Ac. Gibb et al., Tail kinematics of the chub mackerel Scomber japonicus: Testing the homocercal tail model of fish propulsion, J EXP BIOL, 202(18), 1999, pp. 2433-2447
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.