Reconstruction of the phylogeny of the Opisthobranchia (Mollusca: Gastropoda) by means of 18s and 28s rRNA gene sequences

The complete 18S (SSU) rRNA gene and the partial 28S (LSU) rRNA gene of 49 taxa, including representatives of the Opisthobranchia, Pulmonata and Allogastropoda, were sequenced to infer the phylogeny of the Opisthobranchia and their major subordinate taxa Nudibranchia, Pleurobranchoidea, Tylodinoidea, Sacoglossa, Cephalaspidea s.l. and Anaspidea. For the first time the enigmatic taxon Acochlidiacea was included. Several tests were applied to the datasets to investigate relative evolutionary rates of the taxa, saturation of substitution, base composition and the amount of phylogenetic information. Maximum parsimony, maximum likelihood and distance methods of tree construction were used. The opisthobranch subgroups, including the Acochlidiacea, were confirmed as monophyletic in most analyses, whereas monophyly vs paraphyly of Opisthobranchia could not be resolved. All trees showed a sister-group relationship between Nudibranchia and Pleurobranchoidea (Nudipleura). In most trees, sister-group relationships between Cephalaspidea s.s. and Anaspidea, and between Nudipleura and Acteonoidea (Pupa solidula, Hydatina physis), were supported. The latter sister-group relationship supports an affinity that has not been mentioned for nearly 40 years (Ghiselin, 1966, Malacologia, 3: 327–378). It was ‘rediscovered’ in the molecular analyses by Grande et al. (2004, Molecular Biology and Evolution, 21: 303–313) and represents a valid and interesting opisthobranch clade.