Effects of elevated pCO2 on crab survival and exoskeleton composition depend on shell function and species distribution: a comparative analysis of carapace and claw mineralogy across four porcelain crab species from different habitats

Elevated concentration of carbon dioxide (elevated pCO(2)) that cause reduced pH is known to influence calcification in many marine taxa, but how elevated pCO(2) influences cation composition of mineralized structures is less well studied. To a large extent, the degree to which elevated pCO(2) 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 pCO(2) 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/pCO(2) levels of 8.0/418 and 7.4/1850 matm during the intermoult period. Reduced pH/elevated pCO(2) 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 pCO(2) reduced survival probability in low-intertidal but not mid-intertidal species. Overall, the effect of reduced pH/elevated pCO(2) 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 pCO(2) among closely related species adds complexity to predictive inferences regarding the effects of OA.