Mobile epifauna inhabiting the invasive Sargassum muticum and two local seaweeds in northern Spain

The mobile epifauna colonizing the invasive brown alga Sargassum muticum (Yendo) Fensholt was compared with assemblages inhabiting two locally established seaweeds (Fucus vesiculosus L. and Cystoseira nodicaulis (Whit.) Roberts) in a sheltered locality of northern Spain. The epifauna colonizing S. muticum was studied in both the shallow subtidal and intertidal pools at mid shore. The main aims of this study were to: (i) determine the host-plant specificity of the mobile epifauna; (ii) estimate the impact of the invasion on these animal assemblages. The mobile epifauna was not highly host-plant specific; very few taxa were found in just one algal species. Herbivorous species (mesograzers) were very abundant, accounting for 58–98 % of the characterized epifauna. Sargassum muticum was successfully colonized by the assemblages inhabiting local seaweeds, and this was probably due to: (i) the presence of epifaunal species that use plant epiphytes as habitat and food rather than the macroalga itself; (ii) morphological and chemical similarities between the invasive and the local seaweeds; (iii) generalist habits of mesograzers consuming the host plant itself. The impact of S. muticum invasion on the epifauna depends on the degree of host-plant specificity of animals as well as on the effect of the invader on macroalgal assemblages. The results of this study support the existence of a labile association between seaweeds and the invertebrates inhabiting them. ©1999 Elsevier Science B.V. All rights reserved.