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First published online September 5, 2008
Journal of Experimental Biology 211, 2931-2942 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.018572
Pelvic fin locomotor function in fishes: three-dimensional kinematics in rainbow trout (Oncorhynchus mykiss)
Museum of Comparative Zoology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA
e-mail: standen{at}fas.harvard.edu
Accepted 17 July 2008
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Summary |
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 TOP Summary INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Key words: swimming, manoeuvring, locomotion, pelvic fin, evolutionary fin function, stability, rainbow trout, Oncorhynchus mykiss
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INTRODUCTION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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). More recently, paired pectoral, and median dorsal and anal
fins in fishes have been shown to have locomotory function
(Arnold et al., 1991;
Jayne and Lauder, 1996;
Webb et al., 1996;
Westneat and Walker, 1997;
Schrank et al., 1999;
Drucker and Lauder, 2002;
Walker and Westneat, 2002;
Standen and Lauder, 2005;
Standen and Lauder, 2007;
Tytell et al., 2008). Paired
pelvic fin function, however, remains largely unstudied. This paper is the
first to study detailed three-dimensional movements of pelvic fins in fishes
and to evaluate hypotheses for how these fins might function.
Early work amputated pelvic fins and detected no change in body motion
during swimming, suggesting that pelvic fins had little or no locomotive
function (Monoyer, 1866;
Grenholm, 1923;
Harris, 1936). Elegant work by
Harris (Harris, 1937;
Harris, 1938) later refined
our ideas of pelvic fin function. He concluded that fish with basal fin
morphologies, such as sharks (Fig.
1; ventral pectoral fins and ventral pelvic fins posterior of the
centre of mass), had extremely limited pelvic fin function, whereas more
derived fishes, such as perch (Fig.
1; lateral pectoral fins and ventral pelvic fins anterior of the
centre of mass) had pelvic fins with limited trimming function to reduce
pitching and upward body displacement during braking. Regardless of body
position, pelvic fins were thought to be held fairly still, acting as static
trimming foils rather than dynamic moving structures. Researchers concluded
that pelvic fins had limited locomotor function during steady swimming.
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MATERIALS AND METHODS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Behavioural observations
Trout swam in the centre of the working area (28 cm wide, 28 cm deep, 80 cm
long) of a variable speed flow tank under conditions similar to those
described in previous hydrodynamic work
(Standen and Lauder, 2007). A
mirror was placed parallel to the flow inside the right side of the flow tank
to visualize the right pelvic fin (Fig.
2). The mirror lay 2.5 cm inside the working area of the flow tank
wall at its base and was flush with the tank wall roughly 14 cm up the right
tank side. The mirror only minimally reduced the working area of the flow tank
and fish swimming behaviour did not visibly change with the presence of the
mirror. Fish were recorded swimming steadily at speeds from 0.13 to 1.36
BL s–1. Fish also performed yawing turns while
swimming at speeds from 0.21 to 0.84 BL s–1. Turns
were not elicited but occurred as spontaneous feeding behaviours as fish
foraged for particles in the flow tank. I used two synchronized high-speed
video cameras (Photron Fastcam 1280x1024 pixels, Photron, San Diego, CA)
operating at 250 frames s–1 (1/250 s shutter speed) to
visualize the movement patterns of the pelvic fins
(Fig. 2).
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).
Each of these points could be seen from both cameras and were digitized in
both views. By using direct linear transformation, the 2D location of these
points in both views were used to calculate the camera locations relative to
each other and to predict the three-dimensional location of the known points
(Ty Hedrick custom Matlab DLTcalibration program, The Mathworks, Natick, MA,
USA). This calibration allowed the calculation of three-dimensional motions of
fish fins during locomotion.
Kinematic measurements
To quantify the temporal and spatial patterns of fin movement, video
sequences were analyzed using a custom digitizing program in Matlab (Ty
Hedrick DLTdataviewer, Matlab version R2006a). For each fish, left and right
pelvic fin motion was tracked by digitizing four points per fin. Points marked
the four fin corners of each fin. Two points marked the lateral-most fin ray:
one point at the base where the ray attaches to the body and one at the ray
tip. Two other points marked the medial most fin ray: again, one point at the
base and one point at the tip. These points clearly described the motion of
lateral and medial pelvic fin edges, as well as a relative approximation of
the fin area (the area within the four digitized points). Video sequences were
three to five consecutive contralateral fin beats in duration. The
three-dimensional motion of paired pelvic fin oscillation was quantified at 20
ms intervals.
