Movie 3.

The 2D MIP of the z-stack accumulates in a single image the maximum values of all the focal planes, i.e. bright regions for a time sequence. A bright region moves from right-to-left relative to the head as time progresses. At the final part of the video, another spermatozoon appears swimming in the opposite direction, showing the bright region to move consistently in the inverse direction (see also Supporting Information and Fig. S2).