Neither “lumpers” nor “splitters”: A global revision of Flabellinidae s.l. nudibranchs (Gastropoda: Heterobranchia: Nudibranchia)

Nudibranch taxonomy is a prime example of the conflict between “lumpers” and “splitters”: with rampant cryptic diversity, difficult to interpret morphological characters, plastic ecology, and variable levels of COI divergence among groups. As a result, it is extremely challenging to reach a consensus on what constitutes a species (not to mention a genus or a family). In this paper, we tackle a particularly problematic group, the family Flabellinidae s.l. Recent studies have shown that this group is polyphyletic and have suggested reclassifying the 84 described species into 29 genera across 7 families. However, further studies indicated the instability of the phylogenetic relationships between some of these species, genera, and families and suggested the need for further revisions of this group. The primary goal of the present study is a critical revision of the Flabellinidae s.l. diversity, by testing the monophyly of currently accepted genera and re-evaluating morphological synapomorphies for high-rank taxa, with the incorporation of previously overlooked taxa and some from poorly studied regions. We conducted an updated phylogenetic analysis based on four molecular markers. We also performed morphological analyses based on light and scanning electron microscopy. The analyses confirm the monophyly of Coryphellidae, Apataidae, and Paracoryphellidae. At the same time, the monophyly of Flabellinidae s.str., Samlidae, Unidentiidae, and Flabellinopsidae and the genera Edmundsella and Samla was questioned. Our results show that Samla rubropurpurata represents a new genus described herein as Launsina gen. nov . We also identify a new phylogenetic clade represented by specimens from the Kurile Islands, which corresponds to the recently established genus Mgueolia. This group is represented by at least four distinct species, three of which are described herein. We demonstrate that the facelinid genus Bajaeolis and the recently described genus Kynaria are members of the family Flabellinopsidae. Finally, we suggest that the family Piseinotecidae likely represents a senior synonym of the family Unidentiidae. Based on our comprehensive integrative analysis, we propose a new taxonomical scheme for Flabellinidae s.l. and identify consistent common traits and synapomorphies for each high-rank group.