While larval bivalves are highly sensitive to ocean acidification, the basis for this sensitivity and the longer-term implications of this sensitivity are unclear. Experiments were performed to assess the short-term (days) and long-term (months) consequences of larval stage exposure to varying CO<sub>2</sub> concentrations for calcifying bivalves. Higher CO<sub>2</sub> concentrations depressed both calcification rates assessed using <sup>45</sup>Ca uptake and RNA : DNA ratios in <i>Mercenaria mercenaria</i> and <i>Argopecten irradians</i> larvae with RNA : DNA ratios being highly correlated with larval growth rates (<i>r</i><sup>2</sup>>0.9). These findings suggested that high CO<sub>2</sub> has a cascading negative physiological impact on bivalve larvae stemming in part from lower calcification rates. Exposure to elevated CO<sub>2</sub> during the first four days of larval development significantly depressed <i>A. irradians</i> larval survival rates, while a 10-day exposure later in larval development did not, demonstrating the extreme CO<sub>2</sub> sensitivity of bivalve larvae during first days of development. Short- (weeks) and long-term (10 month) experiments revealed that individuals surviving exposure to high CO<sub>2</sub> during larval development grew faster when exposed to normal CO<sub>2</sub> as juveniles compared to individuals reared under ambient CO<sub>2</sub> as larvae. These increased growth rates could not, however, overcome size differences established during larval development, as size deficits of individuals exposed to even moderate levels of CO<sub>2</sub> as larvae were evident even after 10 months of growth under normal CO<sub>2</sub> concentrations. This "legacy effect" emphasizes the central role larval stage CO<sub>2</sub> exposure can play in shaping the success of modern-day bivalve populations.