Identification

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Two morphological features observed in the material studied by Nugyen-Ngoc et al. (2021) may be of taxonomic significance: the shape of the 2'-plate and the micro-morphology of the pores on the thecal plates. The 2'-plate is long and connects to the 3″-plate, thus separating the 3'- and 2″-plates. Most studies of Ostreopsis species including the type species (Schmidt 1901) are not sufficiently detailed to establish the exact shape of the 2'-plate. Verma et al. (2016a) describing O. rhodesiae regarded a long 2'-plate separating the 3'- and 2″-plates (3'- and 3″-plates in the terminology used by Verma et al. 2016a) as a characteristic feature of O. rhodesiae and O. heptagona, but this particular feature seems to occur also in other species. A long 2'-plate is found in Ostreopsis sp. 6 from French Polynesia (Chomérat et al. 2020b) and in O. cf. ovata (Besada et al. 1982; fig. 4) and in the culture strain CBA-6 (as O. cf. ovata) from Indonesia (Penna et al. 2010, fig. 2c; Parsons et al. 2012, fig. 2), which belong the SE Asian clade of O. cf. ovata (Tawong et al. 2014). Rhodes (2011, fig. 2—identical to Faust et al. 1996, fig. 10) shows two cells of Ostreopsis, misidentified as O. lenticularis, with a long 2'-plate. These cells were collected in Puerto Rico and are morphologically similar to the cells identified here as O. siamensis, O. lenticularis has a short obtuse 2'-plate which does not separate the 3'- and 2″-plates (Chomérat et al. 2019, Nguyen-Ngoc et al. 2022). The thecal pores in O. siamensis are lined by a shallow rim which is also seen in Rhodes (2011; Fig. 2) but not in Faust et al. (1996; fig. 15, misidentified as O. lenticularis), nor in species assigned to O. cf. siamensis, see below. Chomérat et al. (2020b) found a similar type of large thecal pores in Ostreopsis sp. 6 from French Polynesia. However, with the current lack of knowledge of the morphological variability within the Ostreopsis, more studies are needed in order to assess the taxonomic value of these features.

Schmidt, J. (1901). Flora of Koh Chang. Contributions to the knowledge of the vegetation in the Gulf of Siam. Peridiniales. <em>Botanisk Tidsskrift.</em> 24: 212-221. 

Nguyen‐Ngoc, L.; Doan‐Nhu, H.; Larsen, J.; Phan‐Tan, L.; Nguyen, X.; Lundholm, N.; Van Chu, T.; Huynh‐Thi, D. N. (2021). Morphological and genetic analyses of <i>Ostreopsis</i> (Dinophyceae, Gonyaulacales, Ostreopsidaceae) species from Vietnamese waters with a re‐description of the type species, <i>O. siamensis</i>. <em>Journal of Phycology.</em> 57(3): 1059-1083.