iHN'SON EFFECTS OF TEMPERATURE AND SALINITY ON ACARTIA CAUFnR.XIENSlS EGGS 



sure to salinity extremes was suggested as a possi- 

 ble cause of induced dormancy. Their conclusion, 

 however, was based upon the hatching behavior of 

 A. californiensis eggs which were spawned at 17.5" 

 C. On the basis of my observations (Figure 5), 

 these latter eggs were most likely all summer eggs 

 which exhibited increasing dormancy below 10' C. 

 It is likely that female-induced dormancy would 

 have been observed if a spawning temperature 

 below 15 C had been used. 



Summer eggs of A. californiensis possess the 

 capacity for short-term facultative dormancy 

 when exposed to temperature below 15° C (Figure 

 5). Hatching resumed within hours following 

 temperature elevation above 15° C. This type of 

 arrested development, temporarily induced by 

 unfavorable external conditions and ended with 

 the return of a favorable environment, represents 

 a state of "quiescence" as the term is used by An- 

 drewartha ( 1952), Lees ( 1955), and Wigglesworth 

 (1972) for other arthropod groups. 



Quiescence of nondormant eggs at low tempera- 

 tures is probably a characteristic of most calanoid 

 species which inhabit highly variable environ- 

 ments such as estuaries and lagoons (Uye and 

 Kasahara 1978). For example, Uye and Fleminger 

 (1976) found that A. tonsa eggs which were 

 spawned at 17.5° C (a favorable temperature) 

 exhibited dormancy only at 7.5° and 5' C, a result 

 which they also demonstrated for A. californien- 

 sis. In both species, survival during dormancy at 

 7.5° and 5°C wasof short duration, since no hatch- 

 ing occurred following a temperature elevation 

 after 28-30 days. The lack of long-term viability is 

 supporting evidence that the respective eggs were 

 in a quiescent state and not true resting eggs. 



In each species, the percentage of quiescent 

 summer eggs increased as temperature decreased. 

 However, quiescence occurred at significantly 

 higher temperatures in eggs of A. californiensis 

 from Yaquina Bay, shown by a 359^ hatch at 10° C 

 (Figure 5D), as compared with 1009c fortTie south- 

 ern California population (Uye and Fleminger 

 1976). Both sets of eggs were spawned at 17 or 

 17.5° C. The considerable difference in thermal 

 induction of quiescence in summer eggs may rep- 

 resent a genetic gradient reflecting the latitudinal 

 separation of the two populations. Selection for 

 quiescence in this warmwater species is probably 

 more important in Oregon estuaries because of 

 lower water temperatures (2°-22° C range) and 

 longer winters (Figure 4). Less of a selective ad- 

 vantage would exist in California estuarine and 



lagoonal waters with a narrower annual range of 

 10°-25° C (from Uye and Fleminger 1976). Fur- 

 thermore, any genetic gi'adient caused by differ- 

 ential selective pressures would be reinforced by 

 the localized confinement of populations within 

 estuaries or lagoons, which must greatly reduce 

 gene flow. 



The adaptive value of quiescence may be 

 greatest in temperate estuaries (e.g., Yaquina 

 Bay) where eggs in the bottom sediments typically 

 experience large variations in temperature over 

 successive tidal cycles. However, since viability of 

 A. californiensis eggs in the quiescent state at low 

 temperatures is limited to 1-2 mo, as shown above 

 (Figures 5, 6) and in figure 5F of Uye and 

 Fleminger ( 1976), the importance of quiescence in 

 overwintering must be considered negligible. 



Summer and resting eggs of A. californiensis 

 may cooccur in equal proportions in the cumula- 

 tive spawn at temperatures below 15 C as shown 

 in the November experiment with females which 

 spawned at their field acclimation temperature of 

 12° C (Figure 7). Zillioux and Gonzalez (1972) re- 

 ported similar spawning results for A. tonsa 

 females at acclimation temperatures of 9°, 11.4°, 

 and 14.5° C. In each case, approximately 50-60'7f of 

 the eggs were nondormant and hatched. It is not 

 known from these data if the same female can 

 produce both egg types at once. It is likewise not 

 known if a female producing only resting eggs at 

 lower temperatures can switch back to summer 

 egg production if temperature increased above 15° 

 C. These questions will need to be resolved by 

 observations on individual females. 



There is an apparent discrepancy between the 

 November and October observations. Females col- 

 lected from 15° C water in October produced exclu- 

 sively dormant eggs when rapidly cooled to 13° or 

 9 C. It is reasonable to have expected all of the 

 November eggs spawned at the field acclimation 

 temperature of 12° C to have also been dormant, 

 given the October results. Perhaps there are ef- 

 fects upon the initiation of dormant egg produc- 

 tion from both low temperature to which a female 

 is fully acclimated and from sudden temperature 

 reductions. Response to the latter stimulus would 

 protect against more than usually moderate cool- 

 ing rates in the fall. Given normal field conditions, 

 however, there probably is considerable variation 

 between individual females in the population in 

 the threshold temperature which induces dormant 

 egg production. As a result, cooccurrence of both 

 egg types would be expected during a significant 



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