Fig. 2. Trajectory of swimming sperm with or without eggs. Sperm movements in Acropora digitifera were recorded. The movement of sperm heads was traced by NIH images and the trajectories drawn. (A) The trajectories of sperm heads in the presence of an egg in ASW. Arrows indicate straight direct paths, a chemotactic behaviour observed close to eggs. (B) The trajectories of sperm heads when no egg was present in Na-free ASW containing 20 mmol l^-1 NH₄Cl. Other Acropora species, A. gemmifera and A. tenuis showed similar movement (data not shown). Scale bars, 50 µm.