Taxonomic review of from the Yellow Sea, China with the description of a new species (Nudibranchia, Doridina, Polyceridae).

Species of Bergh, 1880 are enigmatic Nudibranchia sea slugs, and only two valid species are reported in the northwestern Pacific. (Bergh, 1880) was initially described based on alcohol-fixed specimens. In the latest revision of , it was synonymized with (Cantraine, 1835). Recently, several nudibranchs were collected from Tianheng, Shandong Province, China, and one of them is identified as an undescribed species described here as based on integrated approaches incorporating morphological observations, internal anatomy, and phylogenetic analyses of two mitochondrial (COI, 16S rRNA) genes. The other species is identified as Bergh, 1880 based on the anatomy of the reproductive system. The new species is similar to in having a bright orange-red color pattern but differs significantly in the structure of appendages and reproductive system. can be easily distinguished from other species by its translucent, white-pink coloration and unique features of the female reproductive organ. Both species are supported as distinct species in all molecular analyses. The phylogenetic analyses propose a new estimate of the relationship between and , and the evolution of bioluminescence within Triophinae is discussed. Our results also suggest cryptic biodiversity within the species complex.