Both fin tip velocity and body velocity were calculated for each fin. Body motion is dominated by the mediolateral (ML) oscillation of the propulsive wave moving along the fish. For the purposes of this paper, lateral body velocity was calculated as the time derivative of the displacement of the left medial fin attachment point along the mediolateral axis. Fin tip velocity was calculated by taking the time derivative of the three-dimensional lateral pelvic fin tip motion (body motion subtracted). This paper focuses on the magnitude and timing of the oscillation motion of the paired fins during slow-speed swimming and lateral yawing manoeuvres.
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For manoeuvring data, the axial planes that describe the fish body position were calculated by digitizing the fish's AP axis. The AP axis was defined from the nose tip to a clear mid-ventral point just anterior of the pelvic fins. The ML and dorsoventral (DV) fish axes were calculated using the flow axis in combination with the digitized AP axis. This was done by projecting the AP axis onto the plane perpendicular to the flow z-axis. The flow y-axis was then projected onto the plane perpendicular to this projected AP axis. This makes the AP and ML fish axes perpendicular and aligned with the fish's true heading. This system did not take into consideration any roll the fish may have experienced during its manoeuvre. After much testing, however, body roll was so slight that the error associated with calculating roll from off-axis camera views was larger than the measured roll itself. The DV fish axis was calculated by taking the cross product of the projected AP axis and the projected ML axis. This transforms all digitized coordinates into a right-hand rule system for easy identification of angular direction. These fish axes were used to calculate lateral pelvic fin ray tip position relative to the body (see description below).
During steady swimming trout oscillated their pelvic fins both mediolaterally and anteroposteriorly. These two motions are most easily conceived relative to the animal's mid-sagittal plane (ML motion; Fig. 3A, purple plane) and to the animal's transverse plane (AP motion; Fig. 3A, cyan plane). The terms abduct and adduct are used to describe fin motion relative to the body and within the sagittal plane; angles between the fin and the transverse plane oscillated during abduction and adduction. The terms pronate and supinate are used to describe fin motion relative to the body and within the transverse plane; angles between the fin and the sagittal plane oscillated during pronation and supination. Fin oscillation motion is described relative to the transverse and sagittal planes (for example, fins were abducted when moving towards the transverse plane).
Large angles between the fin edge and each plane occurred when the fin was close to the body (adducted in the mid-sagittal plane and suppinated in the transverse plane; Fig. 4A,B, respectively). Small angles, sometimes negative in the case of the mid-sagittal plane, were observed when the fin was away from the body (abducted in the mid-sagittal plane and pronated in the transverse plane; Fig. 4C,D, respectively). The magnitude of the angles between the fin edge and the mid-sagittal plane were corrected for the contralateral, parasagittal location of the left and right pelvic fins. This means, for both left and right fins, large angles occur at suppination and small angles at pronation. By defining the oscillation of each pelvic fin using these angles, the ML motion during the fin oscillation can be partially separated from the cranial-caudal fin motion, allowing for a discussion of possible force production by fins and resulting fin function.
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Timing
Paired pelvic fins have a complex three-dimensional oscillation when trout
swim at slow speeds. For steadily swimming fish, the beginning of the fin
stroke cycle was arbitrarily chosen as the point at which the lateral pelvic
fin ray tip was fully adducted against the body, roughly at right angles to
the fish's transverse plane. The stroke cycle was considered to be at
mid-stroke when the lateral pelvic fin tip was maximally abducted away from
the body towards the transverse plane. Polar coordinates were used to define
the timing of the stroke cycle. A complete fin beat cycle occurred over 360
deg.; the fin beat cycle started with maximal adduction at phase cycle 0 deg.,
moved through maximal abduction (mid-stroke) at phase cycle 180 deg., and
returned to maximal adduction at phase cycle 360/0 deg. All variables were
plotted relative to these polar coordinates, allowing analysis of the phase
relationships and timing.
