Endosymbionts of dreissena polymorpha in Ireland: Evidence for the introduction of adult mussels

Although zebra mussels (Dreissena polymorpha) have invaded waters across Europe for over 200 years, they colonized Ireland only within the past decade. To test the hypothesis that Ireland was colonized by adult D. polymorpha, we examined mussels from different sites along the Lower Shannon River system in Ireland for the presence of host specific and generalist endosymbionts.Within the mantle cavity and/or associated with zebra mussel tissues we found species specific-ciliates (Conchophthirus acuminatus and Ophryoglena hemophaga) and generalist symbionts (the ciliate Ancistrumina limnica, nematodes, oligochaetes and chironomids). We found a significant difference in the prevalence of symbionts among sites, but all mussels at all sites harboured one specialist species C. acuminatus, and all of the mussels at three of the four sites also had the second specialist, O. hemophaga. Thus, with the introduction of D. polymorpha into Ireland, at least two additional species, their host-specific symbionts C. acuminatus and O. hemophaga, have also been introduced. The presence of these symbionts in Ireland supports the hypothesis that adult zebra mussels were introduced into Ireland, rather than larval stages. This contrasts with the introduction of zebra mussels to North America, where adult zebra mussels are devoid of host-specific symbionts.