DOWIS, Hawkins; DICKSON, Kathryn: Swimming kinematics of the eastern Pacific bonito (Sarda chiliensis): Testing hypotheses about the evolution of thunniform locomotion
Tunas are believed to be among the fastest and most efficient swimmers due to their fusiform body shape, �stiff-bodied� thunniform mode of locomotion, and ability to elevate the temperature of their slow-twitch, aerobic (red) locomotor muscle above ambient water temperature (endothermy). It has been hypothesized that the fusiform body shape and thunniform swimming mode are consequences of the unusual internalized, anterior red muscle position and elongated myotome structure found in tunas. To test the hypothesis that the evolution of thunniform locomotion is associated with the internalization of red muscle, we quantified the swimming kinematics of the ectothermic eastern Pacific bonito (Sarda chiliensis). This species is a member of the tunas’ sister group and their aerobic myotomal muscle is not internalized as it is in tunas. High-speed video recordings of bonito (45.5-50.5 cm FL) swimming at sustained speeds in a swimming tunnel respirometer at 18 C° were analyzed using a Peak Performance motion analysis system. The bonito swam between 50 and 140 cm s-1 (0.99 to 3.18 FL s -1) using tail-beat frequencies of 1.50 to 3.18 Hz. Tail-beat amplitudes ranged from 7.85 cm to 10.58 cm (15.5 % – 24.0% FL). Tail-beat frequency and amplitude increased with swimming speed. When compared to published kinematics data for tunas of similar sizes swimming at similar sustainable speeds, the bonito swam with a lower tail-beat frequency and greater amplitude. These data support the hypothesis that bonitos swim in a mode that differs from that of tunas, and thus suggests that thunniform locomotion is a derived characteristic of the tunas, associated with internalized red myotomal muscle.
