Life cycle stages of the benthic palytoxin-producing dinoflagellate Ostreopsis cf. ovata (Dinophyceae)

The asexual and sexual reproduction of Ostreopsis cf. ovata was studied in the field and in cultures isolated from two locations in the Mediterranean Sea. Asexual division took place in the motile stage by the sharing of theca (desmoschisis). High cell-size variability and differences in division capability were detected in the cultures. Thecal analyses and nuclear division patterns allowed characterization of the different phases of dividing cells obtained during an in situ cell-cycle sampling performed off Llavaneres beach (Northeast Spain). During the 45-h cycle, binucleated cells accounted for 2.6% of the population. Division was initiated with the onset of dusk and reached a maximum 3–4 h before dawn. No dividing cells were detected after 09:00 AM. Sexuality occurred both in cultures and in natural populations of O. cf. ovata. Mating gamete pairs were the only sexual stages that could be distinguished from vegetative stages. The differences between these pairs and dividing cells are described herein. None of the individually isolated gamete pairs underwent fusion nor encystment, instead, in most of them one member of the gamete pair divided. Rather than plasmogamy, there was evidence of nuclear migration from one gamete to the other. Pellicle cysts, thin-walled cysts, and thecate cysts were also identified and studied. These cysts, embedded in abundant mucilage in a bloom-derived incubated sample, were able to germinate for as long as 6 months after their formation. Our results suggest that they constitute the overwintering population that causes recurrent blooms of O. cf. ovata in some areas of the Mediterranean Sea.