Some copepods (Calanus species) in the Arctic routinely encounter a range of seawater pH levels each day as they migrate vertically in the ocean; they were not severely affected when exposed to ocean acidification conditions in the laboratory. In contrast, a copepod species (Oithona similis) that does not vertically migrate, and therfore does not routinely encounter a range of pH conditions, had high mortality of adults and larvae when exposed to ocean acidification conditions in the laboratory. These results support the relatively untested hypothesis that the natural range of pH experienced by copepods and other marine life may determine sensitivity to future ocean acidification.