The effects of hypoxic exposure on whole-blood oxygen-affinity were examined in the antarctic fish Pagothenia borchgrevinki. Fish exposed to POO2 = 60 mmHg for 11–14 days at −1.5°C had a P50 value of 20.6±4.8mmHg (S.D., N=13) at pH8.16, compared with 31.1 ±4.3mmHg (N=10) at pH8.00 for normoxic fish. Exposure to low oxygen levels resulted in a significant (66 %) rise in haemoglobin concentration, and erythrocyte [ATP] decreased by approximately 27%. There was no evidence for erythrocyte swelling. An aberrant gill morphology was observed in six fish and these showed unexpectedly high erythrocyte ATP levels. Oxygen-carrying capacity increased by approximately 40% in hypoxic fish and was correlated with a 34 % decrease in spleen mass. Despite the fact that antarctic fish have exceptionally low demands for oxygen and are unlikely ever to encounterenvironmental hypoxia, this antarctic fish has the necessary machinery to respondto hypoxia in a way that is typical of teleosts that naturally inhabit oxylabile environments. The ability to make short-term adaptive changes in the O2 delivery system in response to hypoxic exposure may be typical for vertebrates in general, rather than a feature seen only in those organisms which encounter environmental hypoxia on a regular basis.

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