Chambered chaetae in nereidiform polychaetes (Annelida)

The nereidiform polychaete taxa Chrysopetalidae, Hesionidae and Nereididae are characterized by the presence of chambered chaetae. The medullae (inner part) of all examined annelid chaetae are provided with internal longitudinal canals, but in these taxa there are additional thin, transverse walls (diaphragms), giving the chaetae a barred or chambered appearance in light microscopy. We investigate this structure in chrysopetalids, hesionids, nereidids, with light, scanning and transmission electron microscopy and compare it to phyllodocids and syllids, which are outside this clade. We conclude that chambered chaetae likely constitute an synapomorphy for chrysopetalids, hesionids and nereidids, although further study are required of some aphroditids and nephtyids.