Fig. 4. Cyclic swimming in zebrafish larvae (3 days post-fertilisation; U=8 s^–1) (A) Contours of the specific lateral displacement (negative values = displacements to the left of the larva) and (B) specific curvature (negative values = convex curvature), plotted along the body axis (x-axis) and in time (y-axis) for two complete tail beats. (C,D) Vertical and horizontal cross sections through the displacement (A) and the curvature contours (B), respectively. The contours correspond to Fig. 2B. The thick, black lines indicate zero lateral displacement (h₀) and zero curvature (₀) in A and B, respectively. The broken black lines indicate the changing position in time and space of local extremes in lateral displacement (h_max) and curvature (_max), which correspond to the amplitude envelope of the displacement (transect A1; the asterisk indicates pivot point) and the curvature wave (transect B1). Horizontal cross sections (red lines) indicate the body wave (A,C, transect A1) and curvature wave (B,D, transect B1), respectively. The distance between the points at which the horizontal cross section crosses the h₀ and ₀ contours, respectively, is equivalent to half the wave length of the body wave and curvature wave, respectively. The vertical cyan lines (C,D, transects A2 and B2, respectively) indicate vertical cross sections. They indicate the propulsive wave in the displacement profile (transect A2) and the curvature wave in the curvature profile (transect B2).