Effects of longitudinal body position and swimming speed on mechanical power of deep red muscle from skipjack tuna (Katsuwonus pelamis)
Douglas A. Syme1,* and
Robert E. Shadwick2
1 Department of Biological Sciences, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4 and
2 Marine Biology Research Division, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0204, USA
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Fig. 1. (A) Lateral view of a skipjack tuna (adapted from Joseph et al., 1988 ) showing the position of anterior (0.44L) and posterior (0.70L) muscle sample sites. L, fork body length. (B) Hemi-transverse sections of the body showing the location of the deep red muscle. S, skin.
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Fig. 2. Work loops from internal red muscle of a skipjack tuna. Results from two different longitudinal body locations and three different cycle frequencies are shown. Muscle length was cycled in a sinusoidal fashion, and the muscle was stimulated phasically during the length cycle. Loops are plots of muscle force against length for a complete length cycle and are traversed in a counterclockwise direction. Net work is equal to the area inside the loop. Left-hand panels: muscle from an anterior (ANT) location (0.45L); resting length 4.0 mm, strain ±5.5 %. Right-hand panels: muscle from a posterior (POST) location (0.69L); resting length 4.5 mm, strain ±8 %. Both muscles are from the same fish. Work loops generated using stimulus parameters that maximized work output (solid lines) are shown together with loops generated when the muscle was stimulated using parameters measured from swimming fish (broken lines). Values inside loops are the net work done using stimulus parameters from swimming fish (broken loops) as a percentage of the maximum (solid loops). Note the similarity between the two types of loop. Only maximal loops are shown at 8 Hz because fish do not normally swim at this speed using only red muscle.
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Fig. 3. Net work per cycle as a function of cycle frequency (tail-beat frequency) from internal red muscle of skipjack tuna at anterior (ANT) and posterior (POST) locations. Filled symbols and solid lines show results using optimized stimulus conditions that maximize work (optimized) and are expressed as a percentage of the largest value recorded over the range of frequencies studied. Open symbols and broken lines are work done when the muscle was stimulated using conditions measured from swimming fish (in vivo) and are expressed as a percentage of the maximum (optimized) value at each of the frequencies indicated; they therefore show the performance of the muscle under in vivo conditions relative to the maximum (optimized) of which the muscle is capable at each frequency. The limited range of frequencies covered using in vivo conditions reflects the range over which fish actually swim using only red muscle. The strain for ANT muscle was ±5.5 % and that for POST muscle was ±8 %, reflecting strains used in vivo. Values are means ± S.E.M., N=3–6.
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Fig. 4. Net power output as a function of cycle frequency (tail-beat frequency). See legend to Fig. 3 for further details. Values are means ± S.E.M., N=3–6.
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Fig. 5. Net power output as a function of strain amplitude from internal red muscle of skipjack tuna at anterior (ANT) and posterior (POST) locations. Strain is the peak amplitude of the imposed length cycle as a percentage of muscle length. The cycle frequency was 4 Hz in all cases. Power is expressed as a percentage of the value at 8 % strain. The muscle was stimulated using conditions measured from swimming fish. Higher strains result in significantly higher power outputs (two-way ANOVA and Tukey’s test, P<0.05). Values are means ± S.E.M., N=3–6.
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Fig. 6. Stimulus phase as a function of cycle frequency. Phase is the time of stimulus onset during the sinusoidal length cycle, in degrees, where 0° is muscle lengthening through resting length and 90° is maximum muscle length just prior to shortening. Data from anterior (ANT) and posterior (POST) locations are shown. Curves show the optimal stimulus phases required to maximize net work output from isolated muscle. Heavy bars show the stimulus phases used by swimming fish (Shadwick et al., 1999). Asterisks indicate optimal phases that are significantly different (P<0.05) from those used in vivo. The limited range of phases from swimming fish reflects the limited range over which fish actually swim using only red muscle. Values are means ± S.E.M., N=3–6.
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Fig. 7. Stimulus duration as a function of cycle frequency. (A) Data from anterior (ANT) locations; (B) data from posterior (POST) locations. Filled lines and solid symbols show optimal stimulus durations required to maximize net work output from isolated muscle (optimized). Dotted lines and open symbols show stimulus durations used by swimming fish (in vivo) (Shadwick et al., 1999); the heavy portions highlight speeds at which fish actually swim using only red muscle; the remainder of the in vivo curve is an extrapolation from these data. Values are means ± S.E.M., N=3–6.
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Fig. 8. Isometric twitch kinetics of internal red muscle from anterior (ANT) and posterior (POST) locations at 25°C. ‘Stimulus to peak force’ is the time from the stimulus until the peak of the twitch. ‘Peak force to 50 % relaxation’ is the time from the peak of the twitch until developed force declined to 50 % maximal. ‘Duration at half-amplitude’ is the period over which force remained 50 % maximal or greater. An asterisk indicates a significant difference (P<0.05) between locations. Values are means + S.E.M., N=4–6. The inset shows isometric twitches of internal red muscle from anterior (ANT, black) and posterior (POST, red) locations at 25°C. Fine broken lines are twitches from individual preparations; heavy solid lines are composite twitches made by scaling recordings from individual preparations and then averaging the data sets.
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Fig. 9. Simulation of relative power output of red muscle from anterior (ANT) (0.42L) and posterior (POST) (0.70L) locations in a 43 cm skipjack tuna during cyclic contractions at 4 Hz (the tail-beat frequency for steady swimming at approximately 3 L s–1). L, fork body length. Plots show instantaneous power versus time (see Altringham et al., 1993), where power (i.e. force x shortening velocity) was calculated from work loop data; velocity is the time derivative of the length trace, with positive velocity occurring during shortening. The traces for POST muscle have been delayed by 40° to show the proper temporal relationship between the two locations. The amplitudes of ANT and POST power curves are scaled to account for the differences in cross-sectional area of red muscle and mass-specific power output at these locations (see text for details).
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© The Company of Biologists Ltd 2002
