Zooplankton 273 



Pond 



10 



20 



15 

 July 1971 



FIGURE 6-11. Percent completion of release of eggs 

 into brood pouches (oviposition) and of birth of young 

 Daphnia middendorffiana in Ponds B and C, July 1971. 



70% complete in 48 hours in Pond C Daphnia. Thus synchrony in the 

 population appeared to be retained at least through the first adult instar. 



Synchrony was not equally sharp in all years of observation, however. 

 In 1973 a sudden cool period occurred after the females had begun to 

 oviposit. The low temperatures slowed the oviposition process and resulted 

 in a spreading out in time of both the ovipositioning of eggs and later 

 hatching of the young. 



The hypothesis that molting was under the control of an endogenous 

 circadian rhythm was also tested. Measurements were carried out in the 

 field in chambers in which day length and temperatures were controlled. 

 Sample populations of Daphnia were observed at intervals of a few hours 

 (1971) or were continuously monitored in the ZEHMAC (1972, 1973). 

 This apparatus was a bottomless cage of Daphnia that revolved on a 

 circular track once every 24 hours. Cast exuviae, dead animals and egg 

 pods (ephippia) fell to the track surface. The cage, equipped with a roller 

 that fitted into the rear wall, pressed them against the bottom as it passed. 



There was a distinct rhythmicity in molting. Under natural conditions 

 a peak in molting intensity during the last 8 hours of a day was followed by 

 a second peak during the first 8 hours of the day following. This 

 bimodality seemed similar to that observed for oscillations in feeding 

 intensity (see above). Only the experimental conditions of constant light 

 and constant temperature or constant light and natural temperature gave a 

 response similar to the results under natural conditions. A treatment of 

 constant temperature plus natural light resulted in a near-uniform spread 

 of molting. 



In 1973, an interesting synchrony occurred in the timing of the molt 

 to the first adult instar of Daphnia. In one of the two monitors operating 



