Early life stages of marine crustaceans respond sensitively to elevated seawater PCO2. However, the underlying physiological mechanisms have not been studied well. We therefore investigated the effects of elevated seawater PCO2 on oxygen consumption, dry weight, elemental composition, median developmental time (MDT) and mortality in zoea I larvae of the spider crab Hyas araneus (Svalbard 79�N/11�E; collection, May 2009; hatch, December 2009). At the time of moulting, oxygen consumption rate had reached a steady state level under control conditions. In contrast, elevated seawater PCO2 caused the metabolic rate to rise continuously leading to a maximum 1.5-fold increase beyond control level a few days before moulting into the second stage (zoea II), followed by a pronounced decrease. Dry weight of larvae reared under high CO2 conditions was lower than in control larvae at the beginning of the moult cycle, yet this difference had disappeared at the time of moulting. MDT of zoea I varied between 45 ± 1 days under control conditions and 42 ± 2 days under the highest seawater CO2 concentration. The present study indicates that larval development under elevated seawater PCO2 levels results in higher metabolic costs during premoulting events in zoea I. However, H. araneus zoea I larvae seem to be able to compensate for higher metabolic costs as larval MDT and survival was not affected by elevated PCO2 levels.