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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

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Pelvic fin locomotor function in fishes: three-dimensional kinematics in rainbow trout (Oncorhynchus mykiss)

E. M. Standen

Museum of Comparative Zoology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA

View larger version (10K): [in this window] [in a new window]   Fig. 1. An evolutionary transformation: paired fin position throughout fish evolution. A representative cladogram of selected fish groups. In the basal condition, paired pectoral fins (red) are ventrally located and paired pelvic fins (green) are located behind the centre of mass. In the derived condition, pectoral fins are located laterally on the body and the pelvic fins are located directly below or even in front of the centre of mass. Black and white circles represent the estimated location of fish centre of mass.

View larger version (17K): [in this window] [in a new window]   Fig. 2. Experimental apparatus. Fish swam in a multi-speed flow tank. High-speed cameras filmed ventral and lateral views simultaneously, enabling three-dimensional analysis of fin motion. A mirror slightly angled along the right side of the fish allowed full visualization of the right pelvic fin. Fin motion was measured relative to the water flow (flow axes) and relative to the fish body position (fish axes, which change as the fish turns during manoeuvres).

View larger version (52K): [in this window] [in a new window]   Fig. 3. Motion of lateral edge of left pelvic fin. The motion of each pelvic fin is described as the change in angle of the fin's lateral edge with the mid-sagittal plane and the transverse plane of the fish. The fin's lateral edge is defined as the line between where the base of the most lateral pelvic fin ray attaches to the body and the tip of the same lateral-most fin ray. (A) Purple represents the fish's mid-sagittal plane; cyan represents the fish's transverse plane. (B) Representative angles are labeled S (sagittal) and T (transverse) to depict the angle measured. Black lines represent the left pelvic fin lateral edge as it moved through its oscillating cycle over four fin beats. (C) Plot of the three-dimensional motion of the lateral pelvic fin-ray tip over one fin beat. Axes dimensions are the same as in B with all units in cm and no temporal offset in the crainio-caudal axis. The circles represent the start of fin stroke when the angle between the fin edge and the transverse plane was minimal. The green line represents the three-dimensional motion of the fin tip. Blue, cyan and black lines represent the motion of the fin tip projected on the three axial planes.

View larger version (41K): [in this window] [in a new window]   Fig. 4. Pelvic fin motion. Paired pelvic fins oscillated in a regular contralateral cycle. (A–D) Four stages of left pelvic fin oscillation are drawn in lateral, transverse and ventral views. The left fin is coloured green, the dorsal side of the fin in light green and the ventral side of the fin in dark green. (A) Column A is arbitrarily assigned as the start of the oscillating cycle when the fin is adducted against the body relative to the transverse plane (phase=0 deg.). (B) As the fin started its cycle the fin's lateral edge began abducting toward the transverse plane and supinated away from the sagittal plane where it reached maximum lateral excursion (phase=120 deg.). (C) As the fin continued abducting toward the transverse plane, the fin's lateral edge started pronating medially toward the fish's mid-sagittal plane (phase=180 deg.). (D) The fin began adducting away from the transverse plane as the fin's lateral edge was pronated maximally toward the mid-sagittal plane (phase=300 deg.). Finally, the fin made its return stroke back to a fully adducted, partially supinated starting position (A, phase=360 deg./0 deg.). Arrows in each of the images note the direction in which the fin is moving or about to move. Background colours of each panel correspond with those in Fig. 3.

View larger version (13K): [in this window] [in a new window]   Fig. 5. Paired pelvic fin oscillation during steady swimming. Oscillation of the pelvic fin's lateral ray tip in the x-dimension (motion in the sagittal plane along the cranio-caudal fish axis) over time. Green line represents the left pelvic fin; red represents the right pelvic fin.

View larger version (35K): [in this window] [in a new window]   Fig. 6. Magnitude of kinematic variables for body and fins during steady swimming. Green bars represent the left fin and red bars represent the right fin. Body and fins moved symmetrically around the midline of the fish [excursion to the left (cm)=excursion to the right (cm), P>0.07]. Fin excursions (body motion subtracted) were larger than body excursions (P=0.0011). For all remaining variables, maximum values were significantly larger than minimum values for each fin (P<0.0001). Right and left fins did not differ in fin area, fin or body velocity (black bars), or fin angle with the transverse plane (P>0.05). Left and right fins did differ in angle with the sagittal plane; right fins had larger pronation angles than left fins (P=0.0001). Large angles (rad) represent fin adduction/supination and small angles represent fin abduction/pronation. Asterisks above the bars denote significantly different values within fins (P<0.0001). Asterisks below the bars denote significantly different values between fins (P<0.001). Fin excursion, fin area and fin angle with the sagittal plane had significant interaction between fish and fin (P<0.0001). The subtle variation in these variables between animals suggests a fine-tuned adjustment of fin area and kinematics to maintain a steady gait.

