Linking muscle metabolism and functional variation to field swimming performance in bluegill sunfish (Lepomis macrochirus)

Skeletal muscle has diverse mechanical roles during locomotion. In swimming fish, power-producing muscles work in concert with the accessory muscles of the fins which augment and control power transfer to the water. Although fin muscles represent a significant proportion of the locomotor muscle mass, their physiological properties are poorly characterized. To examine the relationship between muscle metabolism and the differing mechanical demands placed on distinct muscle groups, we quantified the aerobic and glycolytic capacities of the myotomal, pectoral and caudal muscles of bluegill sunfish. These were indicated by the activities of citrate synthase and lactate dehydrogenase, rate-limiting enzymes for aerobic respiration and glycolysis, respectively. The well-established roles of slow and fast myotomal muscle types in sustained and transient propulsive movements allows their use as benchmarks to which other muscles can be compared to assess their function. Slow myotomal muscle had the highest CS activity, consistent with meeting the high metabolic and mechanical power demands of body-caudal fin (BCF) swimming at the upper end of the aerobically supported speed range. The largest pectoral adductors and abductors had CS activities lower than the slow myotomal muscle, in line with their role supplying thrust for low-speed, low-power swimming. The metabolic capacities of the caudal muscles were surprisingly low and inconsistent with their activity during steady-state BCF swimming at high speeds. This may reflect adaptation to the observed swimming behavior in the field, which typically involved short bouts of BCF-propulsive cycles rather than sustained propulsive activity.