Finlets are small non-retractable fins located on the dorsal and ventral margins of the body between the second dorsal and anal fins and the tail of scombrid 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 examined in the chub mackerel Scomber japonicus. Functionally, S. japonicus has five dorsal and anal triangular finlets (the fifth finlet is a pair of finlets acting in concert). Slips of muscle that insert onto the base of each finlet indicate the potential for active movement. In animals of similar mass, 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 the horizontal and vertical planes. Finlet excursions in the x, y and z directions ranged from 1 to 5 mm, increased posteriorly and were independent of speed. The timing of the maximum amplitude of oscillation was phased posteriorly; the phase lag of the maximum amplitude of oscillation was independent of speed. During some periods of gliding, a finlet occasionally moved independently of the body and the other finlets, which indicated active control 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 flow longitudinally as the tail decelerates and thereby contribute flow to the developing caudal fin vortex.

Block
B. A.
,
Finnerty
J. R.
,
Stewart
A. F. R.
,
Kidd
J.
(
1993
).
Evolution of endothermy in fish: mapping physiological traits on a molecular phylogeny
.
Science
260
,
210
–.
Dewar
H.
,
Graham
J. B.
(
1994
).
Studies of tropical tuna swimming performance in a large water tunnel. III. Kinematics
.
J. Exp. Biol
192
,
45
–.
Dingerkus
G.
,
Uhler
L. D.
(
1977
).
Enzyme clearing and staining of alcian blue stained whole small vertebrates for demonstration of cartilage
.
Stain Technol
52
,
229
–.
Drucker
E. G.
,
Lauder
G. V.
(
1999
).
Locomotor forces on a swimming fish: three-dimensional vortex wake dynamics quantified with digital particle image velocimetry
.
J. Exp. Biol
202
,
2393
–.
Gibb
A. C.
,
Dickson
K. A.
,
Lauder
G. V.
(
1999
).
Tail kinematics of the chub mackerel Scomber japonicus: testing the homocercal tail model of fish propulsion
.
J. Exp. Biol
202
,
2433
–.
Gillis
G. B.
(
1997
).
Anguilliform locomotion in an elongate salamander (Siren intermedia): effects of speed on axial undulatory movements
.
J. Exp. Biol
200
,
767
–.
Jayne
B. C.
,
Lozada
A. F.
,
Lauder
G. V.
(
1996
).
Function of the dorsal fin in bluegill sunfish: motor patterns during four distinct locomotor behaviors
.
J. Morph
228
,
307
–.
Magnuson
J. J.
(
1970
).
Hydrostatic equilibrium of Euthynnus affinis, a pelagic teleost without a gas bladder
.
Copeia
1
,
56
–.
Nauen
J. C.
,
Shadwick
R. E.
(
1999
).
The scaling of acceleratory aquatic performance: body size and tail flip performance of the California spiny lobster Panulirus interruptus
.
J. Exp. Biol
202
,
3181
–.
Ono
R. D.
,
Poss
S. G.
(
1982
).
Structure and innervation of the swim bladder in the weakfish, Cynoscion regalis (Teleostei: Sciaenidae)
.
Can. J. Zool
60
,
1955
–.
Walker
J. A.
(
1998
).
Estimating velocities and accelerations of animal locomotion: a simulation experiment comparing numerical differentiation algorithms
.
J. Exp. Biol
201
,
981
–.
Walters
V.
(
1962
).
Body form and swimming performance in the scombroid fishes
.
Am. Zool
2
,
143
–.
Westneat
M. W.
,
Hoese
W.
,
Pell
C. A.
,
Wainwright
S. A.
(
1993
).
The horizontal septum: mechanisms of force transfer in locomotion of scombrid fishes (Scombridae, Perciformes)
.
J. Morph
217
,
183
–.
Wilga
C. D.
,
Lauder
G. V.
(
1999
).
Locomotion in sturgeon: function of the pectoral fins
.
J. Exp. Biol
202
,
2413
–.
This content is only available via PDF.

Supplementary information