Ocean acidification (OA) in the near future is affecting several marine organisms negatively. To increase our understanding of the effects we studied embryos and larvae of the great scallop. They were exposed to an increasing level of OA, from natural seawater to levels expected in 50–100 years from now. Larvae were negatively affected already after 2 days, and fed larva seemed as sensitive as unfed larvae after 7 days. Adaption and acclimatization may counteract future OA effects on scallops.