Muscle dynamics in skipjack tuna: Timing of red muscle shortening in relation to activation and body curvature during steady swimming

Cyclic length changes in the internal red muscle of skipjack tuna (Katsuwon us pelamis) were measured using sonomicrometry while the fish swam in a wat er tunnel at steady speeds of 1.1-2.3 L s(-1), where L is fork length. Thes e data were coupled with simultaneous electromyographic (EMG) recordings. T he onset of FMG activity occurred at virtually the same phase of the strain cycle for muscle at axial locations between approximately 0.4L and 0.74L, where the majority of the internal red muscle is located. Furthermore, EMG activity always began during muscle lengthening, 40-50 degrees prior to pea k length, suggesting that force enhancement by stretching and net positive work probably occur in red muscle all along the body. Our results support t he idea that positive contractile power is derived from all the aerobic swi mming muscle in tunas, while force transmission is provided primarily by co nnective tissue structures, such as skin and tendons, rather than by muscle s performing negative work. We also compared measured muscle length changes with midline curvature (as a potential index of muscle strain) calculated from synchronised video imag e analysis. Unlike contraction of the superficial red muscle in other fish, the shortening of internal red muscle in skipjack tuna substantially lags behind changes in the local midline curvature. The temporal separation of r ed muscle shortening and local curvature is so pronounced that, in the mid- body region, muscle shortening at each location is synchronous with midline curvature at locations that are 7-8 cm (i.e. 8-10 vertebral segments) more posterior. These results suggest that contraction of the internal red musc le causes deformation of the body at more posterior locations, rather than locally. This situation represents a unique departure from the model of a h omogeneous bending beam, which describes red muscle strain in other fish du ring steady swimming, but is consistent with the idea that tunas produce th rust by motion of the caudal fin rather than by undulation of segments alon g the body.