In this study, the toxicity of CuO (40 nm), alpha-Al2O3 (40 nm), and alpha-Fe2O3 (20-40 nm) nanoparticles was comparatively investigated on Carcinus aestuarii. Crabs were semi-statically exposed to 1mg/L of each for 14 days and their accumulation and distribution in tissue and hemolymph, potential oxidative stress mechanism, total hemocyte counts and types, and the osmoregulatory and ionoregulatory responses were determined. The tissue distribution of CuO nanoparticles was hepatopancreas > hemolymph >= gill > muscle, for alpha-Fe2O3 gill > hepatopancreas > muscle > hemolymph, and for alpha-Al2O3 gill > muscle >= hemolymph > hepatopancreas. While alpha-Al2O3 and alpha-Fe2O3 NPs, induced lipid peroxidation and changes in antioxidant enzyme activity in the hepatopancreas tissue, the oxidative damage caused by the CuO nanoparticles was minimal. All three nanoparticles, copper in particular, elicit osmoregulatory and ionoregulatory toxicity at this concentration, due to the inhibition of Na+, K+-ATPase activity in the gill and depletion of hemolymph and carcass ion concentrations.