A contribution to sedentary polychaete phylogeny using 18S rRNA sequence data

The phylogenetic position of Annelida as well as its ingroup relationships are a matter of ongoing debate. A molecular phylogenetic study of sedentary polychaete relationships was conducted based on 70 sequences of 18S rRNA, including unpublished sequences of 18 polychaete species. The data set was analysed with maximum parsimony and maximum likelihood methods. Clade robustness was estimated by parsimony-bootstrapping and jackknifing, decay index, and clade support, as well as a posteriori probability tests using Bayesian inference. Irrespective of the applied method, some traditional sedentary polychaete taxa, such as Cirratulidae, Opheliidae, Orbiniidae, Siboglinidae and Spionidae, were recovered by our phylogenetic reconstruction. A close relationship between Orbiniidae and Questa received a particularly strong support. Echiura appears to be a polychaete ingroup taxon which is closely related to Dasybranchus (Capitellidae). As in previous molecular analyses, no support was found for the monophyly of Annelida nor for that of Polychaeta. However, we suggest that an increase in taxon sampling may yield additional resolution in the reconstruction of polychaete ingroup phylogeny, although the difficulties in reconstructing the basal phylogenetic relationships within Annelida may be due to their rapid radiation.