A bacterium that invades the nuclei of deep-sea bathymodiolin mussels from cold seeps. This organism was discovered in Gigantidas childressi from the Mississippi Canyon cold seeps at the Gulf of México. Fluorescence in situ hybridization and transmission electron microscopy analyses of the developmental cycle of E. childressii 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. childressii were those of the gill bacteriocytes. These cells contain methane-oxidizing bacteria, suggesting that the mussel symbionts can protect their host nuclei against the parasite. E. childressii belongs to a monophyletic clade of Gammaproteobacteria associated with marine metazoans as diverse as sponges, corals, bivalves, gastropods, echinoderms, ascidians and fish.