Data were processed using DynamicStudio (v.4.10, Dantec) and Matlab (2011b through 2016b, Mathworks). Fluid and larvae move in different directions, so we first separated the PIV images of particles and larvae using techniques for 2-phase flow. Fluid velocities were computed from the particle images with larvae masked out, and larval translational velocities were calculated from larval trajectories. Fluid motion and larval translation differ due to swimming or sinking movement relative to flow outside the larval boundary layer. In the vertical (z) dimension, wb=wo-wf, where wb is the instantaneous behavioral velocity, wf is the instantaneous flow velocity, and wo is the instantaneous translational (observed) velocity of an individual larva. The horizontal behavioral velocities were computed similarly for ub in the x dimension. Trajectories had mean durations of ~2 s in the weakest flow conditions down to ~0.1 s in the strongest flow conditions and were too short to analyze behavioral changes over time.

We used the PIV data to analyze larval swimming mechanics as a response to the instantaneous flow environments around individual larvae (Fuchs et al. 2013, 2015a, 2015b). The relevant hydrodynamic signals are the dissipation rate ε, strain rate γ, horizontal component of vorticity ξ, and fluid acceleration α. We calculated 2D approximations of these signals from fluid velocities and their gradients, interpolated in space and time to the larval observations.

Approximations for ε varied among flow tanks (Fuchs et al. 2013, 2015b). We also calculated the instantaneous fluid forces on individual larvae. The product of larval mass and acceleration is balanced by a vector sum of forces, including gravity, buoyancy, drag, Basset history forces, fluid acceleration, and the force that larvae exert to propel themselves (see Appendix of Fuchs et al. 2015b). Assuming larvae to be spherical, we computed all terms except propulsive force from measured velocities, larval size, and density (Fuchs et al. 2013), then solved the force balance equation for the propulsive force vector Fv, which indicates the magnitude and Cartesian direction of larval swimming effort. The propulsion direction was corrected to larval coordinates by estimating the vorticity-induced larval tilt angle ϕ (Fuchs 2013), and larvae were classified as "swimming" or "sinking/diving" if their propulsive force was directed upward (velum direction) or downward (shell direction), respectively, relative to the body axis.