EFFECTS OF TEMPERATURE AND SALINITY ON 



PRODUCTION AND HATCHING OF DORMANT EGGS OF 



ACARTIA CALIFORNIENSIS (COPEPODA) IN AN OREGON ESTUARY 



John Kenneth Johnson' 



ABSTRACT 



Experimental results indicate that induction of dormancy in Acartia californiensis eggs is temperature 

 dependent and occurs below 15^ C in two ways: 1 ) Cold adapted females spawn true resting eggs which 

 exhibit major differences in hatching and survival rates from nondormant eggs under similar condi- 

 tions. 2) Nondormant eggs spawned above 15° C may become dormant and have short-term viability at 

 temperatures below 15° C. Salinity does not induce dormancy. 



Hatching results offield-collected resting eggs at naturally occurring temperature- salinity combina- 

 tions demonstrate that termination of dormancy is also primarily temperature dependent. Salinity, 

 however, regulates rate and success of hatching. In addition, heavy naupliar mortality occurs following 

 hatching at low salinities. Substantial hatching must occur in the field over much of the year. Since 

 subsequent survival and population growth depends on the presence of favorable temperature and 

 salinity conditions, nauplii which hatch during the low salinity winter and spring months in Yaquina 

 Bay must be lost. This phenomenon is viewed as a "leaky" population diapause. 



Resting eggs were also demonstrated for Epilahidocera longipedata and Eurytemora affinis, an 

 occurrence previously undescnbed in the literature. 



Resting eggs have been known to be a common 

 adaptation in freshwater zooplankton since the 

 turn of the century (see reviews in Hutchinson 

 1967 and Elgmork 19671. The existence of a com- 

 parable resting egg phase in the life cycle of 

 marine neritic species was postulated for many 

 years to explain the seasonal disappearance of 

 coastal species from the water column (e.g.. Fish 

 and Johnson 1937; Barlow 1955: Conover 1956). 

 Preliminary evidence of marine calanoid resting 

 eggs was first reported by Sazhina ( 1968) for the 

 species Pontella mediterranea and Centropages 

 ponficiis. 



Zillioux and Gonzalez 1 1972) conclusively dem- 

 onstrated with laboratory and field evidence that 

 the seasonal disappearance of Acartia tonsa , a 

 common coastal species, coincides with the pro- 

 duction of overwintering eggs as water tempera- 

 tures fall below 14.5' C. Subsequent research has 

 shown that egg dormancy is an important adapta- 

 tion in many boreal and temperate neritic 

 calanoids, including both summer-fall species 

 (e.g., Tnrtanus fnrcipatus, Kasahara, Onbe, and 

 Kamigaki 1975; Lahidocera aestiva , Grice and 

 Gibson 1975) and winter-spring species (e.g., .4. 



'School of Oceanography, Oregon State University, Corvallis, 

 OR 97331. 



clausi , Uye and Fleminger 1976; C. ahdominalis. 

 Pertzova 1974). Egg dormancy probably enables 

 most coastal species to survive periods during 

 which conditions in the water are unfavorable. 



Environmental factors such as temperature or 

 photoperiod usually govern the induction of dor- 

 mancy in arthropods (Lees 1955). Both the adult 

 and/or the egg may be responsive to adverse en- 

 vironmental changes. Egg dormancy may result 

 from a physiological response of the female to a 

 changing milieu which modifies the eggs. Con- 

 versely, dormancy may develop as a response of 

 the egg to changes in conditions as it sinks 

 through the water column into the bottom sedi- 

 ments. There is evidence for both mechanisms in 

 marine copepods. Zillioux and Gonzalez (1972) 

 demonstrated that the production of resting eggs 

 by A. tonsa is a response of the female to low 

 temperatures. However, Uye and Fleminger 

 (1976) examined egg development of four Acar- 

 tiaspecies (including A. tonsa) from southern 

 California waters at various temperature and sa- 

 linity combinations and concluded that dormancy 

 is primarily a response of the egg to the milieu. 

 Their results led them to hypothesize that expo- 

 sure of the eggs to abnormal salinities may be 

 necessary to induce dormancy in at least two of the 

 species of Acartia . Once buried in the sediments. 



Manuscript accepted March 1979 



nSHERY BULLETIN: VOL 77. NO 3. 1980, 



567- 



