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Aerobic capacities in the skeletal muscles of Weddell seals: key to longer dive durations?

S. B. Kanatous¹, R. W. Davis², R. Watson², L. Polasek², T. M. Williams³ and O. Mathieu-Costello^4,

¹ Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA
² Department of Marine Biology, Texas A&M University, Galveston, TX 77553, USA
³ Department of Biology, University of California Santa Cruz, Santa Cruz, CA 95604, USA
⁴ Department of Medicine, University of California San Diego, San Diego, CA 92093, USA

View larger version (17K): [in a new window]   Fig. 1. Skeletal muscle anatomy of Weddell seals. Biopsy samples were collected from the swimming muscles (M. longissimus dorsi and hindlimb complex) and the non-swimming muscle (M. pectoralis). (Originally redrawn by M. Cooley, reproduced from Kanatous et al., 1999.)

View larger version (195K): [in a new window]   Fig. 2. Representative electron micrograph of the muscles from the Weddell seal. A transverse section through the hindlimb muscle, showing myofibrils (f), Z line (Z) and mitochondria (arrows).

View larger version (78K): [in a new window]   Fig. 3. Fiber-type composition in the muscles of Weddell seals. (A-C) Representative sections from metachromatic ATPase-stained muscle samples from (A) M. longissimus dorsi (swimming muscle), (B) M. pectoralis (non-swimming muscle) and (C) hindlimb (swimming muscle). I, Type I (slow oxidative) fibers; IIA, Type IIA (fast-twitch oxidative) fibers; Type IIB (fast-twitch glycolytic) fibers are completely absent. (D-F) Representative sections of specific immunohistochemical staining of the M. longissimus dorsi for the different fiber types. Darkly stained fibers are positive for: (D) Type I myosin (slow oxidative) and (E) Type IIA myosin (fast-twitch oxidative). (F) Type IIB myosin (fast-twitch glycolytic) are completely absent.

View larger version (20K): [in a new window]   Fig. 4. Histogram showing fiber-type composition (% of total fiber number) in the swimming (M. longissimus dorsi and hindlimb) and non-swimming (M. pectoralis) muscles of Weddell seals. Note the lack of fast-twitch glycolytic (Type IIB) fibers in all the muscles measured.

View larger version (18K): [in a new window]   Fig. 5. Plot of muscle mitochondrial volume density against body mass in athletic (filled circles) and sedentary terrestrial mammals (small dots) and pinnipeds (open symbols). The linear relationship (y=6.75-1.34[logx], r2=0.70) was generated from the volume densities of the vastus medialis, a primary locomotory muscle, from various terrestrial mammals ranging in size from the dwarf mongoose to the steer (Kanatous et al., 1999; and terrestrial data from Hoppeler et al., 1987). By contrast to the shorter-duration divers, the volume density of mitochondria in the M. longissimus dorsi of Weddell seals was similar to that predicted for a sedentary terrestrial mammal of comparable size.