Systematic rearrangements in an all-genus phylogeny of side-gilled slugs (Heterobranchia: Pleurobranchida) Author Moles, Juan Department of Evolutionary Biology, Ecology and Environmental Sciences, Faculty of Biology, University of Barcelona, Av. Diagonal 643, 08028 Barcelona, Spain juan.moles@ub.edu Author Brenzinger, Bastian SNSB-Bavarian State Collection of Zoology, Section Mollusca, Münchhausenstrasse 21, D- 81247 Munich, Germany Author Berning, Maria I. SNSB-Bavarian State Collection of Zoology, Section Mollusca, Münchhausenstrasse 21, D- 81247 Munich, Germany Author Martynov, Alexander Zoological Museum, Moscow State University, Bolshaya Nikitskaya Str. 6, 125009 Moscow, Russia Author TatianaKorshunova Koltzov Institute of Developmental Biology RAS, Moscow 119334, Russia Author Schrödl, Michael SNSB-Bavarian State Collection of Zoology, Section Mollusca, Münchhausenstrasse 21, D- 81247 Munich, Germany & Biozentrum Ludwig Maximilians University and GeoBio-Center LMU Munich, Germany text Zoological Journal of the Linnean Society 2024 2023-12-26 202 2 1 11 https://doi.org/10.1093/zoolinnean/zlad162 journal article 10.1093/zoolinnean/zlad162 0024-4082 14422258 Systematics of Tomthompsonia and Pleurobranchaeidae Pleurobranchidaisrecoveredmonophyleticwithfullsupportand includes three main clades. For the first time in molecular phylogeny, Tomthompsonia antarctica ( Fig. 1A ) forms a discrete, early divergent clade with maximum support across matrices and analyses. Thus, we erect Tomthompsoniidae fam. nov. to encompass a single known genus and species endemic to deep waters in Antarctica ( Thiele 1912 , Wägele and Hain 1991, Hain et al. 1993 ). Then, two main clades sister to each other are recovered, Pleurobranchaeidae and Pleurobranchidae (bs = 88, pp = 0.95). Within Pleurobranchaeidae , the type species Euselenops luniceps ( Fig. 1B ) is first found as a sister to a clade composed of the type species of both Pleurobranchaea and Pleurobranchella . Among the four specimens of Euselenops available, a conflicting signal is found in the 18S of the specimens sequenced by Wollscheid-Lengeling et al. (2001) and, unfortunately, their relationships are not always well-recovered (see Supporting Information, Figs S1 , S2). Excluding these from our concatenated analyses rendered Pleurobranchaeidae monophyletic without maximum support (bs = 91). COI p -distances for both remaining specimens of E. luniceps from Singapore and Vietnam seem to represent cases of hidden speciation (86% identity). Considering both type taxa Pleurobranchaea meckeli (Blainville , 1825) ( Fig. 1C ) and Pleurobranchella nicobarica Thiele , 1925 ( Fig. 1D ) were present in our dataset, and Pleurobranchella is placed within a clade composed of P. meckeli , P. maculata (Quoy and Gaimard , 1832), P. californica MacFarland , 1966, and P. japonica Thiele , 1925 (bs = 100, pp = 1). However, Pleurobranchella is morphologically distinct from Pleurobranchaea , including such immediately recognizable features as an ample notum and differences in the reproductive system (Martynov and Schrödl 2009). We, therefore, keep the genus Pleurobranchella distinct and valid. Finally, P. cf. novaezealandiae from the Western Pacific clusters with our P. japonica from Japan with a 99.1% identity, thus suggesting both specimens belong to P. japonica .