JOHNS(JN EFFECTS OF TEMPERATURE AND SALINITY ON ACARTIA (--Al.lFORyiENSIS EGGS 



laboratory and storage in the dark for 1 mo. Rest- 

 ing eggs were removed (10 March 1976) by first 

 sieving the sediments through 119 /xm and 64 /xm 

 mesh nylon screens. This size fraction was diluted 

 with water (5%"), stirred well, and allowed to set- 

 tle. Appro.ximately 5-6 ml of the surface sediments 

 were then slowly introduced by pipette onto the 

 sui-face of 8-10 ml of a 2.8 M aqueous sucrose 

 solution (suggested by Brewer 1964) in each of 

 several 15 ml centrifuge tubes. Following cen- 

 trifugation at 1,000-2,000 rpm for 1-2 min, rest- 

 ing eggs of Acartm spp. and other species (includ- 

 ing calanoids, harpacticoids, and rotifers) were 

 removed by pipette from the water-sucrose inter- 

 face. These eggs were nearly free of detritus. A 

 thorough rinse with Millipore-filtered water (5'!<ki) 

 on a 64 /xm mesh screen removed the sucrose solu- 

 tion. No evidence of egg distortion or crushing was 

 found to result from the high osmotic gradient. In 

 contrast, eggs centrifuged from surface sediments 

 collected in September collapsed under similar 

 conditions. 



Following rinsmg.Acartia spp. eggs were sorted 

 from extraneous material and then mixed in a 

 Petri dish in 5%o water. Approximately 2,000- 

 2,500 eggs were collected in 3-4 h of work. Most 

 eggs had a clear, outer layer which surrounded a 

 dark inner mass, similar to that reported for rest- 

 ing eggs of Labidocera aestivalGrice and Gibson 

 1975). A moderate number of eggs (ca. 10-209( ), 

 lacking the clear layer, had ended diapause and 

 progi'essed to various stages of embryogenesis. 

 These latter eggs were most likely undergoing 

 development in the uppermost sediment layer 

 when collected in February. Only those eggs with 

 a clear outer layer were used for the experiment. 



Thirty-five resting eggs were sorted by pipette 

 into each of 43 small (6 ml) Petri dishes. The ac- 

 companying water at 5'!'(iii (ca. 1 ml) was removed 

 by pipette. Water of 11 salinities, ranging from 

 O'Khi (glass distilled) to 23.5%is was then added to 

 batches of three or more of these dishes. Three 

 complete rinses of appropriate salinity were added 

 and removed before the final addition. The Petri 

 dishes with eggs were maintained at 17' C ( ±0.1°) 

 in covered beakers as described above. Continuous 

 overhead lighting was provided. Water was re- 

 placed every 5th day. 



Egg and naupliar counts were usually made 

 daily. There were some 2-day intervals. Naupliar 

 mortality between observations was also recorded. 

 Salinity was increased in some replicates at vari- 

 ous time intervals to determine viabilitv of re- 



maining dormant eggs. Hatched nauplii were cap- 

 tured and reared to copepodite stages ( 17° C, 20%o) 

 for positive species identification because of uncer- 

 tainty in distinguishing between A. californiensis 

 and A. clausi nauplii. 



Salinity and Temperature Experiment 



The effect of salinity on hatching of res ting eggs 

 was later evaluated at three more temperatures 

 (15°, 12.5°, 10° C) to obtain hatching rates at 

 temperature-salinity combinations normally 

 present in the upper estuary during later fall and 

 early spring months. Resting eggs were obtained 

 by 2.8 M sucrose centrifugation of sediments col- 

 lected at Station 39 on 18 February 1978. Water 

 conditions were 9.4° C and 7.6%o. Transport to the 

 laboratory, storage (2 days), screening, centrifug- 

 ing, and sorting were all done at 10° C. Two or 

 three replicates of 35 resting eggs (clear-layer 

 type) were prepared at 5%o, 10%ii, 15%o, and 25%o 

 for each temperature. Other details were as de- 

 scribed in the salinity experiment. 



Additional beakers were established at all 

 temperature-salinity combinations to determine 

 the approximate ratio of A. califor/uensis to A. 

 clausi resting eggs by rearing hatched nauplii to 

 copepodites. These beakers contained the un- 

 sorted mixture of resting eggs of Acar<;a spp. and 

 other accompanying species plus detritus which 

 collected at the water-sucrose interface during 

 centrifugation. The number of eggs in each beaker 

 was variable, ranging between 50 and 250. 

 Hatched nauplii were removed daily and transfer- 

 red to new beakers at the next higher salinity 

 ( +5%o) and temperature (+2.5° C). Salinity and 

 temperature were increased every 5th day to 

 maxima of 15° C and 25%o. This was done to im- 

 prove survival while avoiding abrupt change. 

 Ratios were determined when most individuals 

 had molted into the copepodite stages. Hatched 

 nauplii from the Petri dishes were also reared 

 when practical to the copepodite stages. 



RESULTS 



Field Population Cycle 



The twice-weekly sampling program was 

 adequate to follow the seasonal cycle and distribu- 

 tion of A. californiensis in upper Yaquina Bay. 

 Only field data for 1972 are presented, since the 

 population had similar cycles in 1973 and 1974. 



571 



