The calanoid copepod Calanus ?nmarchicus is a keystone species in the North Atlantic. Adverse e?ects of ocean acidi?cation on this species could have major implications for ecosystem structure and function, as well as socio-economic impacts on ?sheries. Cohorts of Calanus ?nmarchicus exposed to simulated ocean acidi?cation environments were followed during the course of two consecutive generations. The time of development into the di?erent molting stages were monitored. While the more moderate CO2 concentration (1080 ppm) did not seem to a?ect the development rate, a slower development into molting stages were found in the highest exposure groups (2080 - 3080 ppm CO 2 ), compared to a control treatment group (380 ppm CO 2 ). Measurements of oxygen consumption and feeding rate in sub adult individuals (copepodite stage C5) from the second generation of continuously exposed C. ?nmarchicus were integrated into calculations of the overall energy balance (Scope for Growth) of the animals. Together with biometric measurements that were performed, these results points to an energy depletion due chronic exposure to elevated CO 2 . Calanus ?nmarchicus seems to be tolerant to a more moderate CO 2 elevation (1080 ppm). No signs of adaptation to the treatments were detected over the course of two generations.