(A) Eigenmannia virescens maintains its position, , in a moving refuge. is the position of the moving refuge. (B) A feedback control system model for refuge tracking. The feedback controller maps the error signal, , to a control signal, . The plant , in turn, generates fore-aft movements . We used measurements of the nodal point as to estimate in each fish, which was subsequently used to infer . (C) Eigenmannia virescens partitions its ribbon fin into two counter-propagating waves that generate opposing forces (Sefati et al., 2013; Ruiz-Torres et al., 2013). The rostral-to-caudal and caudal-to-rostral waves collide at the nodal point (green line). Movements of the nodal point serve as a proxy for the control signal, . Gray lines indicate the positions of the peaks and troughs of the traveling waves over time. In this trial, the refuge was following a sinusoidal trajectory at 2.05 Hz. (D) Movement of the nodal point , position of the fish , and position of the refuge of 13 trials at 0.95 Hz. Semi-transparent lines represent data from individual trials, green and blue lines are the means.