Morphological and Molecular Characterization of the Unarmored Dinoflagellate Gymnodinium trapeziforme (Dinophyceae) from Jiaozhou Bay, China

The genus Gymnodinium contains more than 230 extant species, approximately 30% of which have not been reported since their original description. Approximately eight Gymnodinium species have been reported or described in the coastal waters of China. This work reports the presence of Gymnodinium trapeziforme from Jiaozhou Bay, China, in 2020, and its morphological and phylogenetic characterization by using light and scanning electron microscopy and systematic analysis based on partial LSU rDNA sequences. We observed the typical diagnostic features of G. trapeziforme, including a small size, biconical to ovoid shape, and a sulcal extension intruded to the epicone and connected to the horseshoe-shaped apical structure complex (ASC). Additionally, we firstly observed that the ASC consisted of three parallel series of vesicles, with the central one possessing knobs, and having more than 10 amphiesmal vesicles within the ASC. The nucleus was cucurbit-shaped, and the amphiesmal vesicles covering the cell surface, which would be peeled off for the cells in stress. While our molecular phylogeny inferred with the maximum likelihood (ML) and Bayesian inference (BI) confirmed the conspecificity of our isolate with the holotype G. trapeziforme (accession No. EF192414), we found a difference of 14 bases in the D1–D6 domains of the LSU rDNA sequences between the two entities, which indicates a detectable speciation of the two populations. Our work provides a detailed morphological and molecular characterization of G. trapeziforme that was isolated from the coastal water of China, which also broadens the geographical distribution of this species.