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First published online October 21, 2004
Journal of Experimental Biology 207, 4045-4056 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01224

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Metabolic influences of fiber size in aerobic and anaerobic locomotor muscles of the blue crab, Callinectes sapidus

L. K. Johnson¹, R. M. Dillaman¹, D. M. Gay¹, J. E. Blum² and S. T. Kinsey^1,*

¹ Department of Biological Sciences,University of North Carolina at Wilmington, 601 South College Road, Wilmington, NC 28403-5915, USA
² Department of Mathematics and Statistics, University of North Carolina at Wilmington, 601 South College Road, Wilmington, NC 28403-5915, USA

View larger version (14K): [in a new window]   Fig. 1. Arginine kinase (AK) mediates ATP-equivalent flux in crustacean muscle. Diffusive flux of arginine phosphate (AP) occurs over short distances in small white fibers of juvenile crabs (A) but, as the fibers grow, diffusive flux must occur across hundreds of microns (B). Consequently, there is expected to be a reduction in rates of aerobic processes, such as aerobic recovery following exercise, as fiber size increases during development. In large fibers of adult crustaceans anaerobic glycogenolysis occurs following contraction, presumably to speed up phases of the recovery process.

View larger version (186K): [in a new window]   Fig. 2. Hemocyte (H) in the vascular space (V) between subdivisions. Note the basal lamina (arrowhead) at the surface of the subdivision membrane (arrow) as well as a cross section of myofibrils (f) and numerous mitochondria (m). Hemocytes were frequently found in the vascular spaces between subdivisions, indicating that this tissue is highly perfused with hemolymph. Scale bar, 10 µm.

View larger version (73K): [in a new window]   Fig. 3. Dark levator muscle fiber subdivision development. TEM of fiber subdivisions from (A) very small, (B) small, (C) medium and (D) large crabs is shown in the left panels. The middle panels show highlighted mitochondria from the subdivisions, and the panels on the right show highlighted subdivisions found in the micrographs in the left panels. Scale bars, 10 µm.

View larger version (21K): [in a new window]   Fig. 4. Mean mitochondrial fractional areas (open bars; left y-axis) and mean fiber subdivision diameters (filled bars; right y-axis) of the dark levator muscle fibers for each animal size class.

View larger version (20K): [in a new window]   Fig. 5. Mass-specific scaling of citrate synthase (CS) activity per gram of dark levator muscle (filled circles) and white levator (open circles; white levator data from Boyle et al., 2003). (A) The linear regression fit to the dark levator muscle data is the log of the scaling equation CS activity=25.75M-0.19 (r2= 0.45; M is body mass) and in white levator muscle CS activity=5.0M-0.09 (r2=0.39). (B) Mean CS activity grouped by size class. Note the greater fractional differences between size classes in the dark levator muscle than in the white levator muscle. An asterisk indicates that CS activity for that group is significantly different (P<0.05) from the other two groups.

View larger version (21K): [in a new window]   Fig. 6. Lactate concentrations at rest, immediately after exercise and 60 min after exercise in (A) white levator and (B) dark levator muscles. Note that resting lactate values and those immediately after exercise are not significantly different among the size classes. At 60 min, lactate begins to decrease in both tissues from small and medium crabs, while it remains elevated in large dark and white levator muscles. An asterisk indicates that lactate concentrations for that animal size class were significantly different (P<0.05) from the other two size classes (see text for details).

View larger version (21K): [in a new window]   Fig. 7. Time course of lactate concentration changes in (A) white and (B) dark levator muscles and (C) hemolymph following fatiguing exercise. Resting values are plotted first. The 0 min time point is immediately following exercise. Note the increase in lactate following exercise in white fibers from large crabs and the prolonged period of time for lactate removal from all tissues in large crabs. The differences among the size classes are less dramatic in the dark levator muscle than in the white levator muscle.

View larger version (14K): [in a new window]   Fig. 8. The area under the post-contractile lactate concentration recovery curve for dark and white levator muscle. The asterisk indicates that there was a significant difference (P<0.05) between dark and white muscle in the large size class.

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