Mitochondrial densities of skeletal muscles in pinnipeds: Evidence for symmorphosis in diving mammals?
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Pinnipeds have evolved to live in an aquatic environment and to successfully deal with its unique physiological challenges. One adaptation is their ability lo sustain aerobic metabolism during prolonged breath-hold diving in order to minimize time spent at the surface and maximize time spent submerged. The purpose of this study was to assess the aerobic capacity of skeletal muscles in pinnipeds. Samples of swimming (locomotory) and non-swimming muscle were collected from fourteen Stellar sea lions (Eumetopias jubata), five Northern fur seals (Callorhinus ursinus) and ten harbor seals (Phoca vitulina) by a needle biopsy technique. Immediately upon collection, the samples were placed into glutaraldehyde fixative for at least 24 hours and stored in cacodylate buffer until analysis of mitochondrial densities by electron microscopy. The longissimus dorsi; the principal swimming muscle of seals, had an average mitochondrial volume density of 19.2% V/v (mt,0- The pectoralis; the principal swimming muscle of sea lions and fur seals, had an average volume density of 10.5% and 15.3% V/v (mt,f). These values are 24 times greater than those for locomotory muscles of terrestrial mammals of comparable size. Although the increased mitochondrial density in pinniped muscle would be expected to increase maximum oxygen consumption, there is no experimental evidence to indicate that marine mammals have an increased aerobic scope during exercise as compared to terrestrial mammals. The increased mitochondrial densities of pinniped muscle may have less to do with increasing maximum oxygen consumption and more to do with sustaining moderate levels of aerobic metabolism under the hypoxic conditions created during diving.
