<p>Recreational marinas are raising worldwide attention because of the public appeal for recreational activities such as boating. Their floating pontoons provide substrate for fouling assemblages, which include many ecosystem engineers such as the mussel <i>Mytilus galloprovincialis</i>. However, knowledge on vagile assemblages in floating pontoons under different salinity conditions and its drivers is still very limited. In this study, vagile fouling assemblages were investigated in six marinas along the northern coast of Portugal (three brackish and three marine) to test if the <i>M. galloprovincialis</i> population (abundance, size structure) and the vagile fouling assemblage (abundance, taxon richness, multivariate structure) differed between marine and brackish conditions. Furthermore, we tested if there was a relationship between mussel abundance and the aforementioned features of the assemblage. Mussel abundance was about three times greater in marine than in brackish marinas; the former also showed a more diverse mussel size structure. Richness in marine habitats was about twice in comparison to brackish ones. Seventy-six species were exclusively found in marine habitats whereas 27 species were found only in brackish marinas. Moreover, the multivariate assemblage structure showed an average dissimilarity of 81% between brackish and marine conditions. Mussel abundance and salinity interplayed in the structure of vagile fouling assemblages. Salinity potentially has a direct effect on the vagile fauna by influencing their composition according to each species’ tolerance to salinity, but also an indirect effect through its influence on mussels. Our findings suggest that mussels could be useful if the goal is to increase biodiversity in marinas under marine conditions since richness increased with mussel abundance. However, future experimental studies will be necessary to elucidate the role of mussels as biodiversity enhancers.</p>