Waveform dynamics of spermatozeugmata during the transfer from paternal to maternal individuals of Membranipora membranacea.

Analysis of standard (60 frames/s) and high-speed (200 frames/s) video records revealed that unencapsulated sperm aggregates (spermatozeugmata) of the gymnolaemate bryozoan Membranipora membranacea spontaneously generate at least three types of waveforms: small amplitude, large amplitude, and reverse. All three waveforms significantly differed from one another in amplitude. Additionally, small- and large-amplitude waveforms propagated from the base to the tip of axonemes, whereas the reverse waveform propagated from the tip to the base of axonemes. Small-amplitude waveforms, which were generated most frequently by spermatozeugmata in the paternal perivisceral coelom and in the water column after spawning, produced almost no curvature of the axoneme. Large-amplitude waveforms were produced by spermatozeugmata in the water column and within lophophores. Reverse waveforms were produced while spermatozeugmata moved tail-end forward through the paternal tentacles during spawning and after spermatozeugmata had contacted the intertentacular organ (ITO), a tubular structure that spermatozeugmata pass through to enter the maternal coelom and that eggs pass through to enter the seawater. The production of reverse waveforms by spermatozeugmata after reaching the ITO may be evidence for a behavioral response of bryozoan sperm to conspecific maternal individuals.