View larger version (26K): [in this window] [in a new window]   Fig. 7. Pelvic fin kinematic timing during steady swimming. Complete pelvic oscillation cycle of left fin represented in a polar plot. 0 deg. arbitrarily represents the start of the stroke when fin is held against body. 180 deg. represents mid-stroke when the pelvic fin's lateral tip is maximally abducted. Each rectangle represents the 95% confidence interval around the mean peak kinematic variable with error bars depicting angular variance s2 (Batschelet, 1965; Batschelet, 1981). Bar colours define the following: black, maximum amplitude of body at fin attachment site; green, fin area; red, body velocity; yellow, fin velocity; dark blue, fin angle with transverse plane; light blue, fin angle with sagittal plane. Thick bars represent maximum values for each variable and thin bars represent minimum values. The data represent left pelvic fins of all fish during all swimming trials.

View larger version (82K): [in this window] [in a new window]   Fig. 8 Kinematic and timing data for manoeuvres. (A) Representative example of a single manoeuvre. The thick blue line represents the change in lateral body position (left y-axis); the blue dots on the line represent the starting position, the maximum body excursion to the outside of the turn, the maximum body excursion to the inside of the turn and the final body position. Three alternate final body positions are displayed with solid, dashed and dotted lines. Each manoeuvre is divided according to body excursion: stage one (green) is the start of the manoeuvre until the maximum excursion to the outside; stage two (yellow) is the maximum outside excursion to the maximum inside excursion; and stage three (pink) is the maximum inside excursion until the final body position. The black line represents heading over time (right y-axis). The timing of original heading (o), the start and end of maximum heading (ms and me, respectively), and the final heading (f) are noted by dashed vertical black lines. The diagram of fish below represents the general motion of the fish body at the time of original heading, the maximum heading start and end, and the final heading (f). (B,C) Polar plots of mean timing for all manoeuvres color coded to the three stages of the manoeuvre. Black bars represent the timing of heading changes and correspond with A. Only variables with directionality (Raleigh's test P<0.05) are represented on the polar plots. Small dots represent minimum/abduction mean values and large dots represent maximum/adduction mean values. (B) The outside fin had directionality in fin area (green), and fin abduction with fish transverse (blue) and flow transverse (red) planes. (C) The inside fin had directionality in fin area (green), and both fin abduction and adduction with fish transverse (blue) and flow transverse (red) planes. All variables that had consistent timing peaked well after the fish had started turning, suggesting that pelvic fins are not used to initiate manoeuvres but possibly to stabilize and control body position while returning it to a forward heading.

View larger version (57K): [in this window] [in a new window]   Fig. 9. Magnitude of kinematic variables for body and fins during manoeuvres. Fish body sides, and their respective fins, are defined as inside (the side closest to the direction of the turn, yellow) and outside (the side farthest from the direction of the turn, blue). Fin area was measured in cm2, angles were measured in radians, and velocities were measured in cm s–1. Asterisks above bars denote significantly different values within fins (P<0.05). Asterisks below bars denote significantly different values between fins (P<0.05). Solid colours represent maximum values and diagonal lines represent minimum values. All between fin comparisons were not significant, with the exception of fin angle with the flow sagittal plane (abduction and adduction) and with the fish sagittal plane (adduction only, P<0.05). Fin area, and fin angle with the sagittal plane (fish and flow axes) had significant interaction between fish and fin (P<0.03). The differences in these variables between individuals suggest fine-tuned adjustments of fin surface area and mediolateral motion during manoeuvres, which possibly affect roll stabilization.

View larger version (51K): [in this window] [in a new window]   Fig. 10. Pelvic fin steady swimming functional hypotheses. The complete pelvic oscillation cycle of left fin is represented in two identical polar plots. Body excursion (black), fin area (green), fin angle with transverse plane (dark blue), and fin angle with sagittal plane (light blue) are represented on the plots. 0 deg. arbitrarily represents the start of the stroke when the fin is held against the body. 180 deg. represents mid-stroke, when the outside fin tip is maximally abducted. Thick bars represent maximum values for each variable and thin bars represent minimum values. Data represent the left pelvic fins of all fish during all swimming trials. Widened areas on bars represent the mean and 95% confidence interval; thin lines represent the angular variance s2 (Batschelet, 1965; Batschelet, 1981). (A) Motion relative to the transverse plane. (i) As the pelvic fin supinates towards the transverse plane it actively pushes against the oncoming flow producing a braking force. (ii) The fin passively adducts away from the transverse plane owing to water drag, stabilizing and straightening the body in the flow. (B) Motion relative to the sagittal plane. (i) As the pelvic fin adducts away from the sagittal plane it moves in the same direction as the body, actively pushing against the induced flow and producing a lateral force in the direction the body is oscillating. This force may act to dampen body oscillation, helping to slow and reverse body motion. (ii) As the body changes direction the fin continues supinating away from the sagittal plane, passively moved by body induced water flow. This motion maximally supinates the fin, preparing for the next active cycle. (iii) The fin begins pronating towards the sagittal plane, in the same direction as body oscillation, against body induced flow. This produces a lateral force in the direction of body motion with maximum fin area dampening the body oscillation and helping to reverse the body direction. (iv) Body oscillation changes direction while the fin continues pronation. Induced flow due to body motion passively moves the fin to maximize pronation towards sagittal plane, preparing for the lateral force production of the next stroke.

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