Our study focused mainly on the endosymbiosis between bivalves of the family Lucinidae and sulfur-oxidizing bacteria. Lucinids are by far the most diverse and widespread group of bivalves that host chemosynthetic symbionts15. There are at least 400 living species, occupying a range of habitats including mangrove sediments, seagrass beds, coral reef sediments and coastal mud and sand16. In seagrass habitats, lucinid bivalves and their sulfur-oxidizing symbionts are part of a nested symbiosis with seagrasses, which may be essential to the health and ecological success of seagrasses6. We focused on the symbiosis between Loripes lucinalis (Lamarck, 1818) and its endosymbionts. We also investigated a second symbiosis, that between stilbonematid nematodes and their sulfur-oxidizing ectosymbionts, because these symbionts are associated with the family Chromatiaceae, which contains a number of diazotrophic sulfur oxidizers17,18. Nematodes of the subfamily Stilbonematinae (family Desmodoridae) can be found worldwide in marine sulfidic habitats19. All known species have a dense coating of ectosymbionts on their cuticle, which are hypothesized to contribute to their host's nutrition19. The name Candidatus Thiosymbion oneisti will be proposed elsewhere for the nematode symbionts (Gruber-Vodicka et al., in preparation). We propose the name Candidatus Thiodiazotropha endoloripes for the symbiont of Loripes lucinalis, where ‘Thiodiazotropha’ refers to the sulfur-oxidizing (‘thio’) and nitrogen-fixing (‘diazotroph’) metabolism of the symbiont and ‘endoloripes’ (‘Endo-’, Greek from ἔνδον meaning ‘within’, ‘loripes’) refers to the endosymbiotic association with Loripes lucinalis, its bivalve host.