Fig. 3. Plot of vortex formation time (T^*) during the jet ejection phase (t/T_ejection) of the swimming cycle (solid black line). At the end of the ejection phase (black star), the vortex formation time coincides with the range of values producing maximum vortex growth in mechanically generated jet flows with similar aperture kinematics (grey band) (Dabiri and Gharib, 2005b), where the aperture diameter contraction rate ranges between 15% and 30% of the average jet velocity. For comparison, the measured velar aperture contraction rate in N. bachei is approximately 20% of the average jet velocity. Broken lines indicate measurement uncertainty of ±10% associated with calculation of vortex formation time (see Materials and methods). Note that the local plateau in the vortex formation time near t/T_ejection=0.7 is within the error of the measurement trend.