Morphology and molecular characterization of the epiphytic benthic dinoflagellate Ostreopsis cf. ovata in the temperate waters off Jeju Island, Korea

•Here, we have provided the first report about the occurrence of Ostreopsis cf. ovata in Korean waters.•The cingulum of this Korean strain was displaced and undulated in the ventral view.•We have reported, for the first time, the presence of a line of small knobs on the Po plate.•O. cf. ovata Korean strain is genetically distinct among the O. cf. ovata strains reported to date.•In the phylogenetic trees based on D1-D3 LSU rDNA regions, the Korean strain held the basal position of the other O. cf. ovata clades.

Ostreopsis is an epiphytic and potentially toxic dinoflagellate. Here, we have provided the first report about the occurrence of Ostreopsis cf. ovata in Korean waters. In general, the morphology of the Korean strain of O. cf. ovata isolated from the waters off Jeju Island, Korea, was similar to the original description of O. ovata isolated from Japanese water. However, the cell length of the Korean strain was 20-30% smaller than that of the Japanese strain. In addition, the cingulum of the Korean strain was displaced and undulated in the ventral view, but that of the Japanese strain was not displaced. We have reported, for the first time, the presence of a line of small knobs on the Po plate. The plates of the O. cf. ovata Korean strain were arranged in a Kofoidian series of Po, 3′, 7″, 6c, 6s, Vp, Rp, 5‴, 1p, and 2⁗. When properly aligned, the small subunit (SSU) rDNA sequence of the O. cf. ovata Korean strain was 5.8% different to the O. cf. ovata Malaysian strain, which is the closest strain. In addition, compared to the south-east Asian, Pacific, Atlantic, and Mediterranean strains of Ostreopsis sp., O. ovata, and O. cf. ovata, the sequences of the internal transcribed spacer region (ITS1 and ITS2) plus 5.8S, D1-D3, and D8-D10 large subunit (LSU) rDNA of the O. cf. ovata Korean strain showed a difference of 3.6-16%, 15.8-19.7%, and 0-3%. Furthermore, in the phylogenetic trees based on D1-D3 LSU rDNA regions, the O. cf. ovata Korean strain was located in the basal position of the other O. ovata and O. cf. ovata clades. Therefore, we suggest that the O. cf. ovata Korean strain is genetically distinct among the O. ovata and O. cf. ovata strains reported to date.