Do grazers prefer invasive seaweeds?

Sargassum muticum is an invasive alga that affects biodiversity of benthic communities. Its proliferation in recipient communities may be due to several factors including limited grazing effects by native herbivores (“Enemy Release Hypothesis”, ERH). Until now, there is no information about grazing preferences exerted by native herbivores over S. muticum and native seaweeds on the Galician coast. The aim of this study was to determine whether or not different grazers preferred S. muticum compared with native seaweeds. The hypothesis tested was that native meso- and macrograzers would prefer native seaweeds rather than the invasive seaweed S. muticum. Concentrations of polyphenols were also examined to see whether or not these compounds correlated with preferences for types of food. Despite variability of responses within and among herbivorous species, results suggested that grazers preferred native seaweeds over the invasive S. muticum. In most cases, the gastropods Littorina littorea and Aplysia punctata preferred to feed on Ulva sp., whereas L. obtusata fed mostly on Fucus vesiculosus. The sea urchin Paracentrotus lividus, however, showed no preference, feeding indiscriminately on different seaweeds. It is also important to highlight that in many cases, grazers did not show any preference (i.e. consumption when a choice was available could not be differentiated from consumption when there was no choice). These results, therefore, partially supported our prediction and reinforced the notion that well-designed experiments and analyses must be done before preference can be inferred. We suggest that the invader S. muticum is not under substantial pressure from grazers in the area of study.