I observed no regular fin oscillation cycle during manoeuvres. In lateral yawing manoeuvres, however, the body moved in a consistent S-shaped pattern. Left and right turns were grouped, and manoeuvres are described by IN/towards and OUT/away sides relative to fish turning direction. Based on the consistent S-shaped body movement pattern, manoeuvres were divided into three stages: the first stage starts with the original body position and ends with maximum excursion of the body away from the turn; the second stage starts with maximum excursion away from the turn to maximum excursion in the direction of the turn; finally, the third stage continues from maximum excursion in the direction of the turn to the final steady body position, most often slightly away from the turning direction. An example trace of the change in body excursion can be seen in the blue line of Fig. 8A.
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), with angular
variance calculated according to Batschelet
(Batschelet, 1965;
Batschelet, 1981).
Velocity data measurements during steady swimming were treated as
diametrically bimodal distributions, as they peaked twice in a single fin beat
(Zar, 1999). A Rayleigh's test
for circular uniformity was conducted on all maximum and minimum values for
each variable and fin to determine whether variables occurred at predictable
times in the oscillation cycle. The approximation:
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The timing of all variables was tested first between maximum and minimum
values within one fin and then compared between fins. Timing between steady
swimming and manoeuvring data was not compared statistically to avoid
inferring meaning from differences that might be found between non-comparable
data sets. Magnitudes of maximum and minimum values for all variables, fins and behaviours were calculated using standard statistical procedures to calculate mean and standard error. For each fin, maximum and minimum values were compared for each variable. Maximum and minimum values were also compared between fins to assess the symmetry of the paired fin strokes. Simple two-sided t-tests were conducted on variables with equal variance. When a two-sided F-test determined that samples had unequal variance, the Welch ANOVA was used to test for equal means.
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RESULTS |
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The majority of fin motion was driven by the fin's lateral edge. Plotting the fin's lateral edge over time shows the regular fin oscillation relative to the mid-sagittal and transverse planes (Fig. 3B). Following the lateral pelvic fin tip over one oscillation cycle shows the fin path was oval when projected in all three planes (Fig. 3C). The fin's path for the first half of the stroke was different from the return path, possibly due to functional partitioning of fin motion throughout the stroke.
During steady swimming, left and right pelvic fins did not differ significantly in oscillation kinematics (relevant statistics reported in timing and magnitude results below). Left and right fins oscillated roughly 180 deg. out of phase, this means that when the left fin was maximally abducted towards the transverse plane, the right fin was maximally adducted (Fig. 5; circular test for equal means, left fin abduction angle equals right fin adduction angle, F0.05(1),1,95-2calc=0.1288, P=0.7205).
Six variables were used to describe fin motion (Fig. 6; see also Table S1 in the supplementary material). Body amplitude was symmetrical around the midline; body excursion to the left did not differ from body excursion to the right. There were significant differences between maximum and minimum values for all variables (P=0.0001 for all comparisons; Fig. 6). Left and right fins were not statistically different for all variables, with the exception of the angle of the fin with the mid-sagittal plane; in this case the left fin supinated more than the right fin (P=0.0083). Body and fin velocity oscillated between maximum and minimum values in a diametrically bimodal distribution caused by the side-to-side fish body motion.
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Left and right fins did not differ in timing (Fig. 7). All variables had discreet maximum and minimum peaks (abduction/adduction and supination/pronation in the case of angles), which occurred roughly 180 deg. out of phase from one another. Variation in the timing of adduction and abduction was due to the calculation of angles using three-dimensional variables rather than the two-dimensional location of fin's lateral tip oscillation, which was used to define arbitrary phase timing (Fig. 5). Fin area was minimal at the start of the cycle, and maximal at mid-stroke. Body excursion is discussed relative to the fin of interest. Maximum body excursion towards the fin side occurred just after cycle start (at roughly 40 deg.) and maximum body excursion toward the non-fin side occurred roughly 180 deg. later, just after mid-cycle (at roughly 220 deg.). Minimum body velocity was diametrically bimodal in its distribution, and occurred near maximum body excursion (roughly 40 and 220 deg.); maximum body velocity, also bimodal, occurred roughly 180 deg. later, halfway between peak body excursions. Maximum fin supination away from the sagittal plane occurred after maximum fin-side body excursion (at roughly 120 deg.), pronation occurring roughly 180 deg. later, just after maximum non-fin side body excursion, at roughly 300 deg. Fin velocity was also diametrically bimodal in its distribution. Maximum and minimum fin velocity occurred at roughly 35 deg. and 120 deg., respectively, after peak body excursions.
