A new potentially toxic species of Gambierdiscus (Gonyaulacales, Dinophyceae) from the Western Pacific Ocean.

Over the past four decades, at least twenty species of the benthic dinoflagellate genus Gambierdiscus have been described worldwide, with the highest diversity reported from the South Pacific. Species delimitation within this genus is often challenging due to subtle morphological differences, and reliable identification frequently requires molecular confirmation. In the present study, epiphytic dinoflagellates collected along the coast of Viet Nam were examined using an integrative approach combining morphology and molecular phylogenetics. A new, potentially toxic species, Gambierdiscus larsenii sp. nov. was identified, increasing the number of Gambierdiscus species reported from Vietnamese waters to eight. Cells of G. larsenii sp. nov. measured 75.5 ± 5.4 μm in dorsoventral diameter (DV), 80.3 ± 6.1 μm in width (W), and 57.7 ± 3.9 μm in apical-antapical length (L), with a DV:W ratio of 0.94 ± 0.04. The thecal plates were smooth and densely perforated with pores, a broad pentagonal 2⁗, and rectangular 2' plates represented key morphological features. Morphological characteristics and plate tabulation were consistent with those of the genus Gambierdiscus. Phylogenetic analyses based on LSU rDNA (D1-D3), SSU rDNA, and ITS1-5.8S-ITS2 regions demonstrated that G. larsenii sp. nov. was closely related to, but genetically distinct from, other species possessing a broad 2⁗ plate. Toxin analysis confirmed the production of putative 51-OH-CTX3C at a cellular content in the lower pg cell⁻¹ range, indicating that G. larsenii sp. nov. may represent a potential risk for ciguatera poisoning in Viet Nam.