Tissue metal concentrations are widely used as indicators of environmental contamination. High endogenous concentrations of essential metals such as copper (Cu) can mask changes in tissue metal concentrations. Experiments using enriched isotopes can provide greater insight into endogenous Cu exchange and more broadly the exposure-bioaccumulation-response relationship. The aim of the present study was to investigate the effect of Cu exposure through the dietary uptake route in the intertidal gastropod Bembicium nanum. The marine alga Tetraselmis chuii was cultured at different Cu concentrations and fed to B. nanum. The Cu concentrations in T. chuii and B. nanum were analysed. Effects were measured using the biomarkers - total antioxidant capacity, lipid peroxidation and lysosomal membrane destabilisation in B. nanum. Dietary uptake of Cu in B. nanum was investigated with a feeding experiment using a 65Cu isotopically enriched solution. Although there were no significant differences in the Cu concentrations in B. nanum, lysosomal membrane destabilisation increased with the dietary Cu concentration. The 65Cu experiment demonstrated that B. nanum accumulated Cu under the experimental conditions. The study showed that tissue Cu concentrations are not always indicative of effects in organisms and that enriched isotope analysis assists in understanding the exposure-bioaccumulation-response relationship.