Fin kinematics: manoeuvres
None of the variables measured during manoeuvres, including heading change,
showed consistent timing with respect to fin motion (Raleigh's test,
P
0.0767 for all comparisons). Body motion and the fish's heading,
however, were relatively consistent between manoeuvres and can be used to
define manoeuvre stages, thereby providing a template for discussing fin
motion (Fig. 8). Manoeuvres
resulted in a lateral displacement of the fish's body. During manoeuvres, fish
bodies traced an s-shaped path. Manoeuvres began with a change in heading in
which the fish's nose moved in the direction of the turn while its pelvic
girdle was forced away from the turn. The maximum change in heading
corresponded with the maximum body excursion away from the turn. Usually the
maximum heading remained relatively constant for some time as the body surfed
across the flow in the direction of the yawing turn. Both the duration and the
distance of the sideways body displacement were determined by the length of
time the fish held its maximum change in heading. Just before the body was
half way through its maximum lateral displacement the heading began to return
to the centre, reaching final heading sometime after the body reaches maximum
excursion in the direction of the yaw. The final stage of the turn was
somewhat variable; the body did one of three things
(Fig. 8A): (1) it remained at
maximum yawing excursion; (2) it returned slightly towards its original
position; or (3) it continued drifting in the direction of the yaw. These
outcomes were determined by heading changes and might be the result of
variation in fin motion throughout stage two of the yawing manoeuvre.
The six variables used to describe fin motion during steady swimming were also used to describe fin motion during manoeuvres (Fig. 9; see Table S3 in the supplementary material). Angles relative to fish body axes were also included for manoeuvres, as fish heading was no longer equal to that of the flow. In addition, fins were divided according to their location relative to the turning manoeuvre. Fins located on the side toward which the fish turned were considered inside fins and those located on the side from which the fish turned away were outside fins. All comparisons were made between inside and outside fins.
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As expected, body amplitude was not symmetrical around the body midline during manoeuvres. Body excursion in the turning direction was larger than the initial excursion away from the turn (Fig. 9). Body velocities also showed distinct maximum and minimum values. Kinematics were similar between inside and outside fins with two distinct differences (Fig. 9); inside fins had a far greater pronation and a much smaller supination relative to the flow compared with outside fins, and the adduction and abduction angles of the inside fin to the fish's body did not differ (Fig. 9).
Fin timing: manoeuvres
The timing of each variable during manoeuvres is represented using polar
coordinates (Fig. 8; see Table
S4 in the supplementary material). Manoeuvres were divided into three stages
(Fig. 8A). The first stage
bounded by angles 0 deg. to 120 deg. represented the period from the original
heading until the body reached the maximum excursion away from the turn. The
second stage bounded by angles of 120 deg. to 240 deg. was the period between
maximum body excursion away from the turn and maximum body excursion towards
the turn. Finally, the third stage, angles 240 to 360 deg., was variable and
represented the period between maximum body excursion towards the turn until
the body position stabilized. Sometimes this occurred immediately
(Fig. 8A, dashed line), and
sometimes the body continued on a regular drift towards one side or the other
(Fig. 8A, solid and dotted
lines).
Manoeuvres were variable but a general pattern emerged among trials. This pattern consisted of several steps. The fish's heading changed, the inside pelvic fin abducted and supinated away from the body, the rate of heading change slowed and the outside pelvic fin abducted and supinated, the body straightened and both fins weakly adducted and pronated but remained fairly extended. This pattern happened quickly and with temporal variability between manoeuvres making it difficult to find statistical significance between trials. To clarify the timing data, a single value for each variable was used during each manoeuvre to calculate timing. For example, fin area peak timing was determined by the largest maximum peak area and smallest minimum peak area for each manoeuvre.
During manoeuvres not all variables had directionality when plotted onto
the stages of body motion as described above. Timing varied considerably for
many variables, meaning there were no significant calculations of mean timing
angle during a manoeuvre. The variables that did have angular directionality
were inconsistent between fins. For the outside fin, fin area and abduction
angles between the fin's lateral edge and the transverse plane of the fish and
the flow had direction, or predictable timing. For the outside fin, all other
variables were evenly distributed throughout the manoeuvre cycle and had no
significant directionality (Raleigh's test, P0.0647 for all
variables). Inside fin area, fin abduction and fin adduction (relative to both
fish and flow) were directional. For the inside fin, all other variables were
evenly distributed throughout the manoeuvre cycle (Raleigh's test,
P0.1245 for all variables).
