The ecology and co-management of endangered host-parasite associations, particularly between unionid mussels and their host fish, are an important yet underexplored area of research and conservation. Unionid mussels are important ecosystem engineers and are amongst the most globally endangered taxa, reliant on a fish host to complete a generational turnover. Freshwater fish are themselves highly endangered, a noted reason for the decline in unionid populations. The European eel (Anguilla anguilla), one of Europe's most endangered fish, has experienced a 90% population decline in the past half century. Theoretically, eels should make ideal hosts for unionids due to their broad distribution, long upriver migrations, dispersal capabilities within and across catchments, resistance to gill damage and physical proximity to gravid mussels. We tested the function of eels as hosts to the six most widespread European unionid species to assess if there is significant merit in the co-management of these organisms. We demonstrate that eels can host all but one host specialist unionid, suggesting that declines in global eel populations are a likely factor behind the declines of unionid populations. As conservation actions for eels and mussels are broadly similar, and the conservation of one directly affects the conservation of the other, co-managing both could enhance recovery of both. This study further aims to improve unionid conservation, by pairing an uncharismatic mussels with a well-known and charismatic fish.