Systematic rearrangements in an all-genus phylogeny of side-gilled slugs (Heterobranchia: Pleurobranchida)

The systematics of side-gilled slugs face long-lasting incongruencies provided by morphological and molecular data. Parsimonious morphological assessments divided the single superfamily Pleurobranchoidea into the shell-less Pleurobranchaeidae and the internally shelled Pleurobranchidae, including the aberrant helicoid-shelled Tomthompsonia. Molecular evidence has proven untrustworthy to date, revealing contrasting hypotheses. This study’s multilocus phylogeny benefits from a dense taxon sampling of 94 pleurobranchids, with 22 newly sequenced specimens, including type species for all known genera. We further evaluate the effects of masking with several algorithms to better enhance phylogenetic resolution. Our results support that Tomthompsonioidea superfam. nov. are the sister group of all other Pleurobranchida; the absence of an acidic oral gland and a helicoid shell are the diagnostic features. We also provide the first molecular evidence that Euselenops nests within Pleurobranchaeidae, and Pleurobranchella is related to Pleurobranchaea, but remains distinct morphologically. Regarding Pleurobranchidae, the genus Berthella is paraphyletic and now embodied in the four clades: Berthella s.s., Boreoberthella, Pleurehdera, and Tomoberthella gen. nov. The fact that, whereas there is a supposed low Antarctic diversity, we find hidden speciation in several tropical species suggests that side-gilled slugs pose an Antarctic origin or a relic that radiated in shallower, warmer waters.