AbstractWe studied direct and indirect effects of essential fatty acids (EFA) on Daphnia magna reproduction. Daphnia females that received EFA enriched algae over their entire lifespan produced larger broods than females fed control algae. However, EFA-enriched females were also 91% heavier than control females, which potentially explain the larger investment in reproduction. Thus, with such a large difference in female mass, it is difficult to differentiate the direct effect of the EFA addition from the indirect (maternal size) effects on reproduction. To assess the direct effects on reproduction, we performed two experiments in which we enriched female diets with a range of fatty acids. To minimize maternal size differences, we applied the EFA enrichments only to mature daphniids and studied the effects on reproduction during and after two time intervals (15–16 and 7 days). Limiting the enrichment phase until after maturity decreased the maternal size differences over the enrichment phase among the fatty acid treatments from an average of 91% (for life time enrichment) to 29% after 15–16 days maternal enrichment interval, to 18% after the 7 day interval. Minimizing size differences between differently enriched females decreased the differences in brood size and offspring size. Neonates from control females were more severely affected by starvation than offspring from females that received saturated and polyunsaturated fatty acid enrichments. Under low food concentrations, only neonates that had access to polyunsaturated fatty acids had positive growth rates, thus showing that although dietary fatty acids can be used for energy purposes, specific fatty acids are required to build new biomass. One consequence of our findings is that offspring size does not serve as a good estimate of offspring quality when feeding on different resource qualities.