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Comparing manoeuvres with steady swimming
Slow-speed steady swimming in trout was accompanied by remarkably
predictable and regular pelvic fin oscillations
(Fig. 7). Manoeuvres, by
contrast, showed a large variation in pelvic fin motion. Inside and outside
fins did not have a clear phase relationship relative to the body or to each
other (Fig. 8). I used average
body excursion (left and right sides pooled for steady swimming; in and out
sides relative to turning pooled for manoeuvres) to make a conservative
comparison of body motion between steady swimming and manoeuvring behaviours.
Body excursion during manoeuvres (mean excursion=2.68 cm) was far greater than
during steady swimming (mean excursion=0.24 cm; t-test for unequal
variance, d.f.=1, 11.004, P=0.0133). Fin area during manoeuvres (max
1.57±0.11, min 1.36±0.10) was also greater than during steady
swimming (max 1.25±0.04, min 0.94±0.03; manoeuvres vs
steady swimming t-test for unequal variance: max, d.f.=1, 95.061,
P=0.0003; min, d.f.=1, 85.999, P=0.0001). Angles between the
fin's lateral edge and the flow axes were similar between manoeuvres and
steady swimming (t-test for unequal variance for all comparisons,
P0.084; see Tables S1 and S3 in the supplementary material), with
the exception of fin abduction toward the flow transverse plane, which was
smaller for manoeuvres (t-test for unequal variance, d.f.=1, 59.129,
P=0.0341). Fin's lateral edge pronation and supination relative to
the fish's sagittal plane did not differ between manoeuvres and steady
swimming (t-test for unequal variance: supination, d.f.=1, 54.603,
P=0.6313; pronation, d.f.=1, 51.991, P=0.3096). By contrast,
fin's lateral edge adduction and abduction relative to the fish's transverse
plane was significantly higher during manoeuvres than during steady swimming.
This difference in peak adduction/pronation and abduction/supination angles
between behaviours may be due to the slight tilting behaviour of trout when
swimming steadily at slow speeds. The angular difference between
adduction/pronation and abduction/supination was roughly the same for
manoeuvres and steady swimming. Although minimum fin velocity does not differ
between steady swimming and manoeuvres, maximum fin velocity is greater during
manoeuvres (t-test for unequal variance: min, d.f.=1, 55.043,
P=0.1377; max, d.f.=1, 58.791, P=0.0067). The maximum and
minimum body velocity was greater during manoeuvres than during steady
swimming (t-test for unequal variance: min, d.f.=1, 41.372,
P=0.0001; max, d.f.=1, 41.033, P=0.0001).
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DISCUSSION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Paired pelvic fin motion during steady swimming
Of the few studies previously done on pelvic fins, Harris provides the most
complete description of pelvic fin function
(Harris, 1936;
Harris, 1937;
Harris, 1938). For Harris
(Harris, 1938), pelvic fin
function depended on a phylogenetic context
(Fig. 1); fish such as sharks,
with a basal fin morphology (ventral pectoral fins and pelvic fins located
posterior to the centre of mass), showed little to no pelvic fin function,
whereas more derived teleosts (lateral pectoral fins and pelvic fins directly
below or in front of the centre of mass) had pelvic fins with limited
elevating and depressing functions. These hypotheses were supported by earlier
research in which pelvic fins had been amputated and little to no effect on
fish stabilization had been found
(Monoyer, 1866;
Grenholm, 1923). The most
definitive pelvic fin function proposed by Harris, was that pelvic fins
counteracted the upward drift of the body caused by lateral pectoral fins
during braking in derived perciform fishes
(Harris, 1938). Excluding
fishes with highly unique and specialized pelvic fin structures, Harris
described pelvic fin motion as a limited abduction and adduction directly
below the body, resulting mainly in static trimming forces
(Harris, 1938). In this paper,
I show that trout – a member of the actinopterygian sub-class of fishes,
with pelvic fins located well behind the centre of mass – have an active
and complex three-dimensional pelvic fin motion, suggesting that pelvic fins
have a stabilizing and locomotor function beyond the limited scope proposed by
Harris.
