New details from the complete life cycle of the red-tide dinoflagellate Noctiluca scintillans (Ehrenberg) McCartney.

Noctilucid protozoans are among the dinoflagellates that cause red tides. Sexual reproduction may occur in this group, as they sometimes undergo gametogenesis. However, the life cycle, in particular the developmental process after gamete fusion, has not been fully elucidated. We have been able to maintain clonal cultures of Noctiluca scintillans throughout the whole life cycle and have revealed new details of various stages. In trophont populations, a small fraction of cells spontaneously transform into gametogenic cells, which undergo two successive nuclear divisions, without cellular division, probably corresponding to meiosis. The products of nuclear division migrate to the cell surface with a small amount of cytoplasm, and there further synchronously divide 6-8 times, during which the division products are connected by thin cytoplasmic bridges. Thus, numerous gametes with a semi-spindle body shape are released from the mother cell ghost. They retain two flagella that differ in length and motion, as is typical of dinoflagellates. The presence of longitudinal and transverse grooves indicates that dinoflagellate-like characteristics are conserved in the gametes, although they are not present in the specialized trophonts. Zygotes with four flagella result from the fusion of two isogametes. The zygotes change shape from spindle to spherical, with a reduction in flagellar number. The developing cell acquires a tentacle and crust, similar to large trophonts, and begins to develop a cytoplasmic network, thus completing the transformation into a miniscule trophont. These early trophonts grow to maturity as cell size increases. Our observations of the life cycle of N. scintillans may provide clues for understanding the evolutionary origin of noctilucae.