Fig. 4.

Bidirectional motion of the Ray sperm. (A) Images showing sequences of the Ray sperm swimming backward and forward. The sperm rotated right and moved backward in the first 1,600 ms with the tail twining as a circle. In the next 1,040 ms, the sperm transferred its status from back to forward. Then, the tail unwrapped and returned to the helical structure, which drove the sperm moving forward faster (2,720 to 3,680 ms). (B and C) Trajectories and corresponding moving velocities of forward (orange), backward (purple), and first backward then forward (pink) motions. Box: 25th to 75th percentiles, whisker: 10th to 90th percentiles, and horizontal line: median value. Since the tail twined as a circle in the backward motion and almost made no contribution to the propulsive force, backward motions were much slower than forward ones. (D) Rotational speed and propulsive force in forward and backward motions (n = 495). Bar = mean; error bar = SD. The rotational speeds of the head decreased by over 20% in backward motions compared with those in forward motions. The tail wraps on the head without active rotational motion, resulting in no contribution in propulsive force in backward motions. (E) Fitting relationships between the rotational speeds of head and backward speed. The backward speed (mean ± SD) was correlated with the rotational speed of heads linearly with a fitting slope of 0.117, 0.017, 0.248, 0.165, 0.082, and 0.093, respectively.