Fig. 4. Flow generated during a tail beat of a cyclically swimming zebrafish larva (age 3 d.p.f., body length L=3.8 mm). (Left) Vorticity field (colour map) and velocity vector field (black arrows) adjacent to a fish larva. (Right) Sketch of most relevant flow features. The drag wake of the anterior body (elongate ochre area in sketch) is prominent in the vorticity field throughout the tail-beat cycle. The two main propulsive features of the flow along the larva are (1) the jet into the concave bend of the body, which gradually reorients more caudally as it travels down the body (black arrows); (2) several smaller patches of vorticity along the posterior body, which are shed as two vortex pairs per tail-beat cycle (blue and ochre circles). (Same sequence as Fig. 3.)