Autonomous changes in the swimming direction of sperm in the gastropod Strombus luhuanus

The sperm of the gastropod Strombus luhuanus show dimorphism. The eusperm have a nucleus and fertilize the egg, whereas the other type of sperm, parasperm, are anucleate and are thought to assist fertilization. Here we report the autonomous changes in the swimming pattern of S. luhuanus eusperm. In artificial seawater, the eusperm collected from S. luhuanus sperm ducts formed sperm bundles and initially swam backward with asymmetric flagellar waveforms to detach from the bundles. One hour later, the sperm began to swim forward and in a circle. After an additional 1 h incubation, the sperm swam straight, with a change in the flagellar waveforms from asymmetric to symmetric. Spontaneous backward swimming with symmetric waveforms was also observed. The eusperm stored in the female seminal receptacle were motile and showed forward symmetric swimming with spontaneous backward swimming, which appeared necessary for detachment from the wall of receptacle. All of these motility changes were observed in the absence of parasperm, suggesting that these changes autonomously occur in eusperm. Our waveform analysis of these swimming patterns revealed that only the swimming with symmetric waveform showed reverse propagation of the flagellar waveforms. Both types of backward swimming were diminished in Ca(2+)-free seawater and in seawater containing Ni(2+), indicating the regulation of swimming direction by Ca(2+)-dependent signal transduction.