During slow-speed swimming (0.13–1.36 BL
s–1) trout moved their pelvic fins in contralateral
oscillations. Each individual pelvic fin moved in two major oscillations that
overlapped during the fin beat cycle (left and right fins act 180 deg. out of
phase): (1) oscillations relative to the transverse plane (initiated first);
and (2) oscillations relative to the sagittal plane (initiated 120 deg. after
the transverse plane oscillations). Both the direction and timing of these
oscillations suggested that pelvic fin motion is, at least partially, the
result of active muscle use, and not due to body and water motion alone. For
example, fish swam head into the flow; therefore, as pelvic fins oscillated
towards the transverse plane, they moved against the downstream current. This
motion would require muscle activation, particularly as the fin area increased
as they were presented to the flow. Fin oscillation relative to the sagittal
plane also appeared to be active. There were two points in the sagittal
oscillation cycle at which, instead of moving with the lateral flow caused by
body motion, the fin tips moved with the body against the local flow pattern.
Pelvic fins do have a complex musculature: two sets of abductors and adductors
control each pelvic fin surface (Grenholm,
1923), suggesting that fins can be used dynamically to produce
forces in many directions. The contralateral pelvic fin oscillation suggested
that fin musculature is active over 75% of the stroke cycle, although future
studies that record electrical activity in pelvic fin muscles will be needed
to confirm and demonstrate the pattern of pelvic fin muscle use.
Hypothesized pelvic fin function during steady swimming
Based on kinematic analysis it appears that pelvic fin oscillation produced
a series of forces (Fig. 10).
During the stroke phases when the fin was actively pushing against flow, one
can assume the fin produced hydrodynamic force. There were also points in the
oscillation cycle when fins seemed to move with the flow, and were not
necessarily powered actively; these phases may also be hydrodynamically
functional.
As the fin oscillated on approaching the transverse plane, its surface area
increased against the oncoming flow, possibly causing a drag-based force. This
braking force might: (1) help to slow the fish's forward speed [trout are fast
endurance swimmers, comfortable swimming at speeds of 2.5 BL
s–1 (Bainbridge,
1960; Webb, 1971a;
Webb, 1971b), they may use
their fins at slow speeds to control forward velocity
(Webb, 2006)]; (2) act as a
pitch control (providing a drag surface well behind the fish's centre of mass
forces the tail up); and (3) act to pivot the fish around the pelvic fin base,
providing yawing stabilization, which helps to change the fish's heading and
start the next body wave oscillation. The second half of pelvic oscillation
with the transverse plane appears to be passive; oncoming flow pushes the fin
into an adducted position along the body. Although this phase of the stroke
may not require muscle activity, the drag on the fin by the flow, which
adducts the fin, might act as a stabilizing force. The fin, like a dihedral
foil, may produce roll- and yaw-stabilizing lift in much the same way feathers
on an arrow act to straighten the trajectory of the shaft
(Fish, 2002;
Weihs, 1993;
Weihs, 2002).
Fin oscillation with the sagittal plane should produce lateral jet forces
during the part of the cycle in which the fin is moved actively against local
flow. Dorsal and anal fins that oscillate relative to the fish's sagittal
plane have been shown to produce clear lateral jets
(Standen and Lauder, 2007). I
would expect the pelvic fin sagittal oscillation to produce similar
hydrodynamics. For example, the left fin, supinating while moving left with
the body, should produce a left lateral force with the fin's dorsal side. This
force would dampen the leftward body oscillation preparing for the body's
return to the right. Later in sagittal plane oscillation, the fin pronates
towards the right as it moves rightward with the body. This motion should
produce a rightward lateral force with the fin's ventral surface, again
helping to dampen rightward body oscillation. During the portion of the
sagittal oscillation that appears to be passively driven by flow, the pelvic
fin may have two functions: (1) the fin might again be acting as a passive
stabilizing foil as it moves first into maximum supination and later into
maximum pronation away from the body; and (2) the fin might be reducing energy
expenditure by passively allowing water pressure to move the fin into maximum
supination and then pronation positions. This passive motion would prepare the
fin for maximum lateral force production when it begins its active movement
against the flow.
