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Identification of species of the dinoflagellate genus Ostreopsis is difficult because several species have been poorly described, others misidentified in the literature, and the type species, O. siamensis, has not been described by contemporary taxonomic methods. In the present study, it is argued that Ostreopsis sp. 6 as described by previous authors is similar to the type species, and we offer an emended description of O. siamensis by LM, SEM, and molecular analyses of nuclear LSU and ITS rDNA based on material collected a few hundred kilometers from the type locality in the Gulf of Thailand and along the Vietnamese east coast. Ostreopsis siamensis is genetically different from the species reported as O. cf. siamensis in the literature and the latter should be described as a separate species. It is also concluded that with the poor knowledge of the morphological variability of many species of Ostreopsis, O. siamensis may not be distinguished from other similar‐sized species by its morphological features, and hence molecular data are needed for reliable identification. The species Ostreopsis lenticularis and Ostreopsis cf. ovata were also found and described.

Protoperidinium is one of the most important genera of armoured marine dinoflagellates. Despite its importance and many previous studies on this genus, the descriptions of many species are incomplete, their diagnostic features not properly described and illustrated and most species have been illustrated, if at all, only by line drawings and not documented by micro‐photography. As here described, examinations of about 600 phytoplankton net samples collected from Viet Nam and adjacent coastal waters over the last decade revealed about 100 species of Protoperidinium, ten of which belong to the section Oceanica and this paper is the first in a series describing all species of Protoperidinium found in Vietnamese waters. The Oceanica species are described and illustrated by light, and for some also by scanning electron, microscopy. The species are: Protoperidinium claudicans, P. depressum, P. depressum var. claudicanoides, P. larsenii sp. nov., P. murrayi, P. oceanicum f. bisintercalares comb. nov., P. oceanicum var. tenellum comb. nov., P. paraoblongum, P. venustum and P. venustum var. facetum. Protoperidinium larsenii L. Phan Tan, L. Nguyen‐Ngoc, and H. Doan‐Nhu is new to science. It was found in the coastal waters of Rach Gia in Kien Giang Province, western part of south Viet Nam (Gulf of Thailand). The new species is characterized by its broad shape, conical epitheca and short, non‐divergent antapical horns; the 1’ plate is asymmetrical, there are 3 anterior intercalary plates, the 2a plate is quadrangular; and the sutures connecting the 6” plate with the 4’ plate and the 7” form an almost straight line. By these features P. larsenii differs from the similar species P. claudicans and P. latidorsale. Protoperidinium oceanicum (Vanh ffen) Balech () f. bisintercalares (Graham) H. Doan‐Nhu, L. Phan‐Tan, L. Nguyen‐Ngoc, and J. Larsen comb. nov. and Protoperidinium oceanicum (Vanh ffen) Balech () var. tenellum (Graham) L. Nguyen‐Ngoc, L. Phan‐Tan, H. Doan‐Nhu and J. Larsen comb. nov. are two new combinations.

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