Muscle fibre types in transverse myotomal sections from juvenile Arctic charr Salvelinus alpinus L. (A) A laboratory reared planktivorous morph stained with the S58 anti-slow muscle myosin antibody and counterstained with Haematoxylin–Eosin. Note the superficial layer of fibres darkly stained with S-58 (red) centred on the major horizontal septa (s-s). In contrast, the superficial fibres at the dorsal and ventral surfaces of the myotome are weakly stained or unstained with S-58 (s-u). Scale bar, 500 μm. (B) A wild-caught dwarf benthic morph, 4.5 cm FL, stained with S-58 antibody and counterstained with Haematoxylin. The majority of the slow fibres (s) stained intensely for S-58 whereas a small number of the slow fibres (arrowhead) and the fast fibres (f) were unstained. Scale bar, 100 μm. (C) A wild-caught dwarf benthic morph, 5.5 cm FL, stained with S-58 antibody and counterstained with Haematoxylin–Eosin. Note that at this body length all the slow fibres (s) stained with S58. There was no evidence for fibres showing intermediate expression for the antigen to S-58 between the slow (s) and fast (f) muscle layers. Scale bar, 200 μm. (D) A wild-caught dwarf benthic morph, 8.4 cm FL, stained for glycogen. The slow fibres (s) were more intensely stained (magenta) than the fast fibres (f). Scale bar, 100 μm. (E,F) A wild-caught dwarf benthic morph, 8.4 cm FL. (E) Epaxial region of the myotome stained for succinic dehydrogenase (SDHase) activity (blue staining). Slow fibres (s) stained intensely for SDHase whereas fast fibres (f) were weakly stained. (F) Region hypaxial to the major horizontal septum stained for myosin ATPase activity following 1 min preincubation at pH 4.3. The slow fibres (s) were lightly stained and the fast fibres (f) were darkly stained. Scale bars, 100 μm. (G) The fast muscle of a wild-caught dwarf benthic morph, 9.7 cm FL, stained for myosin ATPase activity following 90 s preincubation at pH 4.3. Note myosin ATPase had been inactivated in the immature small diameter fibres (a) but not in the larger diameter fibres (b). Shorter periods of preincubation resulted in uniform dark staining whereas longer periods inactivated the myosin ATPase activity in all diameters of fibres. Scale bar, 100 μm. hs, major horizontal septum; nt, notochord; ll, lateral line nerve; sc, spinal cord; sk, skin.

A double logarithmic plot of the number of slow muscle fibres per myotomal cross-section at 0.7 FL and M_b in Arctic charr from Thingvallavatn. The data points represent dwarf benthic morphs (open circles), planktivorous morphs (open triangles, large benthic morphs (filled circles) and piscivorous morphs (filled triangles). A first order polynomial was fitted to the data with the following equation: log₁₀(slow fibre number)=log₁₀2.98±0.027+0.45±0.114(log₁₀FL) (r²=0.91; d.f. 112; ANOVA; F_1,111=1134.4; P<0.0001).

Transverse sections of myotomal muscle from wild-caught Arctic charr Salvelinus alpinus from Thingvallavatn. All sections were stained with the S-58 anti-slow muscle myosin antibody and counterstained with Haematoxylin. The slow muscle fibres (s) stain red and the fast muscle fibres (f) are unstained. (A) Large benthic morph, 8.0 cm FL. The arrowheads show a zone of small diameter slow fibres adjacent to the fast muscle layer representing a region of stratified hyperplasia. (B) Planktiverous morph, 20.6 cm FL. The arrowhead shows an isolated small diameter fibre within the slow muscle layer. (C,D). Piscivorous morphs, 35.8 cm FL (C) and 50.6 cm (D). The arrowheads show an isolated small diameter fibre within the slow muscle layer. All scale bars, 100 μm. f, fast muscle fibre; s, slow muscle fibre.

(A) The distribution of fibre diameters in the fast myotomal muscle for 46 dwarf benthic morphs ranging in size from 3.7 cm to 27.1 cm FL. Smooth nonparametric distributions were fitted to measurements of a minimum of 1000 fibre diameters per fish using a kernel function (see text for details). The insert shows the detail of the left-hand side of the distribution. The probability density functions of fish containing fibres in the 4–10μ m size category are shown in black (broken lines). Fish with no fibres in this size category that were considered to have stopped recruiting fibres are shown in red. The blue broken lines represent the largest fish examined (27.1 cm FL). (B) A diagrammatic representation of the entire data set showing the presence (closed circles) and absence (open circles) of muscle fast fibres less than 10 μm in relation to fish fork length for the dwarf benthic (DB), large benthic (LB), planktivorous (PL) and piscivorous (PI) morphs.

Transverse sections of fast myotomal muscle from Arctic charr stained with S-58 antibody and counterstained with Haematoxylin–Eosin. (A). Dwarf benthic morph, 25.7 cm FL, showing the absence of fast muscle fibres less than 20 μm diameter. The smallest diameter fibre is labelled (a). (B) Piscivorous morph, 35.8 cm FL, showing the mosaic pattern of muscle fibre diameters indicating recent recruitment. The fibres labelled (a) and (b) are 14 and 18 μm diameter, respectively. Filled arrowheads, muscle nuclei; unfilled arrowheads, connective tissue nuclei. Scale bars, 100 μm. c, capillary.

