How fish power predation fast-starts

Short-horned sculpin (Myoxocephalus scorpius L.) were acclimated for 6­8 weeks to either 5 °C or 15 °C (12 h dark: 12 h light). Fast-starts elicited by prey capture were filmed from above in silhouette using a high-speed video camera (200 frames s-1). Outlines of the body in successive frames were digitised and changes in strain for the dorsal fast muscle calculated from a knowledge of backbone curvature and the geometrical arrangement of fibres. For 15 °C-acclimated fish at 15 °C, muscle strain amplitude (peak-to-peak) during the first tail-beat was approximately 0.16 at 0.32L, 0.19 at 0.52L and 0.15 at 0.77L, where L is the total length of the fish. Fast muscle fibres were isolated and subjected to the strains calculated for the first tail-beat of the fast-start (abstracted cycle). Preparations were electrically stimulated at various times after the initiation of the fast-start using an in vivo value of duty cycle (27 %). Prior to shortening, muscle fibres at 0.52L and 0.77L were subjected to a pre-stretch of 0.055l0 and 0.085l0 respectively (where l0 is resting muscle length). The net work per cycle was calculated from plots of fibre length and tensile stress. For realistic values of stimulus onset, the average power output per abstracted cycle was similar at different points along the body and was in the range 24­31 W kg-1 wet muscle mass. During shortening, the instantaneous power output reached 175­265 W kg-1 wet muscle mass in middle and caudal myotomes. At the most posterior position examined, the muscle fibres produced significant tensile stresses whilst being stretched, resulting in an initially negative power output. The fibres half-way down the trunk produced their maximum power at around the same time that caudal muscle fibres generated significant tensile stress. Fast muscle fibres at 0.37­0.66L produced 76 % of the total work done during the first tail-beat compared with only 14 % for fibres at 0.67­0.86L, largely reflecting differences in muscle mass. The effect of temperature acclimation on muscle power was determined using the strain fluctuations calculated for 0.52L. For 5 °C-acclimated fish, the average power per cycle (± s.e.m.; W kg-1 wet muscle mass) was 21.8±3.4 at 5 °C, falling to 6.3±1.8 at 15 °C. Following acclimation to 15 °C, average power per cycle increased to 23.8±2.8 W kg-1 wet muscle mass at 15 °C. The results indicate near-perfect compensation of muscle performance with temperature acclimation.