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Abstract

1.
Physiology and life history of ex-ephippial and parthenogenetic offspring of Daphnia magna Straus were compared to test the hypothesis that females hatching from resting eggs are well adapted to the predictable high-food conditions of the spring algal bloom.
2.
Although ex-ephippial neonates were smaller than parthenogenetic neonates, they showed higher juvenile growth rates that compensated for the size difference by the time of maturation.
3.
At high food concentration, ex-ephippial females were even larger at maturation than parthenogenetic females, produced more eggs, and reached a higher rate of population increase.
4.
The high activity of the ex-ephippial females was accompanied by higher metabolic rates, which in combination with the low initial mass resulted in reduced survival times under starvation.
5.
Thus, the two phenotypic types of offspring produced by Daphnia are adapted to differing conditions. Ex-ephippial females are superior to parthenogenetic offspring under high food conditions, but inferior when food is limiting.
6.
This pattern selects for synchronous hatching of resting eggs in spring when the spring algal bloom is to be expected.