Elevated concentration of carbon dioxide (elevated pCO2) that cause reduced pH is known to influence calcification in many marine taxa, but how elevated pCO2 influences cation composition of mineralized structures is less well studied. To a large extent, the degree to which elevated pCO2 impacts mineralized structures is influenced by physiological adaptation of organisms to environments where low pH is routinely experienced. Here, we test the hypotheses that elevated pCO2 will differently impact the relative concentrations of divalent cations (Ca2+, Mg2+, Sr2+, and Mn2+) in four closely related species of porcelain crabs distributed across intertidal zone gradients. Cation composition of carapace and claw exoskeleton was determined using inductively coupled plasma mass spectrometry following 24-day exposures to pH/pCO2 levels of 8.0/418 and 7.4/1850 μatm during the intermoult period. Reduced pH/elevated pCO2 caused a 13-24% decrease of carapace [Ca2+] across all species, and species-specific responses in carapace and claw [Mg2+], [Sr2+] and [Mn2+] were observed. During a 24-day exposure, reduced pH/elevated pCO2 reduced survival probability in low-intertidal but not mid-intertidal species. Overall, the effect of reduced pH/elevated pCO2 on exoskeleton mineral composition was muted in mid-intertidal species relative to low-intertidal species, indicating that extant adaptation to the variable intertidal zone may lessen the impact of ocean acidification (OA) on maintenance of mineralized structures. Differences in responses to reduced pH/elevated pCO2 among closely related species adds complexity to predictive inferences regarding the effects of OA.