<i>Hulburtia sanyaensis</i> gen. et sp. nov., a sand‐dwelling dinoflagellate from the South China Sea, and ultrastructure of <i>Pseudocochlodinium profundisulcus</i> and <i>Grammatodinium tongyeonginum</i>

Athecate dinoflagellates have a complex evolutionary history and are considered paraphyletic. Phylogenetic relationships among athecate dinoflagellate species and their higher taxonomic levels remain far from resolved. In the present study, six strains of athecate dinoflagellates were established by isolating single cells or cysts from the South China Sea. Both morphology and ultrastructure were examined in detail, and ribosomal DNA sequences were obtained. Two sand-dwelling strains were assigned to Hulburtia sanyaensis gen. et sp. nov., characterized by a dominant non-motile stage and a semicircular apical structure complex (ASC). Asexual reproduction occurred through binary fission during the immotile phase and formed a cluster of up to 16 cells. The species was also recorded from Kagoshima, Japan. The other strains were identified as Pseudocochlodinium profundisulcus and Grammatodinium tongyeonginum. Nuclear chambers were observed in P. profundisulcus but not in the other two species. The Chinese G. tongyeonginum shared 96.37% sequence similarity in the SSU rDNA region with specimens from South Korea. The molecular phylogeny was inferred using maximum likelihood and Bayesian inference based on concatenated sequences of the SSU, ITS, and LSU rDNA regions. Pseudocochlodinium profundisulcus was nested within Gymnodiniales sensu stricto, while H. sanyaensis and G. tongyeonginum were positioned distantly from this group. The genera Hulburtia and Grammatodinium were closely related to the fossil thecate dinoflagellate Dapsilidinium, but their higher taxonomic ranks remain to be determined. Our results highlight the underestimation of athecate dinoflagellate diversity and emphasize the significance of ultrastructure characteristics to understand their evolution.