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. Under stressful conditions, induced in the laboratory through substantial thermic and nutritive changes, vegetative cells divided repeatedly. Scanning electron and light microscopy of dividing specimens showed that thecal fission began with the separation of the sulcal and ventral epithecal plates and the simultaneous dislocation of the pore plates from the right cell half. The posterior progression of the division led to pairs of cells attached antapically, which produced a new wall of reduced size. This phase of the life cycle coincided with the appearance and development of small forms of D. pavillardi, which displayed cytological features and behavior typical of male gametes, suggesting a process of gametogenesis through depauperating mitotic divisions. Anisogamy occurred at the time of the maximum production of small cells and involved the shedding of thecal components by the smaller gamete and subsequent cytoplasmic fusion and formation of planozygotes. Although the dormancy aspects of this species remain unknown, these observations provide the first evidence of sexuality .