The relationship between fork length (FL) and the number of fast muscle fibres per trunk cross-section in the DB morph (open circles), the LB morph (filled circles), the PL morph (open triangles) and the PI morph (filled circles). The bidirectional error bars represent means ± s.e.m. Fish of a similar size were grouped together in order to show the differences between morphs more clearly. The numbers of fish in each size bin in order of increasing fork length were as follows: LB: 4, 5, 5, 5, 4; DB: 10, 7, 6, 8, 7, 4, 4; PL: 8, 8, 8; PL: 6, 4, 8, 4.

Double logarithmic plots of fibre number and body mass for those fish actively producing muscle fibres. Symbols as for Fig. 7. First order polynomials were fitted to the data for the DB and PL morphs combined and the LB and PI morphs combined, which had the following equations. DB+PL morphs: log₁₀(fibre number)=log₁₀3.88±0.020+log₁₀(M_b)0.52±0.014 (r=0.96; d.f.=51; P<0.0001). LB+PI morphs: log₁₀(fibre number)=log₁₀4.24±0.031+log₁₀(M_b)0.33±0.015 (r²=0.96; d.f.=51; P<0.0001). Values are means± s.e.m.

A 3-D plot of the number of muscle fibres in Arctic charr morphs vs fork length and year class. DB morphs (open circles), LB morphs (filled circles) and PI morphs (filled triangles). The coloured symbols represent individuals that had stopped recruiting muscle fibres.

A double logarithmic plot of the 97^th percentile of fibre diameter calculated from the smoothed distributions of fibre diameter (see text) and body mass for the DB morph (open circles), the LB morph (filled circles), the PL morph (open triangles) and the PI morph (filled triangles) of Arctic charr. First order polynomials were fitted to the data for fish greater than 10 g body mass. The relationships in slow and fast muscle are illustrated and the parameters for the equations were as follows. Fast muscle: intercept=log₁₀1.77±0.013; slope=0.18±0.0060 (r²=0.92; ANOVA r²_1,88=948.4; P<0.0001). Slow muscle: intercept=log₁₀1.42±0.022; slope=0.18±0.010; (r²=0.77; ANOVA F_1,88=297.9; P<0.0001).

The myonuclei content of isolated single fibres from (A) a slow muscle fibre 27 μm in diameter and (B) a fast muscle fibre 157 μm in diameter. The images represent a computed reconstruction of a z-series of 1μ m confocal sections through the fibre. The nuclei (green) were visualised by staining with SYTOX green®. (C) The relationship between muscle fibre diameter (μm) and the number of nuclei per cm in single fibre segments isolated from the slow (red symbols) and fast (black symbols) myotomal muscle from the DB morph (open circles), the LB-morph (filled circles), the PL morph (open triangles) and the PI morph (filled triangles). Fibres were isolated from 4–6 individuals per morph in the size range 20–27 cm FL. The fitted lines represent second order polynomials with the following equations. Slow muscle: nuclei cm^–1=23.38+34.76 (fibre diameter)+0.043(fibre diameter)² (Adj. r²=0.65; residual d.f.=247; P<0.0001). Fast muscle: nuclei cm^–1=231.80+9.00 (fibre diameter)+0.098 (fibre diameter)² (Adj. r²=0.92; residual d.f.=620; P<0.0001).

Immunohistochemistry of molecular markers of myogenic progenitor cells (MPCs) in the fast myotomal muscle of Arctic charr. (A) Pax 7 staining in a transverse section from a piscivorous morph and (B) FoxK1-α staining in a transverse section from a large benthic morph. Arrowheads indicate some of the cells immunopositive for these markers. Scale bars, 50 μm.

The relationship between the density (number mm^–3 muscle) of (A) total myonuclei and (B) Pax 7 immunoreactive cells in tissue sections in relation to fork length for the DB morph (open circles), the LB morph (filled circles), the PL morph (open triangles) and the PI morph (filled triangles). Linear regressions were fitted to the data that had the following equations: density myonuclei= 54029.4–425.8FL (r²=0.45; F_1,26=21.2; P<0.0001). Density Pax7^+ve cells=1653.5–10.54FL (r²=0.17; F_1,26= 5.44; P<0.05).

The relationship between the density (number mm^–3 muscle) of (A) Forkhead box protein K1-α (FoxK1-α) (B) MyoD and (C) Myf-5 immunoreactive cells in tissue sections in relation to fork length for the DB morph (open circles), the LB morph (filled circles), the PL morph (open triangles) and the PI morph (filled triangles).

The influence of rearing temperature on the number and average diameter of fast muscle fibres post-hatch. (A) Log₁₀(fast fibre number per myotomal cross-section) in fry of the dwarf benthic (circles), large benthic (triangles) and pelagic (diamonds) morphs reared at `cool ambient temperatures' of 2.2–3.2°C (open symbols) or in heated water of 4–7°C (closed symbols). A linear regression was fitted to the data with the following equation: log₁₀(fibre number=2.98+0.19F (r²=0.71; F_1,51=125.1; P<0.0001). (B) The average fibre diameter vs fork length. Symbols as for A.