. m. were achieA'ed 

 l)y increasing volumes of the more concentrated 

 stock solution, again giving concentrations of 

 acetone of 25, 50, and 100 p.ji.m. 



The stepwise decrease in survival and growth 

 of rlam huAae at 0.25, 0.5, and 1 p. p.m. Co-Ral, 

 followed by better survival and growth at 2.5 

 p.]:).m. and stei)wise reduction at 5 and 10 i).i).m. 

 indicates that the action of Co-Ral was being 

 synergi/.ed by the acetone solvent (the action of 

 Di-Syston and Phygon was not). The effect of 

 Aldrin on clam embryos sliows some e\idcncc of 

 similar synergism. 



Variable Effects of Endrin and Dieldrin 



Results of tliiferent experiments \\iili these 

 two compounds varied considei'ably even wlien 

 acetone stock solutions were used. The results 

 given for endrin and dieldrin, therefore, are the 

 average values of a munber of experiments but 

 in some experiments tolerances were significantly 

 below these averages. We assume that the ^■aria- 

 tion in the several exjjeriments was caused by 

 differences in jiarticle size and degree of suspen- 

 sion attained in the test cvdtiu'e, since these 

 C(.mi)ounds are essentially insoluble in water. We 

 Would expect field observations on commercial 

 a])plications to yield conflicting data, de])ending 

 ni)on the degree of dispersion attained when the 

 jiesticide reaches the water. 



Possible Indirect Effect of Compounds Through Food 

 Chain 



Ukeles (1962) showed tliat the tolerance for 

 pesticides of some of the best algal foods for 

 bivalve larvae was consideraljly lower than the 

 tolerances of the larvae. Therefore, even a con- 

 centration of a jiesticide that showed no effect 

 on eggs or larvae might, indirectly, if used in the 

 field, markedly reduce the growth of bi\-alve 

 larvae by killing or ])revenling rein'odnctiou of 

 the alsae that serve as foods. 



We believe the residts given in table 1 are, 

 at least primarily, the direct effect of these com- 

 pounds on the embryos or larvae themselves 

 because we are not dependent upon reproduction 

 of the algae in oiir larval cultures. We add the 

 food cells to our experimental cultures daily, and 

 the pesticide would have an indirect effect through 

 the food chain only if it destroyed the food cells. 

 The concentration of Snlmet (sodium sulfameth- 

 azine) used routinely as a bactericide in our 

 larval cultures, for example, is sufficient to inhibit 

 or prevent reproduction of the algae used for food, 

 yet it lias no adverse effect on growth of larvae 

 under our laboratory conditions of feeding. 



Significance of TL,„ Values 



In table 2 we have listed the 24-hour TL^ (the 

 concentration, in p.ii.m. that would cause an 

 apjiroximate 50-percent reduction in the number 

 of eggs developing into normal straight-hinge 

 larvae) for oyster and clam eggs. Also listed are 

 the 12-day TLn, for clam larvae and the 14-day 

 TL„, f()r oyster larvae. We believe the TL^ values 

 listed are of value only for rough comparisons of 

 toxicity because some compounds drasticall}- re- 

 duce the rate of growth of larvae at concentrations 

 too low t(( cause appreciable mortality or may kill 

 embryos at lower concentrations than are required 

 to affect growth or survival of larvae. Both endrin 

 and dieldrin, for example, had 14-day TLn,'s for 

 oyster larvae greater than 10 p. p.m., yet either of 

 these compounds, at concentrations of only 1 

 ]).p.m., reduced the rate of growth of these larvae 

 drastically. Other compounds, such as Nemagon, 

 Aldrin, and toxajjliene, permitted development of 

 embiyos at considerably higher concentrations 

 than those at which the larvae could survive and 

 grow. Conversely, other com]iounds, such as 

 griseofulvin (on clams) and Endotlial (on oysters), 

 almost completely stopped embryonic develop- 

 ment at concentrations too low to afl'ect seriously 

 survival and growth of the larvae. 



In comparison with the TLm values given for 

 other species, the rankings of Amitrol, Endotlial, 

 Omazene, and Phygon are the same for clam and 

 oyster larvae as Bond, Lewis, and Fryer (1960) 

 found for largemouth bass, Mknypterus salmoides, 

 and two si)ecies of salmon, Oncorhynchus kisutch 

 and 0. tshmvyfscha. The median tolerance limits 

 for tlie least tolerant stages in the life cycle of 

 clams and oysters for these comjiounds, however, 



EFFECTS OF PESTICIDES OX DEVELOPMENT OF CLAMS AND OYSTERS 



307 



