A 40 years journey with fish spermatozoa as companions as I personally experienced it

When, in the 1980s, I became interested in the spermatology of fish under the light microscope, active spermatozoa were only visible thanks to their head presenting a sort of "tremor." This situation was quite frustrating given the lack of possible information regarding the motor part called flagellum. We decided to apply simple technologies, including photography. Due to the high speed of the moving fish flagellum, the microscope illumination used a pulsed light strobe combined with a dark field microscope to record the flagellum image despite its small diameter (< 0.5 μm). Then came high-speed cinematographic microscopy up to 200 fps, as well as video cameras. At the end of the 1990s, an automatic moving object video tracking system began to be commercialized (CASA) with main advantages such as (a) a large number of cells tracked, which greatly improves statistics, (b) computer assistance allowing an automatic analysis that provides many motility parameters. Nevertheless, CASA systems are still unable to provide information about fish sperm flagella that move fast. During the 1990s, analog video camera technologies allowed acquisition of flagellum images with high resolution for detailed analysis. Since the 2000s, the use of high-speed video cameras allows the acquisition of images at a much higher resolution and frequency, up to 10,000 frames per second. Since it became possible to visualize the flagella in motion, a noble function was added to that of a propeller: that of a rudder with what a spermatozoon responds to specific signals delivered by the egg for its guidance. In the future, one can wish that an automatic flagella movement analyzer will become functional. This brief anthology puts forward the large amount of progress accomplished during past 40-year period about spermatozoa movement analysis, especially in fish.