ALBERT

Notes: Summary Flagellar waveforms have been studied during the stopping and starting transients of light-induced, Ca2+-mediated, intermittent swimming of live sperm of the sea urchinTripneustes gratilla. Tracings of successive frames of movie film made at about 200 frames s−1 were used to determine the bend propagation velocity, beat frequency, and bend angles during three stopping and four starting transients chosen as representative of the range of variation among sperm in the preparations. A stopping transient begins with a transitional stage in which the asymmetry of the bending waves increases steadily over 2–6 beat cycles (40–120 ms), with the angles of successive fully developed principal bends increasing and those of reverse bends decreasing. This is followed by a blocked stage, lasting one beat cycle (20 ms), in which a principal bend becomes arrested and then decays in the mid-region of the flagellum. The next principal bend forms but remains unpropagated at the base, apparently because no following reverse bend is initiated, and the flagellum becomes quiescent. Quiescent flagella have a characteristic, highly asymmetric waveform consisting of a sharp principal bend of about 3.2 rad at the basal end, a nearly straight mid-region and a gentle principal bend of about 0.4 rad near the tip. After a quiescent period of 0.2–2 s, motility is resumed with the initiation of a new reverse bend at the base. This bend and the proximal principal bend remaining from quiescence begin to propagate but they decay before passing more than halfway along the flagellum. In this blocked stage of the starting transient, which lasts 1–15 beat cycles (20–300 ms), successive principal and reverse bends are propagated progressively further along the flagellum but they decay before reaching the tip and the asymmetry remains at the high value characteristic of quiescence. The first propagation of a principal bend to the tip marks the beginning of the transitional stage of the transient, during which the asymmetry of the bending waves gradually decreases until after 2–5 beat cycles (40–100 ms) it reaches the value characteristic of steady-state beating. In both stopping and starting flagella the beat frequency and the mean of the principal and reverse bend angles remain constant throughout the transient (except for the beat cycle immediately pre- or post-quiescence), indicating that they are regulated by mechanisms almost completely independent of that regulating wave symmetry. The bend propagation velocity remains constant during stopping transients but it is diminished during the blocked stage of starting transients, indicating that the bend velocity, and hence the wavelength, can be altered by changes in the internal resistance to bend propagation.