High-level taxonomic splitting in allopatric taxa causes confusion downstream: a revision of the nudibranch family Сoryphellidae

Abstract

Abstract 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.