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