Jc. Nauen et Gv. Lauder, Locomotion in scombrid fishes: Morphology and kinematics of the finlets ofthe chub mackerel Scomber japonicus, J EXP BIOL, 203(15), 2000, pp. 2247-2259
Finlets are small non-retractable fins located on the dorsal and ventral ma
rgins of the body between the second dorsal and anal fins and the tail of s
combrid fishes. The morphology of the finlets, and finlet kinematics during
swimming in a flow tank at speeds of 0.8-3.0 fork lengths s(-1), were exam
ined in the chub mackerel Scomber japonicus, Functionally, S, japonicus has
five dorsal and anal triangular finlets (the fifth finlet is a pair of fin
lets acting in concert). Slips of muscle that insert onto the base of each
finlet indicate the potential for active movement. In animals of similar ma
ss, finlet length and area increased posteriorly, Finlet length, height and
area show positive allometry in animals from 45 to 279 g body mass. Summed
finlet area was approximately 15% of caudal fin area.
During steady swimming, the finlets typically oscillated symmetrically in t
he horizontal and vertical planes. Finlet excursions in the x, y and z dire
ctions ranged from 1 to 5 mm, increased posteriorly and were independent of
speed. The timing of the maximum amplitude of oscillation was phased poste
riorly; the phase lag of the maximum amplitude of oscillation was independe
nt of speed. During some periods of gliding, a finlet occasionally moved in
dependently of the body and the other finlets, which indicated active contr
ol of finlet movement.
The angle of attack of the finlets averaged approximately 0 degrees over a
tailbeat, indicating no net contribution to thrust production via classical
lift-based mechanisms. However, the timing of finlet movement relative to
that of the tail suggests that more posterior finlets may direct some how l
ongitudinally as the tail decelerates and thereby contribute how to the dev
eloping caudal fin vortex.