VEGETATIVE AND SEXUAL ASPECTS OF DINOPHYSIS-PAVILLARDI (DINOPHYCEAE)

Phases in the life history of the dinoflagellate Dinophysis pavillardi Schroeder from cultured phytoplankton assemblages are described. Unde r stressful conditions, induced in the laboratory through substantial thermic and nutritive changes, vegetative cells divided repeatedly. Sc anning electron and light microscopy of dividing specimens showed that thecal fission began with the separation of the sulcal and ventral ep ithecal plates and the simultaneous dislocation of the pore plates fro m the right cell half. The posterior progression of the division led t o pairs of cells attached antapically, which produced a new wall of re duced size. This phase of the life cycle coincided with the appearance and development of small forms of D. pavillardi, which displayed cyto logical features and behavior typical of male gametes, suggesting a pr ocess of gametogenesis through depauperating mitotic divisions. Anisog amy occurred at the time of the maximum production of small cells and involved the shedding of thecal components by the smaller gamete and s ubsequent cytoplasmic fusion and formation of planozygotes. Although t he dormancy aspects of this species remain unknown, these observations provide the first evidence of sexuality.