JOHNSON: EFFECTS OF TEMPERATLTRE AND SALINITY ON ACARTIA CALIFORMENSIS ECWS 



Incubation Time (days) 



Figure 9. — Hatching success and viabi lity ( "holding" success! of 

 Acartia spp. at 23.5%<i salinity I dashed line), following variable 

 periods of exposure to low salinities i0-5%o) at 17° C. Initial 

 hatching results at 23.5%o used as a control. 



Prehatch mortality also increased gradually 

 with increasing time at low salinity (Figure 9). 

 Total hatch in 23.5%ii after 2.5 and 12 days expo- 

 sure to 5%o was 87% and 79%, respectively. This 

 corresponds to a loss of 8% viability within the first 

 2.5 days and 16% by day 12 as measured against 

 the control hatch at 23.5%o. A comparable trend 

 and loss rate occurred for 0"/oo salinity-exposed 

 eggs later hatching in 23.5%o. Losses in viability 

 on days 4.5 and 12 were 27% and 36%, respec- 

 tively. 



The cause of death in prehatch nauplii is un- 

 known, but presumably is related to exhaustion of 

 energy reserves during the holding period. Mor- 

 tality may also be partially caused by increased 

 osmotic stress at progi'essively lower salinities. 

 Eggs that died prior to hatching usually dis- 

 integrated with 2-3 days at 17° C. 



The occurrence of naupliar mortality during the 

 actual hatching process or within the following 24 

 h substantially increased at salinities below 

 12.5%o. This is shown by a coinparison of cumula- 

 tive hatching and subsequent naupliar mortality 

 as a function of salinity on days 4.5 and 12 (Figure 

 10). Naupliar losses on day 4.5 increased from 1% 

 at 12.5%o to 73% at 5%,i. Mortality was 100% at 

 2.5%o and 0%o. Corresponding losses were 5-10% 

 higher on day 12, indicating that survival follow- 

 ing hatching decreased gradually with increasing 

 time required to hatch. 



Observation of hatching success in salinities 

 below 8"/oo revealed that many nauplii died during 

 the hatching process or immediately thereafter. 

 At 5%o, many of the dead nauplii were found only 

 partially freed from the cracked-open eggshell. 



More typically, the nauplius was found lying next 

 to the empty eggshell, indicating immediate death 

 followed hatching. Some nauplii successfully 

 hatched, but never unrolled. Others escaped the 

 outer egg membrane, but died while still inside 

 the osmotically swollen inner membrane. Eggs 

 which had obviously cracked open prior to full 

 development, and exhibiting an extrusion of cellu- 

 lar debris, were also occasionally seen at 0%u and 

 2.5%o. 



The specific effects of salinity on resting eggs of 

 A. californienfsis cannot be separated from those of 

 A. clausi in the results above. However, support- 

 ive evidence indicates that eggs of both Acartia 

 species hatched at substantial rates at all 

 salinities from OX., to 23.5%ii. For instance, 41% of 

 the copepodites reared from viable nauplii during 

 the course of the experiment were A. californien- 

 sis. While there is no information on possible dif- 

 ferential mortality during culturing, an estimate 

 of 419; is probably low for the percentage of A. 

 californiensis resting eggs in the experiment. Ini- 

 tial rearing conditions actually favored A. clausi. 

 a species that is much moreeuryhalinein Yaquina 

 Bay. Thus, even when considering the lowest 

 hatching rate at 0%" salinity on day 17 (Figure 7), 

 it is clear that the 37% hatch ( 100% mortality ) had 

 to include at least some A. californiensis eggs. 

 Approximately another 40-50% of the holding 

 eggs at 0%o were no longer viable. Therefore, one 

 can conclude that prolonged exposure of A. 

 californiensis resting eggs to extremely low 

 salinities at 17° C will result in death for the 

 majority, whether it be from prehatch or posthatch ' 

 salinitv stresses. 



Salinity (%o) 



FIGITIE 10. — Comparison of cumulative hatch of field-collected 

 resting eggs of Acartia spp. i open circles) and subsequent mortal- 

 ity within first 24 h (solid circlesi at salinities from 0%o to 23.5\(i. 

 Results on day 4.5 (dashed line) and 12 (solid line) based on 3 

 replicates of 35 eggs at each salinity. 



577 



