SANDIFER: LARVAL DEVELOPMENT OF GRASS SHRIMP 



50 n 



30*C 



MOLTS 



Figure 4. — Percentage of animals molting to postlarva 

 at each molt under different temperatures. 



for fewer instars in 15/rf than in other salinities. 

 The temperature-salinity interaction also was 

 not significant. However, the influence of tem- 

 perature was significant at the 19c level, and 

 mean numbers of instars ±: one standard devi- 

 ation were 8.5 ± 1.0 at 20°C, 7.8 ± 1.1 at 25°C, 

 and 8.4 ± 1.0 at 30°C. A multiple mean test 

 showed no difference between the mean numbers 

 of larval instars passed at 20° and 30°C but 

 indicated that animals reared at 25°C passed 

 through significantly fewer instars in larval de- 

 velopment. 



DISCUSSION 



Few previous studies have been concerned 

 with the eflJ'ects of temperature and salinity on 

 Palaemonetes larvae. Sollaud (1919) reared 

 larvae of P. varians microgenitor in the labora- 

 tory and found, as I did for P. vulgaris, that de- 

 velopment was retarded at low temperatures and 



in low salinities and that more instars were 

 passed at the lower than at the more moderate 

 temperature tested. According to Broad and 

 Hubschman (1962), development of larvae of 

 P. intermedms, P. pugio, and P. vulgaris was un- 

 affected by salinity above 20V.V, but below 10%o 

 survival was poor. In the present study, sur- 

 vival in 5',( was very poor, but in salinities of 

 10 to S0'/(( at low and moderate temperatures 

 (20° and 25°C), survival was high. More re- 

 cently, Knowlton (1970) conducted a factorial 

 experiment similar to mine, but he used only 

 five larvae in each temperature-salinity combi- 

 nation. Knowlton (1970) found that at 20° and 

 25°C P. vulgaris larvae seemed to tolerate the 

 entire range of salinity tested (15 to So.'/r ) equal- 

 ly well, with highest survival among larvae 

 reared at 25°C. Lowest survival occurred among 

 larvae reared at 30°C, where no larvae exposed 

 to the low salinities (15 and 20^'^) completed 

 development. The results of the present study 

 were fairly similar, except that some larvae sur- 

 vived through metamorphosis at 30°C in all sa- 

 linities but 5%c. However, Knowlton's (1970) 

 values for mean duration of larval life (37.3 ± 

 2.0 days at 20°C, 30.7 ± 2.0 days at 25°C, and 

 31.1 ± 4.3 days at 30°C) were considerably 

 greater than corresponding values in the present 

 study (30.2 ± 3.8 days, 16.6 ± 2.7 days, and 15.7 

 ± 1.8 days, respectively). Similarly, his values 

 for mean instar duration were greater than val- 

 ues determined here. 



The number of larval instars varied from 8 

 to 16 in Knowlton's (1970) study, while in the 

 present study the observed range was 5 to 12. 

 Knowlton (1965,1970) also found that the num- 

 ber of larval instars increased with increasing 



Table 4. — Summary of analysis of variance for differences in number 

 of larval molts for Palaemonetes vulgaris larvae at different temper- 

 atures and salinities. 



** Significant a\ 1% level. n.s. Not significant. 



1 See Snedecor (1956) for computation of the error mean square in the method of un- 

 weighted means. 



119 



