Selective serotonin reuptake inhibitors (SSRI) are known to increase offspring production in Daphnia magna. This study tested the hypothesis that the increase of serotonin postsynaptic activity by SSRI changes the perception of the food environment and switches life-history responses toward higher food level: females reproduced earlier, producing more but smaller offspring. D. magna reproduction tests, respiration, feeding, and survival-starvation assays and studies of lipids, proteins, and carbohydrate levels of unexposed and exposed females to the SSRI fluoxetine and fluvoxamine and the 5-HT serotonin receptor antagonist cyproheptadine were conducted. Factorial life-history experiments and reproductive assays showed that exposure to SSRI increased juvenile development rate, clutch size, and decrease offspring size at low and intermediate levels of food rations. These effects were reversed by the presence of cyproheptadine, indicating that 5-HT function was essential to the SSRI effects on Daphnia and linking them to the pharmacological effects of SSRI in humans. Respirometry and survival assays and biochemical analyses of lipids, proteins and carbohydrate levels showed that exposure to SSRI increased oxygen consumption rates and decreased carbohydrate levels in adult females. These changes did not affect survival under starving conditions, but they significantly affected the capacity of the exposed animals to survive under anoxic conditions. These results suggest that SSRI increased aerobic catabolism in D. magna making exposed individuals apparently more able to exploit food resources under normoxic conditions, but at the cost of being more sensitive to low oxygen levels, a common situation in natural environments.