JOHNSON EFFECTS OF TEMPERATURE AND SALINITY ON ACARTIA CAUFORME.ySIS EOGS 



tence of dormancy at 5%o was nearly complete with 

 only 2% hatch achieved. 



The nature of continued dormancy at 10° C 

 (Figure 11) was unusual in that both diapause and 

 prehatch holding cooccurred. Dormant eggs re- 

 maining on day 13 at 25%<i, 15%", and 10%o ap- 

 peared to still be in diapause since no apparent 

 change in coloration or internal structure oc- 

 curred during incubation at 10° C. A subsequent 

 1-3 day delay in hatching after temperature was 

 increased to 15° C is also indicative that the eggs 

 were still in diapause (as seen in the October 

 temperature experiments: Figure 5C, D). Cope- 

 podites reared from nauplii hatched at 15° C in the 

 25%o. 15%". and 10'*/im treatments after day 13 were 

 found to be mainly A. californiensis (87%; n = 23). 

 As a result, most of the eggs remaining in 

 diapause at 10° C over the 25-10%o range were 

 probably those of A. californiensis. 



In contrast to results at 25-10"/i>o (Figure IIC), 

 diapause did not persist in eggs of either A. 

 californiensis or A. clausi exposed to 5%o at 10° C. 

 The reason for this difference is not known. While 

 only 2'7f of the eggs had hatched, nearly all re- 

 maining eggs had broken diapause and were in the 

 final prehatch holding state many days before the 

 temperature increase on day 13. The difference in 

 the nature of dormancy is reflected by the rapid 

 hatching rate with no delay period following the 

 temperature increase. In spite of high viability 

 seen after day 13, the termination of diapause and 

 the failure to hatch at 5%o and 10° C demonstrate 

 that egg survival of both Acartia species was 

 short-term and limited by available energy re- 

 serves. 



Posthatch naupliar mortality within the first 24 

 h as a function of salinity at 15°, 12.5°, and 10° C 

 was very similar to that shown for 17° C (Figure 

 10). The mortality range in 15%.., 10%.., and 5%o 

 water, for example, was 2-7%, 12-22%, and 80- 

 100%, respectively, as compared with 2%, 15%, 

 and 78% at 17° C. Typically, the percent survival 

 decreased slightly at a given salinity as tempera- 

 ture decreased. Therefore, at salinities -10%ii, 



survival was high with mortality primarily in- 

 creasing with decreasing temperature. Below 

 lO'Km, the NI nauplii experienced increasingly 

 heavy mortality primarily as a function of de- 

 creasing salinity. 



Additional information on hatching behavior 

 and fraction of A. californiensis resting eggs were 

 obtained by a comparison of the proportions of 

 copepodites reared from nauplii which initially 

 hatched from unsorted egg mixtures at each 

 temperature-salinity combination (Table 3). 

 Hatching at the most optimal of the given experi- 

 mental conditions for both species (15°C, 25-15%o) 

 resulted in copepodite proportions of 51% and 62% 

 A. californiensis. A similar percentage (54%) was 

 observed at 12.5° C and 25"/oo. As these estimates 

 are based on independent rearing treatments, it is 

 reasonable to conclude that roughly 55% of the 

 field resting eggs were those of A. californiensis. 

 This result corresponds very well with other esti- 

 mates of percent abundance determined in earlier 

 unpublished experiments. 



Copepodites of A. californiensis were absent in 

 the cultures which initially hatched in 10"/oo and 

 5'U at both 15° and 12.5° C (Table 3). As nearly all 

 resting eggs were previously found to terminate 

 diapause at these lower salinities (Figure 11 A, B), 

 the absence of A. californiensis is the result of 

 mortality during either prehatch holding or 

 subsequent naupliar stages. This is verified, in 

 part, by the increasing mortality of NI nauplii 

 {Acartia spp.) below 12. 5"/(m (Figure 10). Further- 

 more, hatching rate differences over the range of 

 25-10%ip were small as shown in Figure llA, B. For 

 example, at 15° C, a total hatch of 86% and 81% 

 was observed at 15%o and 10%o, respectively. Yet, 

 the proportion of A. californiensis copepodites was 

 62% at 15"/oo and 0% at 10%.. (Table 3). Similarly at 

 12.5° C, only 6% survived to copepodites when 

 hatched at 15"/.... as compared with 54% at 25%o. It 

 must be reemphasized that the temperature and 

 salinity values referred to here and in Table 3 are 

 hatching conditions only. Rearing was under more 

 favorable conditions (see Methods). Therefore, as 



Table 3. — Proportion of Acar/m californiensis and A. clausi copepodites that survived following hatching at 

 12 temperature-salinity combinations. Temperature and salinity levels for rearing to copepodite stages were 

 gradually increased to 1.5° C and 25%o to increase p.">sthatch survival (see text for further details.. 



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