English: Iberiotoxin slowing down response. Iberiotoxin reduces the number of speractinduced Ca fluctuations that occur during the 5 s immediately after stimulation. Swimming Fluo-4 loaded S. purpuratus spermatozoa exposed to speract through the photo-release of Caged speract (10 nM) with a 200 ms UV flash alone (left panel) or in the presence of Iberiotoxin (100 nM, right panel). An optic liquid guide of 4000 m internal diameter was used as light path between the UV lamp and the microscope. Pseudo-color scale representing maximum (red) and minimum (blue) relative fluo 4 fluorescence intensity. Five times slower: 30 frames s, 40× objective.