Molecular phylogeny of siboglinid annelids (a.k.a. pogonophorans): a review

Siboglinid, or pogonophoran, annelids are tubicolous worms that rely on chemoautotrophic endosymbionts for nutrition. Three clades within the siboglinids are recognized: Frenulata, Vestimentifera, and Monilifera. As a group, these worms have received considerable attention from molecular phylogenetists. Most studies have focused either on the evolutionary origins of the group or on the relationships within vestimentiferans, which live at hydrocarbon seeps and hydrothermal vents. Here I review the literature to date on siboglinid molecular phylogeny and summarize the clade's evolution. The vestimentiferans have been well studied, especially in the eastern Pacific. The seep taxon Lamellibrachia is basal in the clade with vent species being more derived. Recent studies of seeps are finding new species and suggest that habitat depth can be correlated with species boundaries. In contrast to the vestimentiferans, frenulate evolution has been poorly studied. Despite their greater apparent diversity, frenulate specimens have not been sampled so extensively, and thus little is known about their evolution. Sclerolinum, also referred to as Monilifera, is a recognized genus of siboglinids that forms the sister group to Vestimentifera. Like the frenulates, little is known about the history of this group. Our present understanding of siboglinid phylogeny has, in large part, been dictated by insufficient sampling effort.