Aequorea taiwanensis, a new hydrozoan species from the Taiwan Strait Was described using Morphological and molecular characteristics. Both morphological and mitochondrial cytochrome oxidase subunit I (mtCOI) data supported A. taiwanensis n. sp. as a valid species. Sequence divergence and genetic distance of A. taiwanensis n. sp., A. papillata and A. conica were analysed based oil the mtCOI gene sequences. The mtCOI sequences from these three species of the genus Aequorea showed high variation frequency, with sequence divergences ranging from 9. 10% to 11. 9%, and pairwise genetic distances ranging from 0. 097 to 0. 130. MtCOI sequence analysis provided diagnostic molecular systematic characteristics for accurate identification and discrimination of the species of Aequorea or their populations, and will be used to resolve evolutionary relationships among them. It was Suggested that 10%-20% pairwise mtCOI sequence differences indicated the species-level divergence among congeneric species in the Hydromedusae
