Any marine tube-dwelling polychaete worm of the former phylum Pogonophora, now placed in the annelid family Siboglinidae. They are typically found in ocean sediment, some at great depths near hydrothermal vents or cold seeps. Many have a mass of feathery tentacles at the anterior end; hence their popular name of ‘beardworms’. Adults lack a digestive system and details of their nutrition are uncertain; they may rely on endosymbiotic bacteria utilizing inorganic or organic compounds as sources of energy.