The combined effects of predicted ocean acidification and global warming on the larvae of the cold-eurythermal spider crab Hyas araneus L. were investigated in 2 populations: a southernmost around Helgoland (North Sea, 54° N) and a northernmost at Svalbard (North Atlantic, 79°N). Larvae were exposed at temperatures of 3, 9 and 15°C to present day normocapnia (380 ppm CO2) and to CO2 conditions predicted for the near or medium-term future (710 ppm by the year 2100, 3000 ppm by 2300 and beyond). Larval development time, growth and C/N ratio were studied in the larval stages Zoea I, II, and Megalopa. Permanent differences in instar duration between both populations were detected in all stages, likely as a result of evolutionary temperature adaptation. With the exception of Zoea II at 3°C and under all CO2conditions, development in all instars from Svalbard was delayed compared to those from Helgoland. Most prominently, development was much longer and fewer specimens morphosed to the first crab instar in the Megalopa from Svalbard than from Helgoland. Enhanced CO2 levels (particularly 3000 ppm) extended the duration of larval development and reduced larval growth (measured as dry mass) and fitness (decreasing C/N ratio, a proxy of the lipid content). Such effects were strongest in the zoeal stages of Svalbard larvae, and during the Megalopa instar of Helgoland larvae. The high sensitivity of megalopae from the Svalbard population to warming and of those from Helgoland to enhanced CO2levels suggests that this larval instar is a physiologically sensitive bottleneck within the life cycle of H. araneus. © Inter-Research 2010.