The in vivo responses of C. dubia to nanoparticles exemplified by a photoactive titanium oxide (TiO2) and a non-photocatalytic aluminum oxide (Al2O3) were studied. Both nanomaterials inhibited the growth of C dubia at concentrations ca. > 100 mg/L The EC50 value was 42 and 45 mg/L in the presence of TiO2 and Al2O3, respectively, based on 3-brood reproduction assay. Results implied that reactive oxygen species (ROS) may not be totally responsible for the adverse effects exerted on the invertebrate. Aggregation and interaction among nanoparticles, C. dubia, and algal cells, major food source of Daphnia, played a significant role on the responses of C. dubia to nanoparticles. Dynamic energy budget (DEB) analysis was used to assess the impact of nanoparticles on the energy allocation of C. dubia. Results indicated that nanoparticles could disrupt the assimilation and consumption of energy in C. dubia dramatically. The assimilation energy was negatively correlated to the concentration of nanomaterials, a reduction from 11 to near 0 mu g-C/animal/day in the presence of TiO2 or Al2O3 nanoparticles at a nanoparticle concentration of 200 mg/L. The energy consumed for life-maintenance increased also with increase in the concentration of nanomaterials. Results clearly demonstrated the importance of energy disruption in determining the toxicity of nanoparticles toward C. dubia. (C) 2011 Elsevier B.V. All rights reserved.