Molecular evidence that Sclerolinum brattstromi is closely related to vestimentiferans, not to frenulate pogonophorans (Siboglinidae, Annelida)

Siboglinids, previously referred to as pogonophorans, have typically been d ivided into two groups, frenulates and vestimentiferans. Adults of these ma rine protostome worms lack a functional gut and harbor endosymbiotic bacter ia. Frenulates usually live in deep, sedimented reducing environments, and vestimentiferans inhabit hydrothermal vents and sulfide-rich hydrocarbon se eps. Taxonomic literature has often treated frenulates and vestimentiferans as sister taxa. Sclerolinum has traditionally been thought to be a basal s iboglinid that was originally regarded as a frenulate and later as a third lineage of siboglinids, Monilifera. Evidence from the 18S nuclear rDNA gene and the 16S mitochondrial rDNA gene presented here shows that Sclerolinum is the sister clade to vestimentiferans although it lacks the characteristi c morphology (i.e., a vestimentum). The rDNA data confirm the contention th at Sclerolinum is different from frenulates, and further supports the idea that siboglinid evolution has been driven by a trend toward increased habit at specialization. The evidence now available indicates that vestimentifera ns lack the molecular diversity expected of a group that has been argued to have Silurian or possibly Cambrian origins.