Fig. 1. Effects of osmolality and extracellular [Ca²⁺] on motility in sperm of tilapia Oreochromis mossambicus. Sperm were suspended in solutions of 10 mmoll^-1 Hepes-NaOH, pH 8.0, containing different concentrations of electrolytes and a nonelectrolyte (mannitol) to give the required osmolality. The percentage of motile sperm was measured from video recordings. (A) Motility in the absence of CaCl₂. Electrolytes were NaCl (filled squares) and KCl (filled triangles). Filled circles, nonelectrolyte (mannitol). (B) The effect of Ca²⁺ on motility. Filled squares, NaCl alone; filled triangles, NaCl + 2 mmoll^-1 CaCl₂; filled circles, NaCl + 10 mmoll^-1 CaCl₂; open squares, NaCl + 5 mmoll^-1 EGTA. Arrows indicate the osmotic pressure of seminal plasma, i.e. isotonic osmolality. Values are means ± S.D.; N=150 sperm from 5 fish for each point.