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Fig. 3 The pectoral fin musculature during the late larval period consists of only fast muscle fibers. A: Transverse section of juvenile pectoral fin (15 mm SL). There are small populations of S58-positive slow fibers at the periphery of the individual muscles. B: Lateral view of the pectoral fin of a 6.5 mm larva stained in whole-mount for slow fibers (S58, green) and all fibers (MF20, red). There are no slow fibers stained within the pectoral fin (bracket), but slow fibers are apparent within the trunk (arrows). C: Transverse section of a 6.5 mm larva demonstrating that the pectoral fin musculature consists of only fast fibers. Slow fibers within the trunk are labeled by S58 at this stage (arrows). Scale bars = 100 μm in A,B, 25 μm in C.