Volumetric imaging of fish locomotion

Abstract

Fishes use multiple flexible fins in order to move and maintain stability in a complex fluid environment. We used a new approach, a volumetric velocimetry imaging system, to provide the first instantaneous three-dimensional views of wake structures as they are produced by freely swimming fishes. This new technology allowed us to demonstrate conclusively the linked ring vortex wake pattern that is produced by the symmetrical (homocercal) tail of fishes, and to visualize for the first time the three-dimensional vortex wake interaction between the dorsal and anal fins and the tail. We found that the dorsal and anal fin wakes were rapidly (within one tail beat) assimilated into the caudal fin vortex wake. These results show that volumetric imaging of biologically generated flow patterns can reveal new features of locomotor dynamics, and provides an avenue for future investigations of the diversity of fish swimming patterns and their hydrodynamic consequences.

Figure 1.

Instantaneous volumetric wake visualization of vortices produced by the symmetrical tail of freely swimming fishes. (a) Bluegill sunfish (Lepomis macrochirus) swimming with the tail, dorsal and anal fins within the laser light volume and the vorticity isosurface (at 5.0 s⁻¹), to show the linked vortex rings produced by the tail. (b) White asterisks denote trailing edge vortices separating from the caudal fin of the bluegill. (c,d) Cichlid fish Pseudotropheus greshakei swimming just upstream of the laser volume and the linked chain vortex wake produced by the tail. (e) Angled view from below to show the vorticity isosurface and the three tail vortices (numbers correspond to rings shown in panel (d)). (f) Two orthogonal slices through the tail vortex wake to show the three-dimensional structure of the wake and the alternating jet flows (green arrows).

Figure 2.

(a) Vortical wake produced by dorsal (red), anal (blue) and caudal (green) fins of freely swimming bluegill sunfish (at 1.5 L s⁻¹) isosurfaced by absolute vorticity (5.0 s⁻¹). Alternating dorsal and anal fin vortices are visible due to location of these fins within the capture volume. (b) Slice of isosurface in (a), contoured by Z vorticity. (c) Three-quarter view of wake interaction of vortices produced by dorsal (red), anal (blue) and caudal (green) fins of live swimming bluegill (1.5 L s⁻¹) isosurfaced by absolute vorticity (5.0 s⁻¹). Velocity vectors are drawn from XZ planes bisecting the dorsal and caudal fin vortices, with every third velocity vector shown for clarity. (d) Close view of the dorsal fin vortex loop in (c) joining the caudal vortex ring. (e) Sections through the vortex wake to show the dorsal, caudal and anal fin vortices in the YZ plane and the resulting side jet (directed to the right in this figure). (f) Horizontal slice through the vortex wake to show the side momentum produced by the dorsal fin. Fish image has been inserted into panels a, b and c to indicate the location of the fins.