The Acteonoidea are a taxon of about 150 species with uncertain systematic affinity. They have been variously regarded as basal Opisthobranchia, as Architectibranchia, or placed basal to Opisthobranchia together with other heterobranch clades with uncertain interrelationships. We performed molecular phylogenetic analyses employing four gene markers (nuclear 18S rDNA and 28S rDNA; mitochondrial 16S rDNA and COI) to reassess the phylogenetic position of Acteonoidea and provide the first comprehensive study on interrelationships of the constituent families. Our analyses reveal a sister-group relationship of the Acteonoidea to the ‘lower’ heterobranch taxon Rissoelloidea, implying a basal placement outside the Opisthobranchia. However, the Acteonoidea/Rissoelloidea clade is sister group to the morphologically derived opisthobranch clade Nudipleura, implying an opisthobranch origin. Regarding the interrelationships of the Acteonoidea, the current division into three families is challenged by recovery of only two clades. The genus Bullina (sole genus in Bullinidae) resolves as a member of the Aplustridae, which is thus rendered paraphyletic. The Acteonidae are paraphyletic due to the strongly supported position of Rictaxis punctocaelatus basal to the Aplustridae. The first detailed investigation of the morphology and histology of R. punctocaelatus revealed similarities with both Acteonidae and Aplustridae, implying an intermediate position. Owing to the placement of R. punctocaelatus basal to the Aplustridae, the shared acteonid characters are therefore considered plesiomorphic for the whole Acteonoidea and are probably the features of the last common ancestor of the clade. |
