Exposure of M. edulis to 8 ppb (ppb = μg 1 - 1 ) added copper during the veliger larval or post-larval stages has no significant effects on survivorship or shell growth. However, previous exposure to 8 ppb added copper during the embryo stage significantly increases veliger growth rate and decreases spat survivorship in a population from the Menai Strait, Wales, UK, in which this level of copper causes a significant increase in embryo abnormality. No such effects of pre-exposure are seen in a population from the Westerschelde, The Netherlands. These preincubation effects have a substantial effect on estimates of the toxicity of copper over the life cycle of M. edulis.