Is the biofouling community of a subtropical estuary resistant to the recruitment of an exotic hydroid?

In sessile marine communities, recruitment is the initial phase of new environment colonization and represents a measure of how vulnerable the community is to non-native species' arrival. In invasion biology, it is argued that efficiency in resource exploitation and space occupation is the main form of a resident community to withstand the establishment of alien species. Our goal was to compare the colonization patterns of the alien hydrozoan Podocoryna loyola between early-stage communities and mature communities, both growing on artificial substrates , in an estuarine area in the south of Brazil. The number of recruits and the hydroid coverage were recorded in bimonthly intervals. Podocoryna loyola occurred on both communities, with greater abundance along the warmer bimesters. During the recruitment peak, there was little variation in the number of colonies between early-stage and mature communities (two to five colonies), but coverage of P. loyola recruits was lower in the mature ones. These results indicate that, in bimonthly recruitment intervals, the mature community did not provide resistance to settlement and metamorphosis of P. loyola larvae but limited the initial growth of recruits. However, after the first year of settlement, P. loyola reached higher cover percentages (up to 50%) in the mature communities, probably interfering with their composition and dynamics. We conclude that recruitment and initial growth of P. loyola are independent of a pre-existing community. • The number of hydroid colonies was similar in early-stage and mature communities, during the peak of the recruitment. • The coverage of colonies was lower in mature than early-stage communities, during the peak of the recruitment period. • The alien hydroid reached higher cover percentages becoming one of the dominant species in the mature communities. • Recruitment and initial growth of P. loyola are independent of a pre-existing community.