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 musc ulature and dorso-ventrally symmetrical external and internal morphology. B ecause of this symmetrical morphology, it has often been assumed that scomb rid caudal fins function as predicted by the homocercal tail model. Accordi ng to that model, the caudal fin moves in a dorso-ventrally symmetrical man ner and produces no vertical lift during steady swimming, To test this hypo thesis, 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 vide otaped from lateral and posterior views, allowing a three-dimensional analy sis of tail motion. Analysis of tail kinematics suggests that, at a range o f 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 excurs ion of the dorsal tail-tip also increases significantly by 32 % over this r ange of speeds, indicating a substantial increase in tail-beat amplitude wi th speed. In addition, if the tail were functioning in a dorso-ventrally sy mmetrical manner, the tail should subtend an angle of 90 degrees relative t o the frontal (or xz) plane throughout the tail beat. Three-dimensional kin ematic analyses reveal that the caudal fin actually reaches a minimum xz an gle of 79.8 degrees. In addition, there is no difference between the angle subtended by the caudal peduncle (which is anterior to the intrinsic tail m usculature) and that subtended by the posterior lobes of the tail. Thus, as ymmetrical movements of the tail are apparently generated by the axial musc ulature and transmitted posteriorly to the caudal fin. These results sugges t that the caudal fin of the chub mackerel is not functioning symmetrically according to the homocercal model and could produce upward lift during ste ady swimming.