To identify the properties of taxa sensitive and resistant to ocean acidification (OA), we tested the hypothesis that coral reef calcifiers differ in their sensitivity to OA as predictable outcomes of functional group alliances determined by conspicuous traits. We contrasted functional groups of eight corals and eight calcifying algae defined by morphology in corals and algae, skeletal structure in corals, spatial location of calcification in algae, and growth rate in corals and algae. The responses of calcification to OA were unrelated to morphology and skeletal structure in corals; they were, however, affected by growth rate in corals and algae (fast calcifiers were more sensitive than slow-calcifiers), and by the site of calcification and morphology in algae. Species assemblages characterized by fast growth, and for algae, also cell wall calcification, are likely to be ecological losers in the future ocean. This shift in relative success will affect the relative and absolute species abundances as well as the goods and services provided by coral reefs.