Short- and long-term consequences of larval stage exposure to constantly and ephemerally elevated carbon dioxide for marine bivalve populations

Abstract. 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 CO₂ concentrations for calcifying bivalves. Higher CO₂ concentrations depressed both calcification rates assessed using ⁴⁵Ca uptake and RNA : DNA ratios in Mercenaria mercenaria and Argopecten irradians larvae with RNA : DNA ratios being highly correlated with larval growth rates (r²>0.9). These findings suggested that high CO₂ has a cascading negative physiological impact on bivalve larvae stemming in part from lower calcification rates. Exposure to elevated CO₂ during the first four days of larval development significantly depressed A. irradians larval survival rates, while a 10-day exposure later in larval development did not, demonstrating the extreme CO₂ sensitivity of bivalve larvae during first days of development. Short- (weeks) and long-term (10 month) experiments revealed that individuals surviving exposure to high CO₂ during larval development grew faster when exposed to normal CO₂ as juveniles compared to individuals reared under ambient CO₂ 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 CO₂ as larvae were evident even after 10 months of growth under normal CO₂ concentrations. This "legacy effect" emphasizes the central role larval stage CO₂ exposure can play in shaping the success of modern-day bivalve populations.