Biomechanics of Thunniform Swimming: Electromyography, Kinematics, and Caudal Tendon Function in the yellowfin tuna Thunnus albacares and the skipjack tuna Katsuwonus pelamis

This research project was undertaken on two species of tropical tuna: the yellowfin Thunnus albacares and the skipjack Katsuwonus pelamis, to explore the dynamic physiological design features of tunas which underly the highly-developed thunniform propulsion mechanism. To achieve an integrated understanding of tuna swimming mechanics, a combination of electromyography, kinematics, and direct force measurements was used to probe the dynamic function of the myotomal red muscle as fish swam over a range of sustained, aerobic swimming speeds in a large water tunnel.In both species, onset of red muscle activation proceeds sequentially in a rostra-caudal direction, while deactivation is nearly simultaneous at all sites, coincident with peak force in the caudal tendons. In yellowfin, there is complete segregation of contralateral activity, while in skipjack there is slight overlap. In both species, all red muscle on one side is active simultaneously for part of each cycle. Comparison with other fish species shows that the tuna EMG patterns culminate a spectrum of activation patterns underlying swimming modes from anguilliform to thunniform.Coupling kinematics with EMG data revealed that muscle activation occurs after the midline has reached its maximum convexity at any given axial location; therefore, it was concluded that midline curvature is not an accurate indicator of local strain in the myotomal cone muscle. Activation and curvature waves travel down the body at the same rate, so muscle function is predicted not to vary by axial location. In yellowfin, peak force occurs in the caudal tendons as the tail tip crosses the midpoint of its sweep; in skipjack, when the tip is about three-quarters through its sweep.Internal force measurements were made for the first time in any fish species by fitting a stainless steel buckle force transducer around the pair of deep caudal tendons on one side. At cruising speeds, mean forces were 1.3-3.8 N in yellowfin and 1.6-4.1 N in skipjack. At restrained burst speeds, forces were approximately 10 times higher. The biomaterial properties of the tendons show that they function as inextensible linkages, rather than biological springs, in transferring muscle force to the tail