Fig. 4. Optimal vortex formation during fast-swimming of N. bachei. Image from dye flow visualization showing vortex formation in the wake of the animal. Image height is 7.2 mm. No trailing flow exists directly behind the vortex (see also movie in supplementary material), supporting the conclusion that the animal generates a single vortex (shown schematically in inset) per swimming cycle, despite the fact that T^*_max8.