Energy allocation rules in Daphnia magna: clonal and age differences in the effects of food limitation

The allocation of energy to carapace formation, respiration, growth, and reproduction were examined in two parthenogenetic clones of Daphnia magna (Cladocera) cultured at two levels of food (Chlorella) concentration. Clonal differences in energy allocation were more apparent at high ration (1.5 μg C mL^-1) than at low ration (0.3 μg C mL^-1). These differences included respiratory and molting costs, and the timing of energy allocation to growth and reproduction. A comparison of active vs. anesthetized animals revealed that the interclonal difference in respiration rate was the result of a difference in activity level. In both clones mass-specific rates of respiration, growth, and brood production all decreased at low vs. high ration levels, whereas mass-specific molt-loss rate increased. Lowered food concentration decreased the relative allocation of energy to growth and reproduction, but increased allocation to maintenance (respiration and carapace formation). These allocation responses to food limitation indicated that for both clones the highest energy priority was carapace formation. However, the relative priority of respiration, growth and reproduction varied with age and clone. In juveniles (instars 1-4) the priority ranking of growth was essentially equal to that of respiration, whereas respiration always had higher priority in adults (instars 5-9). All three possibilities for the relative ranking of growth and reproduction (i.e., growth>reproduction, growth=reproduction, and reproduction>growth), as specified by different models in the literature, were observed depending on age and clone. The energy allocation rules were also shown to vary between other daphniid species. Furthermore, metabolic responses to chronic food limitation may be different from responses to acute food deprivation. In this study, one clone showed a greater decrease in respiration rate as a result of lifetime food limitation than did the other, but the opposite was true when these clones were exposed to 48 h of starvation. These differences in allocation rules and in acute vs. chronic responses may have to be considered when using physiological data to model Daphnia populations.