Small mytilids of the genus Ildas are related to the large mussels found worldwide at deep-sea hydrothermal vents and cold seeps. They are therefore keys to a better understanding of the colonization of vents and seeps by symbiont-bearing organisms, but still little is known about their biology. For this study, specimens of a mytilid referred to the genus Idas were collected from various substrates in a cold seep area near the Nile deep sea fan. Based on molecular and morphological data, all specimens are confirmed to belong to a single species of the genus Idas, which was previously shown to host six distinct bacterial symbionts. Its larval shell characteristics indicate a long planktonic phase, which could explain its close relationship to a mussel species that occurs in the Gulf of Mexico. 3-D FISH indicates the dominance of sulfur-oxidizing, methane-oxidizing and methylotrophic symbionts in all specimens analysed.