![]() ![]() It is suggested that Atlantic cod at such high temperatures may solely depend on increases in cardiac output and blood O 2 capacity, or thermal acclimatisation of metabolic rate, for matching circulatory O 2 supply to tissue demand.Ītlantic cod ( Gadus morhua) are widely distributed in coastal and shelf seas throughout the North Atlantic, but stocks near the southern, equatorward upper thermal margin of their historic distribution limit in the Irish and southern North Sea have declined over the past decades, which has in part been ascribed to warming seas ( Brander, 2005 Drinkwater, 2005 Perry et al., 2005 Beggs et al., 2014 Deutsch et al., 2015). Furthermore, there was no evidence for an increase of maximal Sa–v O 2 with temperature. Modelling of Sa–v O 2 at physiological pH, temperature and O 2 partial pressures revealed a substantial capacity for increases in Sa–v O 2 to meet rising tissue O 2 demands at 5.0 and 12.5☌, but not at 20☌. This was paired with strongly pH-dependent affinity and cooperativity of red blood cell O 2 binding (Bohr and Root effects). Red blood cells had an unusually low O 2 affinity, with reduced or even reversed thermal sensitivity between pH 7.4 and 7.9, and 5.0 and 20.0☌. The results showed statistically indistinguishable red blood cell O 2 binding between the three HbI genotypes in wild-caught Atlantic cod from the Irish Sea (53° N). The present study assessed the thermal sensitivity of O 2 binding in Atlantic cod red blood cells with different Hb genotypes near their upper thermal distribution limit and modelled its consequences for the arterio-venous O 2 saturation difference, Sa–v O 2, another major determinant of circulatory O 2 supply rate. ![]() Limitations to circulatory O 2 transport, in particular cardiac output, and the geographic distribution of functionally different haemoglobin (Hb) genotypes have separately been suggested to play a role in setting thermal tolerance in this species. Atlantic cod are a commercially important species believed to be threatened by warming seas near their southern, equatorward upper thermal edge of distribution. ![]()
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