In summary, when both fins are oscillating 180° out of phase, three major forces appear to always be in effect. (1) A powered braking force (as one fin or the other abducts into the oncoming flow). (2) A passive stabilization force (fins producing static drag and possibly lift, like a dihedral foil, as they are adducted by the flow). And (3), lateral thrust forces that combine to dampen body oscillation (when the left fin is pushing with its dorsal side towards the left, the right fin, 180° out of phase, is also pushing towards the left but with its ventral side). Finally, the phase lag between individual fin oscillations ensures that pelvic fins never lie directly along the fish's belly. When either fin is fully adducted relative to the transverse plane it is partially supinated relative to the sagittal plane and vice versa, providing a stabilizing roll reducing foil at all points in the cycle. During slow-speed steady swimming in trout, pelvic fins have a complex, active motion that appears to have both a dynamic-powered and a static-trim force producing function.
Manoeuvring
Pelvic fin kinematics were highly variable during manoeuvres. Fish
voluntarily manoeuvred while feeding on food pellets moving through the flow
tank. Consequently, the distance of pellets from the fish's original position
varied. Moreover, the pellet's drifting speed varied depending on its position
in the flow tank. Judging the distance and speed of target food particles,
fish actively modulated pelvic fin kinematics to help control body position
during food capture.
Despite these differences between manoeuvres, a general movement pattern
existed. Manoeuvres began with a change in the fish's heading followed by
inside fin abduction. Next, there was a reduction in heading change with
abduction of the outside fin. Finally, fish heading returned close to the
original heading and often was accompanied by a secondary inside fin abduction
(Fig. 8). Fish use their
pectoral fins and body musculature to initiate heading changes when turning
(Drucker and Lauder, 2003).
Peak pelvic fin oscillation occurs well after the initiation of heading change
(Fig. 8), suggesting that
pelvic fins contribute to body posture control after turn initiation.
The timing patterns of fin motion during manoeuvres suggested that two mechanisms of control are used by pelvic fins: variable and consistent. Peaks in pronation and supination of fins occurred irregularly throughout manoeuvres, suggesting that pelvic fins are capable of fine tuning their motion and timing to offset perturbations. Conversely, peaks in abduction, adduction and fin area have consistent timing, and can be predicted to occur at particular points during a manoeuvre, suggesting coordinated function. Consistent pelvic fin motion during manoeuvres implies asymmetric functions between inside and outside pelvic fins.
The motion of the fin on the inside of the turn suggests two main functions. (1) Abduction just before and during maximum heading increases inside fin drag. Because pelvic fins are located behind the centre of mass, this may pivot the body into the turn, helping to maintain and stabilize maximum heading amplitude. And (2), rapid fin adduction as the fish heading returns to centre, again, might pivot the body in the reverse direction, allowing the body to realign behind the centre of mass, ultimately returning the fish heading to centre. Maximal fin area at this point also provides a larger stabilizing foil surface.
Concurrently, the outside fin motion also suggests two unique functions. (1) Just prior to maximum heading change, the outside fin has minimal area, possibly reducing deleterious drag and thereby reducing the turn angle and (2) at the end of maximum heading, the outside fin abducted with maximal fin area; this may increase drag on the outside of the turn, continuing to pivot the body around the centre of mass, bringing the heading to centre and completing the turn.
Steady swimming and manoeuvres compared
Webb defines two types of correction forces that aquatic organisms use to
maintain stability: powered corrections and trimming corrections
(Webb, 2002). Powered
correction forces are active motions of fins, independent of body motion, to
produce forces. Trimming correction uses induced flow over a relatively
stationary fin to produce forces as the body is moving through the fluid.
Trout pelvic fins appear to produce powered correction forces during
slow-speed swimming when the body is not moving fast enough to produce
trimming forces over the fins, and trimming correction forces during
manoeuvres when the lateral speed of the body is increased.
Conclusions
Harris (Harris, 1938)
concluded that pelvic fins, at best, produced weak trimming forces; fish with
basal morphologies having less functional pelvic fins than those with derived
morphologies. The data from this paper clearly show complex three-dimensional
motion in trout pelvic fins, which suggests complex pelvic fin function in an
actinopterygian with relatively basal fin morphology
(Fig. 1). Future
electromyographic and flow visualization experiments will clarify the
activation pattern of muscles driving the pelvic fins and the resultant
hydrodynamic forces produced, testing the hypotheses of fin function set forth
in this paper.
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Acknowledgments |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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