Tomthompsonia, Wagele & Hain, 1991

Systematics of Tomthompsonia View in CoL and Pleurobranchaeidae

Pleurobranchidaisrecoveredmonophyleticwithfullsupportand includes three main clades. For the first time in molecular phylogeny, Tomthompsonia antarctica ( Fig. 1A View Figure 1 ) 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 View Figure 1 ) 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 View Figure 1 , 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 View Figure 1 ) and Pleurobranchella nicobarica Thiele , 1925 ( Fig. 1D View Figure 1 ) 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 .