Phocid and cetacean blueprints of muscle metabolism

Large seals, such as northern and southern elephant seals and Weddell seals, are able to dive for unexpected lengths of time and to enormous depth. The current dive-duration record is 120 min (recorded for the southern elephant seal); the current depth record is 1.5 km (recorded for the northern elephant seal). Equally striking is the widespread observation that these seals, when at sea, spend close to 90% of the time submerged and often at great depth. For practical purposes, these species can be viewed as true mesopelagic animals when they are at sea. Analysis of current knowledge indicates that enzyme adaptations in chronic hypobaric hypoxia are directed mainly towards up-regulation of metabolic efficiencies. Evidence that similar metabolic adjustments are utilized by seals was obtained by profiling the maximum enzyme activities of four phocid species (harbor seal, Weddell seal, crabeater seal, leopard seal) and one cetacean (fin whale). In the seals, the patterns obtained were strikingly similar to those of hypobaric hypoxia adaptations. The extensive enzyme data obtained on seals, however, showed notably different patterns from those found in whale muscles. The data from the large seals were consistent with the concept that low power output but high-efficiency metabolic functions of skeletal muscles coupled with inherently low (and potentially further suppressible) metabolic rates constitute strategic biochemical components in the design of a mesopelagic mammal.