A scaleless scale worm: Molecular evidence for the phylogenetic placement of Pisione remota (Pisionidae, Annelida)

Pisionidae is a group of interstitial worms whose phylogentic affinities have been enigmatic. They have been allied to different Phyllodocida taxa. Although originally associated with Glyceridae and Phyllodocidae, they are more recently considered to be related to scale worms. Scale worms are a well-defined taxon, Aphroditiformia, within Annelida due to the unique possession of dorsal scales called elytra. Pisionidae lack elytra but they have been grouped with scale worms because they possess two pairs of jaws with venom glands, also found in Glyceridae. Determining the phylogenetic position of Pisionidae is important for understanding if features such as elytra and venomous jaws are evolutionarily labile in annelid history. Therefore, we explored 18S rDNA and Cytochrome c Oxidase subunit I data from several Aphroditiformia, Pisionidae, and other Phyllodocida to determine the phylogenetic placement of Pisionidae. Maximum likelihood and Bayesian inference of separate and combined data sets were conducted. All analyses support a derived position of Pisionidae within Aphroditiformia, close to Pholoidae and Sigalionidae. The loss of elytra in Pisionidae is probably due to adaptation for interstitial life. Furthermore, the results reject a monophyletic Aphroditoidea comprising Acoetidae, Aphroditidae, Eulepethidae and Polynoidae. Thus, the possession of only simple chaetae is either symplesiomorphic or convergent.