In larval scallops,ocean acidification (pH 7.4–7.6) reduced survivorship by more than 50 percent. Low-oxygen water inhibited growth and metamorphosis. When exposed to both low oxygen and ocean acidification at the same time, scallops fared worse than under either one by itself. In early life stage clams, low oxygen led to 30 percent higher mortality, and acidified waters reduced growth by 60 percent. Later stage clams were resistant to low oxygen or acidification separately but experienced a 40 percent reduction in growth rate when exposed to both conditions simultaneously. Collectively, these findings demonstrate that the consequences of low oxygen and acidification for early life stage bivalves, and likely other marine organisms, are more severe than would be predicted by either individual stressor and thus must be considered together. (Laboratory study)