A brief review of holopelagic annelids.

Annelids are one of the most successful major animal lineages in terms of number of species and of habitats occupied. Despite annelids being common in terrestrial, aquatic, and marine environments, only a limited number of lineages have evolved a holopelagic existence. Most of these holopelagic lineages belong to Phyllodocida (nereidids, syllids, scale worms, and jawed worms) and more particularly often within the family Phyllodocidae. These worms generally appear to retain many characteristics of adult annelids. Moreover, we provide molecular evidence showing that the well-known alciopids are derived from within Phyllodocidae. In contrast, at least two lineages, Poeobius meseres/Flota flabelligera and probably Chaetopterus pugaporcinus, are derived through paedomorphic processes acting on larvae from lineages that have sedentary adult forms. Herein, we will briefly review the known diversity of holopelagic annelids with discussion of their evolutionary origins.