Integrative systematics and phylogeographical patterns suggest the Zelentia pustulata species complex (Gastropoda: Nudibranchia) represents a single species with a complex evolutionary history

Recent revisions of the family Fionidae s.l. led to the erection of a number of new taxa on species, generic and even family-levels. However, in several cases the boundaries between these taxa remain unclear. In this study, we performed an updated molecular and morphological analysis of the Zelentia pustulata species complex in the Atlantic and the Arctic to clarify its identity and provide a new prospect for further revision of this group. The molecular phylogenetic study included three markers (COI, 16 S, H3) and implemented Bayesian and Maximum likelihood phylogenetic approaches. Species delimitation was investigated with ASAP, PTP and GMYC methods for the COI dataset. The population structure was studied using TCS-based haplotype analysis of COI and 16 S markers. This was also accompanied by the analysis of ITS1-ITS2 region. We reconstructed TMRCA of recovered clades based on gastropod COI substitution rates by a strict molecular clock scenario implemented in BEAST v. 2.7.3. Morphological analysis included standard anatomical techniques and SEM. Based on our integrative results and phylogeographic approach we show that Z. pustulata and Zelentia roginskae represent a single species, which is characterized by a continuous range of morphological variation and high heterogeneity in mitochondrial DNA. Divergence times estimation suggests a complex evolutionary history for this species, following Pleistocene glacial conditions in the Arctic and Atlantic region that is reflected by the mtDNA divergence. This study impacts forthcoming Arctic biodiversity studies as it highlights the necessity to account for complex evolutionary history and phylogeographical patterns in delimitation of taxa inhabiting regions undergoing rapid environmental changes.