| Summary: | Transcellular oxygen flux was modelled mathematically in the aerobic skeletal muscles of perciform fish species living at widely different temperatures (Antarctica, sub-Antarctica and the Mediterranean Sea). Using structural data derived from stereological analysis of electron micrographs, mean fibre P O 2 was calculated on the basis of temperature-corrected rates of mitochondrial respiration and oxygen diffusion. The mean muscle fibre diameter (MFD) among Antarctic notothenioids was in the range 17–61 μm and mitochondrial volume density, V v(mit,f), was 0.27–0.53, but capillary-to-fibre ratio varied only between 1.2 and 1.5. For a mean capillary P O 2 of 6 kPa, the model predicted a mean tissue P O 2 in the range 0.7–5.8 kPa at the estimated maximum aerobic capacity ( M ˙ O 2 max ). The lowest levels of tissue oxygenation were found in the pectoral muscle fibres of the icefish Chaenocephalus aceratus , which lacks the respiratory pigments haemoglobin and myoglobin. Red-blooded notothenioids found in the sub-Antarctic had a similar muscle fine structure to those caught south of the Antarctic Convergence, with an MFD of 20–41 μm and V v(mit,f) of 0.27–0.33, resulting in an estimated mean P O 2 of 4–5 kPa at M ˙ O 2 max . Mean tissue P O 2 in the sub-Antarctic icefish Champsocephalus esox , with greater MFD and V v(mit,f), 56 μm and 0.51, respectively, was calculated to exceed 1 kPa at winter temperatures (4°C), although oxidative metabolism was predicted to be impaired at the summer maximum of 10°C. At the high end of the thermal range, related perciform species from the Mediterranean had a negligible drop in intracellular P O 2 across their small-diameter fibres, to a minimum of 5.4 kPa, comparable with that predicted for Trematomus newnesi from the Antarctic (5.6 kPa) with a similar MFD. These data suggest that, within a single phylogenetic group, integrative structural adaptations potentially enable a similar degree of tissue oxygenation over a 20°C range of environmental temperature in the red-blooded ...
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