Deep-sea methane seeps are dynamic sources of greenhouse gas production and unique habitats supporting ocean biodiversity and productivity. Here, we demonstrate new animal-bacterial symbioses fueled by methane, between two undescribed species of annelid (a serpulid Laminatubus and sabellid Bispira) and distinct methane-oxidizing Methylococcales bacteria. Worm tissue delta 13C of -44 to -58 per mil suggested methane-fueled nutrition for both species and shipboard experiments revealed active assimilation of 13C-labelled CH4 into animal biomass, occurring via engulfment...