• energetics;
  • sperm behaviour;
  • swimming variables;
  • velocity

In most species, fish spermatozoa activate their motility on contact with the external medium (sea or fresh water depending of their reproductive habitat). Their flagella immediately develop waves propagated at high beat frequency (up to 70 beats s−1), which propel these sperm cells at high velocity (6–10 mm min−1), but for a quite short period of time, usually limited to minutes. Their specific inability to restore their energy content (mostly adenosine triphosphate) fast enough relatively to their high rate of energy consumption by flagellar contributes mainly to the activity arrest of motility, as the spermatozoa need to rely on early accumulated energy prior to activation. This review of the published data explains the present understanding of physico-chemical mechanisms by which flagellar motility is activated (mostly through osmotic and ionic regulation) and then propels sperm cells at speed. It aims also to describe the gradual arrest of their motility much of which occurs within a few minutes.