First occurrence of the genus Ostreopsis (Dinophyceae) in the Sea of Japan

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Abstract
Preliminary studies on dinoflagellates epiphytic on macrophytes in a bight of Peter the Great Bay, Sea of Japan revealed the presence of two species of the genus Ostreopsis, O. cf. ovata and O. cf. siamensis. Morphometric features, such as the length of the pore plate, and thecal ornamentations were used for species identification. The two species differed from each other in cell size and the length of the pore plate. Cells of O. cf. ovata were smaller in length and width, with a dorsoventral diameter of 36–60 μm and a width of 24–45 μm, in comparison with the cells of O. cf. siamensis, which were 63–78 μm in dorsoventral diameter and 36–54 μm in width. Cells of O. cf. siamensis are slightly flattened in comparison with O. cf. ovata; the dorsoventral to anteroposterior diameter ratio varied from 1.9 to 2.7 for O. cf. ovata and from 1.8 to 3.5 for O. cf. siamensis. The length of the pore plate in O. cf. ovata (6.3–8.3 μm) is shorter than that in O. cf. siamensis (11–13 μm), while the diameter of the thecal pores in O. cf. ovata and O. cf. siamensis overlapped markedly (0.12–0.25 μm and 0.16–0.20 μm, respectively). The two species were present from August to October at water temperatures of 7–25°C on almost all macrophytes collected. The maximum abundance of Ostreopsis spp. reached 10,970 cells g-1 dry weight in late September at a water temperature of 9°C. This is the first occurrence of the genus Ostreopsis in Russian waters, which are characterized by cold temperatures during the winter season.

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