New insights on the life cycle stages of the toxic benthic dinoflagellate Ostreopsis cf. ovata

•Two kinds of gamete mating seem to occur.•A process possibly involved in mating mechanism was observed on top of epitheca.•Meiosis may have occurred as suggested by tetrad formation.•Different cyst types were recognized including both non-dormant and resting cysts.•Double-walled cysts after 5 months were able to germinate only at 25 °C.

New observations on the life cycle stages of Ostreopsis cf. ovata are reported in the field and in cultures samples from the northern Adriatic Sea (Mediterranean Sea). Cultures of O. cf. ovata were performed using both replete and N-free growth media and analyzed for 5 months. Ostreopsis cells displayed a high morphological variability. Some cells were characterized by the presence of orange accumulation bodies, which represented a signal of stress conditions. Two mechanisms of gamete mating seem to occur. In the first (already reported) vegetative cells conjugated through the epitheca without plasmogamy; in this regard we observed a new ‘process’ on the top of the epitheca which might represent a structure involved in this mating mechanism. In the second, small cells acting as gametes were aligned laterally with the two cingula perpendicular to each other. Meiosis possibly occurred as suggested by tetrad formation, originating four vegetative cells. At least two types of cyst were formed, a non-dormant (pellicle) cyst germinating within 2 days and a resting cyst which is able to germinate after a 5-month dormancy only at temperatures over 25 °C.