Is the dinoflagellate Takayama xiamenensis a synonym of Takayama acrotrocha (Kareniaceae, Dinophyceae)?

The naked dinoflagellate Takayama acrotrocha was identified as responsible for a bloom in Shenzhen Bay, Guangdong, China, in early spring 2021. The identification was confirmed by light, scanning, and transmission electron microscopy and molecular data based on the LSU ribosomal DNA (rDNA) and ITS rDNA sequences. This is the first record of T. acrotrocha bloom in the South China Sea. The sulcus of T. acrotrocha was wide and extended onto the epicone as a short intrusion in general, sometime the intrusion was not apparent and some were finger-like. The apical groove was deeply sigmoid. The nucleus was large, ovoid to cup-shaped and occupied most of the epicone. A large, rounded pyrenoid surrounded by a starch sheath was located at the left side to the centre of the hypocone. Under epifluorescence illumination, a row of large vesicular knobs was observed on the upper border of the cingulum. The intraspecific morphological variabilities in the clonal cultures of T. acrotrocha were investigated carefully. Cells that share the same diagnostic characters used for the description of Takayama xiamenensis such as the finger-like sulcus, a large nucleus located in the epicone and the similar pyrenoid type were observed. The LSU rDNA sequences of T. acrotrocha and T. xiamenensis only differed in 3 base pairs (bp) for a sequence length of 673 bp (with a similarity of 99.55%). For these reasons, we propose T. xiamenensis as a junior synonym of T. acrotrocha .