Nearly 13% of opisthobranch gastropods in the Atlantic Ocean are regarded as amphi-Atlantic (i.e. species occurring on both eastern and western coastlines of the Atlantic realm). This assumption has been broadly based on morpho-anatomical similarities and has rarely been tested within a molecular phylogenetic framework. Among opisthobranchs, the order Sacoglossa is renowned for its difficult and confusing species taxonomy, particularly in some genera like Elysia, where several cases of cryptic speciation have been documented. Moreover, the width of the Atlantic realm poses a serious challenge to the idea of trans-Atlantic dispersal, particularly in nonplanktotrophic species. The species Bosellia mimetica, Elysia timida and Thuridilla picta are three sacoglossan opisthobranchs with amphi-Atlantic status, but doubts have been raised about the conspecificity of eastern and western populations. Here we used methods to test molecular phylogenetic whether the Atlantic Ocean constitutes a barrier for dispersal and hence whether eastern and western populations belong to the same species. We used the criteria of divergence and reciprocal monophyly, supported by concordant genealogically independent genetic markers, to define species. Maximum-likelihood and Bayesian analyses of partial DNA sequences of the mitochondrial cytochrome c oxidase subunit I and 16S rRNA genes and nuclear gene Histone-3 (H3) were used to produce phylogenetic trees. We conclude that the eastern Atlantic species B. mimetica is not amphi-Atlantic, although the H3 gene did not recover reciprocal monophyly between western and eastern populations, perhaps in a result of incomplete lineage-sorting. Elysia timida is the only species showing an amphi-Atlantic distribution, but possibly due to recent introduction in the tropical western Atlantic. Thuridilla picta is restricted to the western Atlantic and therefore not amphi-Atlantic. Populations of ‘T. picta’ inhabiting the Macaronesian archipelagos of Azores and Madeira are a colour form of the eastern Atlantic T. hopei. Our results highlight the difficulty surrounding systematics of Sacoglossa sea slugs and provide further evidence that cryptic speciation is common in the group.

Aeolidida is one of the largest clades of nudibranchs with at least 560 known species. However, its systematics has not been studied in a comprehensive manner. Phylogenetic analyses of larger clades such as Nudibranchia or Cladobranchia have usually included a poor sample of aeolids. Furthermore, phylogenetic studies at the family or generic level in Aeolidida are a few and far between. The first molecular phylogeny of the aeolid family Aeolidiidae is presented here. This study, the most comprehensive for Aeolidida to date, uses new sequences of two mitochondrial (COI and 16S) genes and one nuclear gene (H3). 251 specimens from members of seven families of Aeolidida, including 39 species of Aeolidiidae were studied. Excluding Pleurolidia juliae , Aeolidiidae is monophyletic. Our results resolve the systematic relationships within the Aeolidiidae at a generic level, requiring changes in the systematics of this family. Spurilla, Anteaeolidiella, Limenandra and Aeolidia are well-supported and monophyletic clades. Aeolidiella stephanieae is transferred to Berghia and Aeolidiopsis ransoni and Spurilla salaamica to Baeolidia , to maintain the monophyletic lineages reflected in this study. The systematics of Cerberilla remains unclear. Some species earlier attributed to Aeolidiella are now grouped in a previously unnamed clade that we designate as Bulbaeolidia gen. nov.

Tergipedidae represents a diverse and successful group of aeolid nudibranchs, with approximately 200 species distributed throughout most marine ecosystems and spanning all biogeographical regions of the oceans. However, the systematics of this family remains poorly understood since no modern phylogenetic study has been undertaken to support any of the proposed classifications. The present study is the first molecular phylogeny of Tergipedidae based on partial sequences of two mitochondrial (COI and 16S) genes and one nuclear gene (H3). Maximum likelihood, maximum parsimony and Bayesian analysis were conducted in order to elucidate the systematics of this family. Our results do not recover the traditional Tergipedidae as monophyletic, since it belongs to a larger clade that includes the families Eubranchidae, Fionidae and Calmidae. This newly recovered clade is here referred to as Fionidae, the oldest name for this taxon. In addition, the present molecular phylogeny does not recover the traditional systematic relationships at a generic level, and therefore, systematic changes are required. We recognize the following clades within Fionidae: Calma, Cuthona, Cuthonella, Eubranchus, Fiona, Murmania, Tenellia, Tergipes, Tergiposacca gen. nov., Rubramoena gen. nov. and Abronica gen. nov. The type species of Tergiposacca, T. longicerata nov. sp. is described. The other two new genera have a previously described species as their type species. Most of these taxa, with the exceptions of Eubranchus, Tergipes and Fiona are composed of radically different constituent species from their traditional membership, but appear to be supported by morphological synapomorphies as well as molecular data. Aenigmastyletus, Catriona, Phestilla, Tenellia and Trinchesia are nested within other clades and, thus are here considered as synonyms of the larger clades. The phylogenetic position and validity of Myja, Guyvalvoria, Leostyletus and Subcuthona still need to be tested in future studies when material becomes available.