Fig. 1. (A) Schematic drawing of the experimental set-up, a goldfish and a vortex ring. The pipette tip used to generate the vortex is at the left. The elevation of the pipette (y-axis) was always at the height of the fish's trunk lateral line canal. The rostro-caudal distance (x-axis) between the pipette tip and the neuromast was adjusted to 5 cm. The lateral distance (z-axis) of the pipette tip to the operculum of the fish was 0.5 cm, if not stated otherwise. A laser sheet was used to illuminate the particles added to the water. In all cases in which we obtained PIV data and neuronal data, the laser sheet illuminated the x–y plane (yellow), i.e. a plane that was perpendicular to the fish's rostro-caudal axis. The x–y plane was placed as close to the skin as possible. For a further characterization of the water motions caused by a vortex ring, the laser sheet was also positioned in the z–x plane (red) or in the z–y plane (gray). (B) Vectors (arrows) and vorticity (blue, counterclockwise; red, clockwise) of the particle motions recorded 400 ms after valve opening measured in the horizontal (z–x) plane (see A) in the absence of a fish. (C,D) Velocities (v) directed parallel to the x-axis, calculated from the vectors highlighted by the longitudinal boxes in B. The plot in C transects the vortex through its midline and reveals the central flow. The plot in D transects the laser sheet in the z-axis. Here, the central flow is bordered by two vortex cores of minimal velocity and a region where the flow direction is opposite to the jet flow. The diameter of the vortex ring is approximately 3 cm.