transformed values (G) for measurements of 

 growth are generally accurate to about ±10 per- 

 cent for oyster larvae and to about ± 5 percent for 

 clam larvae. Tlie confidence limits for oyster larvae 

 were not narrowed appreciably- even by increasing 

 N to 300; therefore, it was considered impractical 

 to increase the number of measurements of oyster 

 larvae sufficiently t-o reduce the error to that of 

 measurements of clam larvae. 



Errors involved in our teclmiciues for determin- 

 ing numbers of larvae developing from fertilized 

 eggs to straight-hinge larvae or the number sur- 

 viving in growth experiments liave been found to 

 be about ±10 percent. 



In any event, the effects of test compounds on 

 larval growth and survival are readily distin- 

 guished, from random variation, by the regular 

 stepwise rediu'tion at each successive increase in 

 concentration of the test cliemical. 



EFFECTS OF DIFFERENT COMPOUNDS 

 ON EMBRYOS AND LARVAE 



Table 1 shows the relative percentage of fertil- 

 ized oyster and clam eggs that developed through 

 normal embryonic stages into straight-hinge 

 larvae, the relative percentage of larvae that 

 survived, and the relative percentage increase in 

 mean length when exposed to various pesticides 

 and chemicals. We calculated the relative per- 

 centages, as has been previously stated, by using 

 the survival and rate of growth of larvae in the 

 control cultures of each experiment as 100 percent. 

 The values given (except where noted) are aver- 

 ages for duplicate cultures at each concentration 

 in each experiment. When more than one experi- 

 ment was run, we combined the results of all 

 experiments. 



Some compounds were more toxic to embryos 

 than to larvae (despite the much shorter exposure 

 period of the embryos), although the reverse is 

 generally indicated in our tests. The differences 

 between the tolerance of developing embryos and 

 larvae of oysters to the same pesticide are strik- 

 ingly evident from the effects of the weedicides, 

 Amitrol and Endothal (table 1). Embryos de- 

 veloped normally in higher concentrations of 

 Amitrol (500 p. p.m.) tiian those at which larvae 

 showed good growth (100 p. p.m.). In contrast, 

 eggs could tolerate only 10 p. p.m. of Endothal, 

 whereas larvae showed about normal growth at 

 concentrations as high as 50 p. p.m. 



The compounds we used in our tests differed 

 widely in chemical composition and presumably 

 have different modes and sites of action. It is not 

 too svu-prising, therefore, that although the toxic 

 levels of most compounds are about the same for 

 clams and oysters, some are appreciably more 

 toxic to one than to the other. The tolerance of 

 oyster larvae to Endothal, for example, was con- 

 siderably greater than that of clam larvae (al- 

 though oyster embryos were slightly less tolerant 

 tlian Were clam embryos). Oyster larvae showed 

 fair survival and normal growth at 25 p. p.m., 

 wliereas this concentration caused 100 percent 

 mortality of clam larvae. In general, however, the 

 rates of growth of clam larvae are less affected by 

 toxicants than growth of oyster larvae. 



As has been reported previously (Davis, 1960), 

 some of the lower concentmtions of certain com- 

 pounds significantly accelerated growth of larvae 

 (notably Seviu, Endothal, 2-4-D salt, phenol, and 

 Sulmet — table 1). Although tlie reasons for this 

 ]5lienomenon are not clear, we believe it is the 

 result of the bacteriostatic or, possibly, chelating 

 effect of these compounds. Because growth of 

 clam larvae is less affected by bacterial and 

 algal toxins than is growth of oyster larvae, any 

 bacteriostatic or chelating effect these compounds 

 might have would be expected to have a less 

 marked eft'ect on growth of clam larvae than on 

 growth of oyster larvae. 



Synergistic Action of Solvent With Some Compounds 



With certain pesticides, the solvent may act 

 as a synergist and increase the toxicity of the 

 compound, but with other pesticides the same 

 solvent may show no such action. Acetone ap- 

 peared to act as a synergist with Co-Ral but not 

 with Di-Syston and Phygon. In exj)eriments 

 when the stock solutions of these three water- 

 insoluble compounds were made up in acetone 

 (appendix), the pair of control cultures receiving 

 100 p. p.m. acetone (the maximum concentration 

 used in any of the experimental cultures ) showed 

 no significant reduction in growth or survival of 

 either clam or oyster larvae. Survival and growth 

 of clam larvae receiving Di-Syston and Phygon 

 decreased progressively as the concentrations of 

 these compounds increased, just as it did in 

 various concentrations of the water-soluble toxi- 

 cants (table 1). The toxic eft'ects of Co-Ral, how- 

 ever, show a definite break in the middle of the 



396 



U.S. FISH AND WILDLIFE SERVICE 



