THE LIFE-HISTORY CONSEQUENCES OF RESOURCE DEPRESSION IN CERIODAPHNIA-QUADRANGULA AND DAPHNIA-AMBIGUA

An analysis of the life history and energetics responses of two small planktonic cladocerans, Daphnia ambigua and Ceriodaphnia quadrangula, to food limitation is presented. The results are consistent with an earlier study of D. pulex in that the size-specific instar durations and proportional investments in reproduction are independent of the food level. Compared to D. pulex, these smaller species spend more time in early instars but less time in adult instars. Of the energy available for (growth + reproduction), a much greater fraction is allocated to reproduction than in the case of similar-sized D. pulex, and it quickly reaches an asymptote of 94-97%. As in the case of D. pulex, the net rate of energy intake reaches a plateau shortly after maturity. This, combined with the increased cost of molting as the animal grows, causes age-specific reproduction to level off and eventually decline. The maximum rate of net energy intake is much lower and the cost of molting much higher for D. ambigua and C. quadrangula than for the larger D. pulex. This puts an upper limit on body size for the two smaller species that is much lower than that attainable in D. pulex. The data suggest the possibility that the sizes of planktonic cladocerans may be less a direct consequence of size-selective mortality than a correlated response to selection operating on energy-related traits.