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First published online July 6, 2005
Journal of Experimental Biology 208, 2661-2671 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01708

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Spatial and temporal patterns of water flow generated by suction-feeding bluegill sunfish Lepomis macrochirus resolved by Particle Image Velocimetry

Steven W. Day^1,*, Timothy E. Higham¹, Angela Y. Cheer² and Peter C. Wainwright¹

¹ Section of Evolution and Ecology, University of California, One Shields Avenue, Davis, CA 95616, USA
² Department of Mathematics, University of California, One Shields Avenue, Davis, CA 95616, USA

View larger version (31K): [in a new window]   Fig. 1. Schematic of the experimental setup, showing experimental tank, position of laser sheet, optics, mirrors, camera, tank divider and door.

View larger version (29K): [in a new window]   Fig. 2. Representative PIV measurements of the fluid field in front of the feeding fish at four times; opening (A), prey entering (B), peak gape (C), and closing (D) during the strike. Color contours represent the magnitudes of fluid speed and streamlines show the direction of velocity. The position of both the predator and prey are shown overlaid on the PIV measurement. Note that the region of significant fluid velocity induced by suction is constrained to a region in close proximity to the mouth and extends approximately equidistant in all directions from the mouth, and that the magnitude of speed and size of the affected region both increase during mouth opening and are maximum at peak gape. Measurements with a signal-to-noise ratio less than 2.0 are removed, but the more rigorous validation scheme that was applied to the extracted profiles has not been applied, resulting in some erroneous measurements near and within the mouth aperture in frames A and B.

View larger version (19K): [in a new window]   Fig. 3. Profiles of speed (A) and scaled speed (B) measured along the centerline transects for three different feedings of the same individual. (A) The magnitude and shape of the velocity profile are affected both by variation in peak gape (PG) and time to peak gape (TTPG). (B) Data for the same three feedings as in A, in addition to 13 others from this individual, after scaling. Spatial distances are scaled by gape at the time of the velocity measurement. Fluid speeds are scaled by the fluid speed measured at a distance of gape (FSgape) in front of the fish. A polynomial fit to the data of all 16 feedings is shown as a red broken line (r2=0.986). Error bars represent the S.D. of the residuals about the fit line and are shown at every 0.1 scaled distance. The equation for speed along the centerline from the theoretical model of Muller (1982) is shown as the blue broken line (r2=0.986). Note that although variations in PG and TTPG have a significant effect on the absolute values of speed, the scaled profiles for all transects are very similar to one another.

View larger version (19K): [in a new window]   Fig. 4. Mean profiles of scaled speed along the centerline transect for each of the three individuals in addition to the polynomial fit to the pooled data set. SSpooled=0.348x4–2.49x3+6.61x2–7.78x+3.56. Error bars represent S.D. of the residuals about the fit lines. Note that all mean profiles fall within the error bars of the fit to the pooled data at all locations measured. Fluid speed at the mouth is somewhere between 3 and 4 times that at gape and fluid speed at a distance of 1.0 gape is approximately 0.25 the speed at gape.

View larger version (16K): [in a new window]   Fig. 5. Comparison of velocity profiles along the centerline and off-centerline transects within the mid-sagittal (MS-30 and MS-60) plane and frontal plane (F-30 and F-60) for individual #3. All transects share a common intersection at the center of the mouth opening. The region of significant fluid velocity induced by suction is constrained to a region close to the mouth. Note that the similarity between the velocity profiles shown in the mid-sagittal and frontal plane demonstrates that the distribution of velocity is essentially axi-symmetric, despite a laterally compressed fish body.

View larger version (132K): [in a new window]   Fig. 6. Outline of the parcel of water ingested by the feeding fish during a suction-feeding event. All particles suspended within the white line were ingested during this strike. Inset shows the relative position of the fish at peak gape to the boundary of ingested fluid. Note the prey in the center of the parcel of water.

View larger version (26K): [in a new window]   Fig. 7. Fluid speed compared to gape distance and ram speeds as a function of time for the two representative sequences. Fluid speed is located at a constant distance in front of the fish equal to PG. (A) A relatively slow strike (TTPG=42 ms) and (B) a fast strike (TTPG=14 ms). Note the different x-axis scales for the two feedings. Although the magnitude of speeds and duration of events is different between the two, the relative timing is similar in that peak jaw speed precedes peak fluid speed, which in turn slightly precedes peak gape. Body ram speed continually decreases throughout the strike.

View larger version (10K): [in a new window]   Fig. 8. Relative timing of kinematic events to measured peak fluid measured fluid speeds. To account for variation in absolute speed of the event, all times are shown normalized to TTPG. The time of each event is relative to the time of 20% opening and expressed as a fraction of the time from 20% opening to 95% opening (TTPG) so that both fast and slow strikes may be compared in the analysis. Because of the definition of TTPG used, the kinematic events of 20% PG and 95% PG are necessarily located at 0 and 1, respectively. All other symbols and error bars show the mean ± S.D. for all 42 feeding analyzed. Note that peak fluid speed occurs at approximately the same time as 95% opening, slightly preceding peak gape and peak protrusion. Events that have some duration, such as mouth opening more than 20% or 95% and the prey entering are represented as a filled bar with error bars (S.D.) for the start and finish of these events.

View larger version (22K): [in a new window]   Fig. 9. DPIV data from 42 feeding sequences from three bluegill showing that time to peak gape (TTPG) is closely related to peak fluid speed (FS; measured at PG in front of the fish on the centerline), illustrating one mechanism of enhancing fluid speed during suction. Individual #1, peak FSPG=4.6 xTTPG–0.86 (r2=0.86); Individual #2, peak FSPG=4.0 xTTPG–0.84 (r2=0.84); Individual #3, peak FSPG=4.2 xTTPG–0.80 (r2=0.92); pooled data from all 42 feedings from three individuals, peak FSPG=4.5 xTTPG–0.85 (r2=0.87); speed, m s–1; time, ms.