English: Delay time for the onset of the first Ca fluctuation of S. purpuratus spermatozoa is increased by the presence of Verapamil. Swimming Fluo4 loaded S. purpuratus sperms were expose to speract through the photorelease of caged speract (10 nM) with a 200 ms UV flash alone (left panel) or in the presence of Verapamil (10 M, right panel). An optic fiber of 4000 m internal diameter was used as light path between the UV lamp and the microscope. 20 times slower: 120 frames s, Scale bar = 25 m, 40× objective. Yellow circles indicate spermatozoa at the beginning and progression of the first Ca fluctuation until reach the averaged delay time of the whole population is reached.