Abstract: Ruppia is a cosmopolitan genus and the species identity within the genus has been often confusing due to high plasticity in morphology. We used nuclear ribosomal DNA internal transcribed spacer and seven chloroplast DNA fragments combined with morphological traits to distinguish Ruppia species along the coastline of China, and to infer the potential mechanisms shaping their distribution patterns. Molecular and morphological data distinguished three congruent clades corresponding with three species: R. cirrhosa (Petagna) Grande, R. maritima L., and R. megacarpa Mason. The geographical ranges of R. cirrhosa and R. maritimacoincided closely with coastal current breaks by the Taiwan Strait; R. cirrhosa was found in 17 locations mainly in the north, whereas R. maritima was common in the south, most likely to be the result of differentiated adaptation to optimal temperatures of growth and/or seed germination. Ruppia megacarpa, usually considered as an endemic species of Australia and New Zealand, was first reported in China at two bird stopover sites, Qingdao and Yancheng. The transoceanic dispersal by migratory birds may contribute to the presence and distribution of R. megacarpa in China.

Key words: cpDNA, environmental factors, ITS, long distance dispersal, Ruppia, R. megacarpa.