Tail kinematics of the chub mackerel Scomber japonicus: Testing the homocercal tail model of fish propulsion

Scombrid fishes possess a homocercal caudal fin with reduced intrinsic musculature and dorso-ventrally symmetrical external and internal morphology. Because of this symmetrical morphology, it has often been assumed that scombrid caudal fins function as predicted by the homocercal tail model. According to that model, the caudal fin moves in a dorso-ventrally symmetrical manner and produces no vertical lift during steady swimming. To test this hypothesis, we examined the tail kinematics of chub mackerel, Scomber japonicus (24.8±1.3 cm total length, L). Markers were placed on the caudal fin to identify specific regions of the tail, and swimming chub mackerel were videotaped from lateral and posterior views, allowing a three-dimensional analysis of tail motion. Analysis of tail kinematics suggests that, at a range of swimming speeds (1.2-3.0 L s^-1), the dorsal lobe of the tail undergoes a 15% greater lateral excursion than does the ventral lobe. Lateral excursion of the dorsal tail-tip also increases significantly by 32% over this range of speeds, indicating a substantial increase in tail-beat amplitude with speed. In addition, if the tail were functioning in a dorso-ventrally symmetrical manner, the tail should Subtend an angle of 90°relative to the frontal (or xz) plane throughout the tail beat. Three-dimensional kinematic analyses reveal that the caudal fin actually reaches a minimum xz angle of 79.8°. In addition, there is no difference between the angle subtended by the caudal peduncle (which is anterior to the intrinsic tail musculature) and that subtended by the posterior lobes of the tail. Thus, asymmetrical movements of the tail are apparently generated by the axial musculature and transmitted posteriorly to the caudal fin. These results suggest that the caudal fin of the chub mackerel is not functioning symmetrically according to the homocercal model and could produce upward lift during steady swimming.