Functional correlations of axial muscle fiber type proportions in the waterfall‐climbing Hawaiian stream fish Sicyopterus stimpsoni

Assessing the factors that contribute to successful locomotor performance can provide critical insight into how animals survive in challenging habitats. Locomotion is powered by muscles, so that differences in the relative proportions of red (slow‐oxidative) vs. white (fast‐glycolytic) fibers can have significant implications for locomotor performance. We compared the relative proportions of axial red muscle fibers between groups of juveniles of the amphidromous gobiid fish, Sicyopterus stimpsoni, from the Hawaiian Islands. Juveniles of this species migrate from the ocean into freshwater streams, navigating through a gauntlet of predators that require rapid escape responses, before reaching waterfalls which must be climbed (using a slow, inching behavior) to reach adult breeding habitats. We found that fish from Kaua'i have a smaller proportion of red fibers in their tail muscles than fish from Hawai'i, matching expectations based on the longer pre‐waterfall stream reaches of Kaua'i that could increase exposure to predators, making reduction of red muscle and increases in white muscle advantageous. However, no difference in red muscle proportions was identified between fish that were either successful or unsuccessful in scaling model waterfalls during laboratory climbing trials, suggesting that proportions of red muscle are near a localized fitness peak among Hawaiian individuals. Micrograph of cross‐section from posterior tail segment of a juvenile from the Hawaiian waterfall‐climbing gobiid fish Sicyopterus stimpsoni, stained for slow oxidative (red) muscle fibers. Comparisons showed no difference in axial red fiber proportions between successful and unsuccessful climbers, but did identify differences between fish from different islands that face different primary functional demands (predator escape on Kaua'i vs. sustained climbing on Hawai'i).