The shell calcification rates of small hard clams in five size classes (0.39, 0.56, 0.78, 0.98, and 2.90 mm shell height) decreased with increasingly severe ocean acidification conditions (pH 8.02, 7.64, and 7.41). Clams in the larger sizes were able to deposit new shell material even under corrosive conditions. However, the smaller clams were unable to overcome the tendency of calcium carbonate to dissolve, likely because they were still developing organs and undergoing developmental shifts in shell mineralogy. In addition, the researchers used hard clams from two different sources and found that they had different calcification rates, most likely due to genetic differences. The findings confirm the susceptibility of the early life stages of this important bivalve to ocean acidification. (Laboratory study)