Ocean acidification does not limit squid metabolism via blood oxygen supply

Ocean acidification is hypothesized to limit the performance of squid owing to their exceptional oxygen demand and pH sensitivity of blood-oxygen binding, which may reduce oxygen supply in acidified waters. The critical oxygen partial pressure (P_crit), the P_O  below which oxygen supply cannot match basal demand, is a commonly reported index of hypoxia tolerance. Any CO₂-induced reduction in oxygen supply should be apparent as an increase in P_crit In this study, we assessed the effects of CO₂ (46-143 Pa; 455-1410 μatm) on the metabolic rate and P_crit of two squid species - Dosidicus gigas and Doryteuthis pealeii - through manipulative experiments. We also developed a model, with inputs for hemocyanin pH sensitivity, blood P_CO  and buffering capacity, that simulates blood oxygen supply under varying seawater CO₂ partial pressures. We compare model outputs with measured P_crit in squid. Using blood-O₂ parameters from the literature for model inputs, we estimated that, in the absence of blood acid-base regulation, an increase in seawater P_CO  to 100 Pa (≈1000 μatm) would result in a maximum drop in arterial hemocyanin-O₂ saturation by 1.6% at normoxia and a P_crit increase of ≈0.5 kPa. Our live-animal experiments support this supposition, as CO₂ had no effect on measured metabolic rate or P_crit in either squid species.