Vestimentiferan tubeworms, once erected at a phylum level, are now known to comprise a part of the specialised deep-sea polychaete family Siboglinidae. Their widespread and abundant occurrence at hydrothermal vents and hydrocarbon seeps has fostered numerous studies of their evolution and biogeography, ecology and physiology. Harbouring autotrophic, sulphide-oxidising, intracellular bacterial symbionts, they form large populations of ‘primary’ producers with contrasting characteristics, from fast-growing, short-living species at vents, to slow-growing, long-living species at seeps. These different life strategies and the ways they modify the biogeochemistry of their respective environments have consequences on the macro- and meiofaunal assemblages that develop within vestimentiferan bushes. New findings indicate that postlarval recruits get infected through the skin by free-living bacteria for which growth is rapidly and specifically limited by the host to mesoderm cells around the gut that further transform into the characteristic trophosome. The resulting internal location of symbionts prompts specific adaptations of the hosts to fulfil their metabolic requirements, including unusual sulphide and carbon dioxide assimilation and transport mechanisms. Symbiont genome sequencing has improved our knowledge of potential bacterial metabolism and should rapidly open the way for new research approaches to resolve the intricate physiological relationships between a eukaryotic host and its chemoautotrophic bacterial symbionts.