HALANYCH, Kenneth M: Evolution and habitat preference in Siboglinids (formerly Pogonophora)

Siboglinids, previously referred to as pogonophorans, are marine protostome worms which lack a functional gut and harbor endosymbiotic bacteria. Of the two major recognized clades, frenulates and vestimentiferans, the evolutionary history of the vestimentiferans is one of the best studied for any annelid. Recent data from both morphological and molecular (16S rDNA and 18S rDNA) data suggest that Sclerolinum is a sister group to vestimentiferans. In contrast, Sclerolinum has traditionally been thought to be a basal siboglinid that was originally regarded as a frenulate and later as a third lineage of siboglinids (i.e., Monilifera). Frenulates and vestimentiferans are usually treated as sister taxa. Whereas, frenulates usually live in deep sedimented reducing environments, and vestimentiferans inhabit hydrothermal vents and sulfide-rich hydrocarbon seeps.The present phylogenetic analyses further support the idea that siboglinid evolution has been driven by a trend toward increased habitat specialization. The evidence now available indicates that vestimentiferans lack the molecular diversity expected of a major animal clade that has been argued to have Silurian or possibly Cambrian origins.