The Dexiarchia (from ancient Greek dexios=right; archos=anus) is a group of nudibranchs recently established by Schrödl et al. (2001) that includes traditional cladohepatic nudibranchs (eolidaceans, arminaceans and dendronotaceans) plus the enigmatic group of Doridoxidae, all characterized by having the anus in the right lateral position and a more or less branched digestive gland.
Janolus cristatus by Enric Madrenas
Cladobranchia is diverse group of marine molluscs within the Dexiarchia that includes approximately 1000 species, but the lack of studies results in that this group is poorly understood.
According to Goodheart et al. (2015), to date, there have been only two large-scale phylogenies published specifically on Cladobranchia: (Pola & Gosliner, 2010) which used the three genes most commonly used in nudibranch phylogenetics and (Goodheart et al, 2015), that used all data publicly available through GenBank to understand the evolutionary relationships of members of this group, thus adding two additional genes and increasing taxon sampling by approximately 200 taxa. In both of these phylogenetic inferences of Cladobranchia, the majority of relationships remained unclear, both between and within the three traditional taxonomic divisions of Cladobranchia (Arminida, with about 100 species; Dendronotida, with about 250 species; and Aeolidida, with about 600 species).
In addition to the phylogenies specifically aimed at understanding relationships within Cladobranchia, phylogenetic inferences have been attempted to address the evolutionary history of Nudibranchia or the larger clade Euthyneura (Wollscheid E & Wägele H. 1999; Wollscheid et al. 2001; Thollesson, 1999; Wägele & Willan, 2000; Wägele et al, 2013; Mahguib & Valdés, 2015 and Schrödl et al, 2001). The results regarding the major groupings within Cladobranchia were inconsistent in all of these studies, with all three major divisions considered both paraphyletic and monophyletic in different publications. The most recent and comprehensive work by Wollscheid-Lengling et al. 2001 suggested that Aeolidida is monophyletic and both Dendronotida and Arminida are paraphyletic.
Though the more basal evolutionary history of Cladobranchia has been problematic for phylogenetic inference, a number of studies (both morphological and molecular) have provided evidence to support relationships at both the family and generic levels. Phylogenies of individual families have been published on Aeolidiidae, Arminidae, Bornellidae, Glaucidae, Scyllaeidae and Tritoniidae, and at the genus level publications have focused on Antaeolidiella, Babakina, Berghia, Burnaia, Dendronotus, Limenandra, Melibe, Phyllodesmium and Spurilla. All of these studies were focused on particular aspects of the Cladobranchia tree, and although they have supported some relationships, it is clear that the overall relationships of higher level groups in Cladobranchia are not well understood. In addition, individually these studies cover very little of the overall diversity of Cladobranchia.
The Cladobranchia, as a group, lack an homogeneous body organization: Aolidaceans have an elongated body and groups of papillae called “cerata” on both sides of the body. The Arminaceans have a roughly triangular shaped body and live in soft bottoms. Dendronotaceans have an elongated body and dendritic extensions in both sides of the body. The Doridoxidae have the body shape of doridacean nudibranchs, that is, dorsoventrally flattened.
Current taxonomy of Dexiarchia infraorders and parvorders is as following:
- Suborder Dexiarchia Schrödl, Wagele & Willan, 2001
- Infraorder Cladobranchia Sars, 1878
- Parvorder unassigned Cladobranchia Willan & Morton, 1984: molecular phylogeny can not stablish yet the relations among some families of Cladobranchia nudibranchs, as Charcotiidae, Dironidae, Goniaeolididae, Heroidae, Madrellidae, Pinufiidae and Proctonotidae (traditionally considered as Arminaceans), Dotidae (traditionally considered as Dendronotaceans) and Embletoniidae (tradicionally considered as Aeolidaceans) so this clade gathers them together until more data is available. The former infraorder Pseudoeuctenidiacea Tardy, 1970 is also included in this group, despite it only included the Doridoxidae family, deep water dwelling nudibranchs with only two known species of the genus Doridoxa Bergh, 1899, still to be found in Iberian waters.
- Parvorder Aeolidida Odhner, 1934: includes the traditional Aeolidaceans, slender bodied nudibranchs, with digitiform dorsal papillae where the digestive gland penetrates. Numerous families, genus and species worldwide.
- Parvorder Dendronotida Odhner, 1934: includes the traditional Dendronotaceans, elongated body nudibranchs with a row of dendritic extensions to each side of the dorsum.
- Parvorder Euarminida Odhner, 1939: these are the true Arminaceans, with the genus Armina Rafinesque, 1814 and Dermatobranchus van Hasselt, 1824 among others.
- Infraorder Cladobranchia Sars, 1878
Main Dexiarchia groups: