FISHERY BULLETIN: VOL 



diapause was ended at all salinities at 12.5°-15°C, 

 it is evident that individuals hatching from rest- 

 ing eggs of A. californiensis exposed to salinities 

 below 15%o experienced early death, even when 

 temperature and salinity were increased to more 

 favorable levels following hatching. As a result, 

 these data can be used to define minimal hatching 

 conditions for growth to maturity and can be corre- 

 lated with the fall disappearance and summer re- 

 population of A. californierislfi in Yaquina Bay. 



Acartia californiensis copepodites were absent 

 at all salinities at lO'C (TableS). Absence over the 

 range of 25-10%(i was probably the result of con- 

 tinued diapause of the resting eggs of A. califor- 

 niensis . as previously demonstrated (Figure 1 IC). 

 Some of the latter eggs may have hatched at 25%o, 

 given the likelihood of a 62% hatch (Figure IIC) 

 and an estimated resting egg ratio of 55% (Table 

 3). However, egg hatching at 15"/im and 10"/on (40- 

 33%; Figure IIC) can be completely attributed to 

 A. clausi since it composed ca. 45% of the resting 

 eggs. 



An unrelated but important observation con- 

 cerns the hatching of a few Epilabidocera lon- 

 gipedata Sato (= E. amphitrites McMurrick) 10° 

 C, 25%o) and Eurytemora affinis (Poppe) (10-15° 

 C, 5-25%o) from the unsorted resting egg mixtures 

 used to obtain Acartia spp. ratios. Identification 

 was made at the late copepodite stages. Neither 

 species has previously been reported as possessing 

 a resting egg stage. Both species are absent at 

 Station 39 in Yaquina Bay during the winter 

 months, insuring that the eggs were in diapause 

 when collected. 



DISCUSSION 



Environmental Conditions Resulting 

 in Egg Dormancy 



Many shallow-water neritic and estuarine 

 calanoid species with multivoltine life cycles are 

 now known to survive long periods of adverse 

 environmental conditions by facultative produc- 

 tion of resting eggs. Field observations and 

 laboratory results have demonstrated that this is 

 true for A. californiensis in Yaquina Bay. After 4 

 or 5 successive generations with substantial 

 recruitment (July-September 1972; Figure 2), the 

 planktonic population declined rapidly and was 

 gone by mid-November. The failure of recruitment 

 in early October coincided with a decline in 

 temperature below 15° C. Salinity remained rela- 



tively high and stable at 25-30%.., during the popu- 

 lation disappearance, implying temperature de- 

 pendence for the production of resting eggs (Fig- 

 ures 3, 4). 



Experimental results confirmed the hypothesis 

 that diapause in A. californiensis eggs is essen- 

 tially a response of the spawning females to low 

 temperatures, similar to that shown for A. tonsa 

 (Zillioux and Gonzalez 1972), Tortanus forcipatus 

 (Kasahara, Onbe, and Kamigaki 1975) and possi- 

 bly Pontella meadi (Grice and Gibson 1977). The 

 shift from summer egg to resting egg production 

 occurred between 15° and 13° C (Figure 5), a tem- 

 perature range comparable to that observed in the 

 field. Salinity was not a factor, since it was main- 

 tained at a constant and favorable level (25%o) in 

 all treatments. Food quantity and quality were 

 excluded as factors by daily providing the adults 

 with an ample ration consisting of three prey 

 species. Photoperiod is known to influence induc- 

 tion of diapause in some cladocerans (cf.: Daphnia 

 magna, Bunner and Halcrow 1977; D. pulex, 

 Stross and Hill 1965). However, it was eliminated 

 as a possible extrinsic factor by the use of continu- 

 ous lighting. 



The production of overwintering eggs was most 

 likely initiated by changes, possibly hormonal, 

 within the female in response to the extrinsic 

 stimulus of adverse temperature. Extensive re- 

 search on the physiology of insects has confirmed 

 the regulation of diapause by hormones ( e.g.. Lees 

 1955; Slama et al. 1974). Moreover, Carlisle and 

 Pittman (1961) found differences in forebrain 

 neurosecretions between summer and over- 

 wintering "dormant" CV copepodites of Calanus 

 finmarchicus that resembled diapause in insects. 

 Watson and Smallman (1971) similarly reported 

 significant changes in small tissue patches in the 

 head region of the cyclopoid copepod, Diacylops 

 navus, that correlated with induction and cessa- 

 tion of diapause. Since dormant egg production 

 can be rapidly induced in A. californiensis by low- 

 ering water temperature, it is probable that the 

 controlling physiology is somewhat different. 



The temperature-dependent maternal role in 

 the induction of dormancy in A. californiensis eggs 

 is contrary to results reported by Uye and 

 Fleminger ( 1976) for A. californiensis ( = Acartia 

 sp. I) in a southern California lagoon. They con- 

 cluded that egg dormancy in A. californiensis 

 must occur independently of maternal influence 

 since 90-100% of the eggs hatched at all tempera- 

 tures in the annual field range (10°-25° C). Expo- 



580 



