280 



R. G. Stress et al. 



lOOr 



-lis 



-12 



Moternal Length 



Brood Size 



Temperature 



I I 



o 



a. 



E 



.0) 



0600 1200 1800 



Time, hr 



2400 



0600 



FIGURE 6-14. Simulation of brood size 

 and maternal length of Daphnia midden- 

 dorffiana by the model sunda y. Relative 

 length and brood size are shown to de- 

 pend on the phase of an endogenous 

 rhythm of activity relative to the phase of 

 a daily oscillation in temperature. Length, 

 and especially brood size, are maximum 

 when the phase of peak activity coincides 

 with the time of day when pond tempera- 

 tures are intermediate to the extremes. At 

 these times the temperature is at the bio- 

 logical optimum for the species, as deter- 

 mined by Chisholm et al. 1975. (After 

 Stross et al. 1979.) 



reach reproductive length. When temperatures were allowed to oscillate, 

 the same food concentrations permitted attainment of large maternal 

 lengths and large brood sizes (Stross et al. 1979). 



Available food influences both adult length and brood size in 

 Daphnia. They in turn reflect the concentration of food available and the 

 initial length of the individual. Therefore the increased reproductive effort 

 in Pond C over a three-year interval suggests either an increase in food 

 availability or a larger size at birth. Despite the increase, mean brood size 

 was less than saturation as shown experimentally (see below). 



Model simulation of reproductive characteristics typical of a real 

 arctic population reveals the rapid increase in brood size with increase in 

 maternal length (Figure 6-13). Three conditions contribute. First, a larger 

 intake of food is accompanied by, second, the more efficient conversion of 

 foodstuffs to growth. The third factor is the larger proportion of the 

 growth fraction that goes into synthesis of gametes. The reader will note 

 that the simulated regression of brood size on maternal length is biphasic. 

 This has been shown in real populations of D. middendorffiana at Barrow 

 but not in D. pulex. 



