Coryphellina rubrolineata (Gastropoda: Nudibranchia: Flabellinidae) was believed to be a widespread tropical species demonstrating high diversity in external and internal morphological traits. In this paper, we perform an integrative analysis of the C. rubrolineata species complex based on samples collected in Vietnam waters, combined with available data from other localities of the Indo-West Pacific. The methods of the study include morphological analysis of external and internal traits using light and scanning electron microscopy and the molecular analysis of four markers (COI, 16S, H3, and 28S). The phylogenetic hypothesis was performed using Bayesian and maximum likelihood approaches, and the species delimitation analyses included ASAP, GMYC, and bPTP. Our results support the validity of the genus Coryphellina as a distinct taxon and confirm that Coryphellina rubrolineata is restricted to the type locality and adjacent waters, while in the Indo-West Pacific, it represents a complex of pseudocryptic species. Based on our integrative analysis, we describe four new species: Coryphellina pseudolotos sp. nov., Coryphellina pannae sp. nov., Coryphellina flamma sp. nov., and Coryphellina aurora sp. nov. For the first time, Coryphellina lotos is reported in Vietnam waters. All five species differ in combination of coloration and other external traits and show minor differences in internal morphology.

Conformity of trans-Arctic Coryphellidae genera with the most recent revised taxonomy for the group was tested. Both morphological and molecular data were used to compare more than 200 specimens from various localities in the North Pacific, North Atlantic and Arctic Oceans. The molecular study included four markers (COI, 16S, H3 and 28S) and implemented Bayesian and maximum likelihood phylogenetic approaches. Species delimitation was investigated with automatic barcode gap discovery (ABGD), Poisson tree processes (PTP) and generalized mixed Yule coalescent (GMYC) methods. The inter- and intraspecific divergence were studied using the TCS-based haplotype analysis and calculation of p-distances. Morphological analysis included standard anatomical techniques and scanning electron microscopy. Based on integrative analysis, we conclude that Coryphella verrucosa and C. pseudoverrucosa represent a single species (C. verrucosa), which is highly heterogeneous at morphological and molecular levels. For Microchlamylla gracilis and M. amabilis, their synonymization under the name M. gracilis is suggested, but increased sample size is needed for further clarification of their identities. Finally, the amphiboreal species Borealea nobilis and the North Pacific B. sanamyanae are distinct species displaying secondary sympatry in the north-west Pacific. Based on these results, we propose a new classification system for the family Coryphellidae uniting all currently considered genera in the single genus Coryphella, and suggest a new approach for further revision of Flabellinidae s.l. taxa.