Sessile hydrozoans constitute a common component of marine rocky communities. We compared the hydrozoan assemblages occurring on sea-walls of commercial harbours with those on natural rocky cliffs along the southern Iberian Peninsula, to identify differences in the multivariate structure of the assemblages and species richness. Harbour hydroid assemblages significantly differed from natural ones mainly due to their qualitative composition. Medusa-less taxa, optimized for low dispersal and long-term persistence on the substratum, are barely represented in harbours, but abundant at natural sites. “Port species” assemblages were composed of (1) small, short-living species with typical opportunistic characteristics; (2) cosmopolitan large-size taxa, significantly represented both in harbours and in natural habitats; (3) non-indigenous species. Contrarily to the expected lower richness of communities in confined areas, our results demonstrate that richness of hydroid assemblages in harbours is comparable to that of natural habitats.
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