Fig. 4. Effect of Ca²⁺ on trajectories and swimming speed. Trajectories in the presence or absence of Ca²⁺ are shown. Sperm trajectories of Acropora digitifera in Na-free ASW containing (A) 5 mmol l^-1 EGTA and 10 µmol l^-1 of the Ca²⁺ ionophore A23187, (B) 10 mmol l^-1 CaCl₂ and 10 µmol l^-1 A23187, and (C) 10 mmol l^-1 CaCl₂. (D) Swimming speed in each of the solutions. Other species showed similar patterns (data not shown). Values are mean ± s.d. (N=150 sperm from five individuals). Scale bars, 50 µm.