A bacterium that invades the nuclei of deep-sea bathymodiolin mussels from hydrothermal vents and cold seeps. This organism was first discovered in Bathymodiolus puteoserpentis from the Logatchev hydrothermal vent field on the Mid-Atlantic Ridge. Fluorescence in situ hybridization and transmission electron microscopy analyses of the developmental cycle of E. puteoserpentis showed that the infection of a nucleus begins with a single rod-shaped bacterium which grows to an unseptated filament of up to 20 μm length and then divides repeatedly until the nucleus is filled with up to 80 000 bacteria. The greatly swollen nucleus destroys its host cell and the bacteria are released after the nuclear membrane bursts. Intriguingly, the only nuclei that were never infected by E. puteoserpentis were those of the gill bacteriocytes. These cells contain the symbiotic sulfur- and methane-oxidizing bacteria, suggesting that the mussel symbionts can protect their host nuclei against the parasite. E. puteoserpentis belongs to a monophyletic clade of Gammaproteobacteria associated with marine metazoans as diverse as sponges, corals, bivalves, gastropods, echinoderms, ascidians and fish.