Taxonomic reassessment of Ulva prolifera (Ulvophyceae, Chlorophyta) based on specimens from the type locality and Yellow Sea green tides

Abstract Since 2008, the green seaweed Ulva prolifera has caused the world's largest green tide in the Yellow Sea, China. It has subsequently attracted considerable research interest. However, species identification is an essential step for advancing this research. Based on phylogenetic analyses using molecular sequences such as internal transcribed spacer (ITS) or ribulose bisphosphate carboxylase large chain (rbcL), specimens of U. prolifera collected worldwide were separated into a European clade and the Ulva linza–procera–prolifera (LPP) complex clade that included the Chinese bloom-forming strains and Japanese brackish strains. This has resulted in considerable controversy as to the identity of U. prolifera and the bloom-forming species in the Yellow Sea. To resolve this issue, we examined populations of U. prolifera from the type locality at Lolland Island, Denmark, and globally significant sites including sites from Japan and China using morphological, developmental, molecular and crossing studies....