A growing body of evidence indicates that some engineered nanoparticles (ENPs) are toxic to organisms that perform important ecosystem services in terrestrial and aquatic ecosystems. However, toxicity can be influenced by the biotransformation of contaminants, including ENPs, as it may alter the fate and transport of these substances. In turn, fate and transport can influence their bioavailability. To understand how biotransformation influences the fate and transport of ENPs in marine ecosystems, we exposed suspension-feeding mussels, Mytilus galloprovincialis. to two common nano-metal oxides, CeO2 and ZnO, over a range of concentrations from 1 mg L-1 to 10 mg L-1, in a laboratory experiment. Mussels exposed to 10 mg L-1 accumulated 62 mu gg(-1) of Ce and 880 mu gg(-1) of Zn on a dry tissue basis but rejected 21,000 mu gg(-1) for Ce and 63,000 mu gg(-1) for Zn in pseudofeces. Scanning electron microscope evidence indicates CeO2 remained as ENPs but ZnO did not after being rejected by the mussels. Mussels filtered most of the CeO2 from the aqueous media, while a significant fraction of Zn remained in solution. Differences in ENP solubility affect ENP uptake, excretion, and accumulation in mussels. Our study highlights the potential role of marine suspension feeders in biotransformation of ENPs. (C) 2012 Elsevier B.V. All rights reserved.