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

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