Holopelagic Poeobius meseres ("Poeobiidae," Annelida) Is Derived from Benthic Flabelligerid Worms

Phylogenetic relationships among and within the more than 70 recognized families of Annelida are poorly understood. In some cases, such as the monotypic Poeobiidae, derived morphology hinders the ability to find convincing synapomorphies that help elucidate evolutionary origins. In such cases, molecular data can be useful. Poeobiidae consists of the holopelagic polychaete Poeobius meseres, which is typically found in midwater depths off California. Morphologists have speculated that it is close to or within Flabelligeridae, but definitive evidence was lacking. Herein we use maximum likelihood phylogenetic reconstruction methods to examine the nuclear 18S rDNA (SSU) gene and the mitochondrial cytochrome b gene. Our results strongly support the hypothesis that P. meseres is a highly derived flabelligerid annelid closely related to Therochaeta. Thus, Poeobiidae is a junior synonym for Flabelligeridae. This result raises interesting questions about the evolution of the holopelagic P. meseres from a benthic ancestral flabelligerid.