Molecular systematics of vestimentiferan tubeworms from hydrothermal vents and cold-water seeps

Vestimentiferan tubeworms inhabit sulfide-rich environments associated with deep-sea hydrothermal vents and cold-water seeps at continental margins. Twelve species have been described, and several more await formal descriptions. As a group, these worms are best known for their lack of a digestive system in adults and their dependence on endosymbiotic bacteria that supply nutrients derived from chemoautotrophism. The taxonomic status of Vestimentifera has been debated since their discovery. Furthermore, relationships within the Vestimentifera have been difficult to determine by morphological criteria. Several species display considerable phenotypic plasticity, further confounding efforts to establish evolutionary relationships. We used a fragment of the mitochondrial cytochrome c oxidase subunit I gene to examine evolutionary relationships among vent-endemic species (Riftia pachyptila, Oasisia alvinae, Ridgeia piscesae, Tevnia jerichonana) and seep-associated species (Escarpia laminata, E. spicata, Lamellibrachia barhami, L. columna, and an undescribed species) of these worms. The molecular data placed these vestimentiferan taxa within the phylum Pogonophora. The pogonophoran clade (including vestimentiferans) was then linked to the Annelida. Examination of sequence divergence suggests that extant vestimentiferans constitute a recent evolutionary radiation that diversified as a paraphyletic assemblage of seep-associated taxa (including the genera Lamellibrachia and Escarpia) and then gave rise to a clade of vent-endemic taxa (genera Riftia, Oasisia, Ridgeia and Tevnia).