Muscle growth in polar fish: a study of Harpagifer species with sub-Antarctic and Antarctic distributions

Muscle growth was investigated in Harpagifer species with sub-Antarctic (Harpagifer bispinis, environmental temperature (ET) 4-11°C) and Antarctic (Harpagifer antarcticus, ET -1.5 to +1°C) distributions. The trunk musculature of 1 d-old larval H. antarcticus, 0.84-0.93 cm standard length (SL), contained 1025 :t 76 fast muscle fibres per cross-section (Mean :t SE, n=6). Zones of muscle fibre formation were restricted to the dorsal and ventral cones of the myotomes. The maximum diameter of fast muscle fibres increased linearly with standard length and was 270 ~m at 10.1 cm SL. The density of myogenic progenitor cells, determined by staining sections for c-met and myogenic regulatory factors (MyoD, myf-5, myf-6, myogenin), decreased with increasing body length. The number and size distribution of fast muscle fibres with respect to standard length was indistinguishable in H. bispinis and H. antarcticus, although the number of c-met+ve cells and the maximum body size was higher in the Antarctic species. Comparative studies indicate that a large maximum fibre diameter is a general characteristic of the Notothenioideii. This may reflect the radiation from a small bodied ancestor with a relatively low fibre number, although adaptive explanations related to metabolic rate reduction cannot be excluded.