New records of Ulva spp. (Ulvophyceae, Chlorophyta) in China, with special reference to an unusual morphology of U. meridionalis forming green tides

Green tides, dominated by Ulva, are on the rise worldwide, and invasive Ulva spp. tend to bloom in new habitats. Over the last decade, green tides have caused serious problems in all marine regions of China, especially the Yellow Sea, but there has been no study emphasizing alien species or new records of Ulva in China. In this study, based on a molecular investigation spanning over 10 years, we found six new records of Ulva in China and analysed their distribution patterns. Three northern temperate species – Ulva simplex, U. splitiana and U. partita – were also distributed in temperate marine areas in China. Meanwhile, three subtropical species – U. meridionalis, U. tepida and U. chaugulii – were found to be dominant in the green tides in the South China Sea, and have also successfully exploited temperate areas, especially U. meridionalis which has a wide distribution in China. U. meridionalis has an unusual morphology distinct from the other three types reported in Ulva: along the middle of the longitudinal axis, both sides of the thallus are connected, forming a thin sheet in the middle with bilateral tubular structures, like a bow-tie in transverse view. We found a rapid Ulva bioinvasion and we provide fundamental new data for the future monitoring and risk assessment of green tides in China.