Anatomy & Performance: Effects of Muscle Architecture on Waterfall-Climbing Ability in a Hawaiian Gobioid Fish

Hawaiian freshwaters impose many physiological demands upon resident fish, including numerous tall waterfalls (>300m) that amphidromous juveniles must climb to reach adult habitats. Differences in muscle fiber type distribution in Hawaiian fish have previously been correlated with locomotor performance.  Using an ATPase immunohistochemical stain, we examined the architecture of three climbing muscles (adductor pectoralis, adductor ventralis, abductor dorsalis) in Sicyopterus stimpsoni, a waterfall-climbing gobioid fish. Fish were collected from four waterfall-separated segments (n=12/segment) of Nanue stream. Red muscle fiber content of the abductor dorsalis was significantly higher in the two sites furthest upstream than in the lower sites (p=0.002), while no significant differences in fiber content were found in the adductor pectoralis or adductor ventralis muscles between sites. The adductor pectoralis muscle displayed the highest white fiber content of all muscles (p=0.001). Results suggest that the abductor dorsalis, which powers the pelvic sucking disk used by S. stimpsoni to adhere to the waterfall, may be the most physiologically relevant muscle in climbing. The high white fiber content of the adductor pectoralis correlates with physiological function, as it is only active to initiate each climbing bout.  Taken together, differences in muscle architecture of S. stimpsoni explain in-stream distribution.