Abstract: This study reports a new record species of Ulva algae from China, Ulva ohnoi Hiraoka & S. Shimada 2004, collected from Fangchenggang, Guangxi and Zhanjiang, Guangdong. The morphological characteristics are described in detail, and multi-gene marker analysis was performed. The results show that Ulva ohnoi has both attached and floating individuals. The thallus is light green, ranging from 15 to 60 cm in length, and can be solitary or tufted. The tissue is brittle and easily torn, with a shape that is round, ovate, or irregular. The surface cells of the thallus are square or polygonal, arranged irregularly, with jagged protrusions along the edges. The upper and middle parts of the thallus are similar, with cell dimensions of 10-20 × 5-15 µm, and thickness of 20-45 µm. The basal cells are nearly round, measuring 10-30 × 10 22 µm, with a thickness of 70-90 µm. Analysis of rbcL and tufA gene sequences revealed that the samples collected in this study clustered with the type specimens of Ulva ohnoi, supported by high bootstrap values and posterior probabilities, with a genetic distance of 0 between them. This study enriches the species diversity of the Ulva genus in China, emphasizes the importance of molecular biology techniques in the identification of green algae species, and provides fundamental taxonomic data for the protection and utilization of marine algae resources in China.

Key words: Ulva ohnoi, New record species, Morphology, Multi-gene markers, rbcL, tufA