])priiKl (if 10 (lay> lur clam lar\ ae oj' 12 days for 

 oyster larvae. Thus, when ihf' f|iianlitati\ e 

 samples were taken at the end nf the peridd, llie 

 lai\ ae wci-e I'J and 14 days did. ies|ieiMi\'ely. 

 These periods i-epresent the norma! time of settiui; 

 ■for the two specie> undei' ^ood environmental 

 conditions at the tempei-atures used (24°±1° C). 

 We feil the test larv-ae a mixture of li\'e flagellates, 

 "■enerally at a rate of 0.01 ml. of packed cells to 

 each 1-liter cnltnrc per day. Sea water and test 

 compound were I'enew ed in the cultures every 

 second day. Tlie contents of a culture vessel were 

 washed onto a siaiidcss steel sci'een that retained 

 the larxae but allowed the sea water and dis- 

 solved ])esticide to pass through, 'i'he Pyre.x '' 

 culture A'essels used in the experiments were then 

 ihorouuhlv w a>hed before the lar\ae and the fresh 



solution of pesticide in sea water were again added. 

 The renewii.1 of tlie culture medium every second 

 day in this manner mininuzed the buildup of 

 hai'uifui metaiiiilites and made it possibk' to 

 maintain a<-cm'ately the concentration of tc^st 

 compound at the desired level. 



Quantitative samples, consisting of 1.(1 pei'ceul 

 of the total lar\al population of a culture, wei'e 

 taken at the end of the 2-day and the 12- or 14-day 

 experimental periods and preserved for inicrosco[)ic 

 exannuation. We determined the survival by 

 coiniting the number of larvae that had been 

 living at the time of preservation. Survival Aalues 

 (table 1, cols. 1, 2) are expressed as a |)ercentage 

 (R) of the sur\i\'.-d in control cultures and were 

 calcidated as follows: 



,j__Av(rage number of larvai' in cxpcriniintal '-'"Hares 

 Average number of laivae in control cultures 



Growth, or in<'rease in mean lengths in the 12- mean length of l.ar\ae in control cultures and was 

 and 14-day experiments (table 1. col. 3), is calculated as follows; 

 exjiressed as a i>ercentage (G) of the increase in 



, _ Mean length of larvae in ex perimental cultures — mean le ng th at 2 days ^^ 

 Mean h'nffth of larvae in i-onlrol cultures — mean lensth at '2 days 



Mean length was determined l)y measuring the 

 maxinmni length of the shell, parallel to the hinge 

 line, of ll)l)-oy>U'i' lar\ae oi' of 50-clam lar\ ae 

 Friuu the preser\e(.l samples. The expression of 

 results as percentages of the increase in mean 

 length and sui'\ i\ al of lar\ae in ciuitrol cultures 

 uuide possible the dilcct comparison of data from 

 the different experiments. 



Measurements of elfccts of toxicants on growth 

 and survival of larvae aic subject to ciuisiderable 

 error due to randcuu >am])hug err(U' and x'ariatiiuis 

 introduced b_y the slight unconi lolled cnviriui- 

 UK'iital differences between lailtuics. In additiiui, 

 changes in sea Water and food ipialitx' between 

 successive ex])eriments undcuibtedly caused vaiia- 

 tion in observed efTects. These factors must be 

 l.-iken into considerali<ui in judging reliability of 

 the data and in ascertaining safe and harmful 

 levels of the test compound. 



The standard error in nu>asiireinents of growth 

 is considerably greater for oyster larvae than hir 

 clam larvae. A length-frec(uency distribution of 

 clam larvae receiving a given treatment is highly 

 kurtotic, whereas that for oyster larvae exhibits 



this centi'al tendency to a much lesser degree. 

 Figure 1 allows the 95 percent confiilence limits of 

 the nu'an (±2SE;„) in micnuis. with N = 100for 

 oyster larvae and i\ = 5() for clam larvae, at the 

 xaiious mean lengths encountered in luu' 

 exi^erinuuital cultures. 



After considering the variations encountered, 

 in addition to sampling errors, we believe our 



ME4N LENGTH (MICRONS) 



3 Trade names refeiied to ia tliis publication do not imply endoiseinent of 

 commercial products. 



FuaiiE 1. — The 9.5 percent confidenee limits of mia--iiii- 

 meiits of mean lengths of oyster and clam lar\;ie of 

 different sizes. \'aln<'s indicate ±2 SK,„. X= lOfl for 

 oyster larvae; N = 50 for clam lar\ae. 



KFFECTS OF rESTICIDES ON DEVELOl'MEXT OF CL.V.MS AND OYSTERS 



Hur, 



