Changes in Mediterranean Posidonia-associated polychaete assemblages due to a new alien Australian seaweed

The Mediterranean Sea is particularly affected by biological invasions. Alien seaweeds of the genus Caulerpa cause various direct and indirect negative effects on native Mediterranean biota, including changes in benthic assemblages. The northeastern Australian algae C. taxifolia var. distichophylla is a recent invader recorded along the Turkish and southern Sicilian coasts, especially on rocky bottoms and seagrass meadows. The impact of this newcomer on the native Mediterranean benthic communities has not yet been assessed. Herewith the results of the survey carried out in summer 2013, aiming to evaluate the effects of the invasion on the native polychaete assemblages associated to the endemic Mediterranean seagrass Posidonia oceanica are presented. Samples were taken by hand corers on Posidonia meadows borders (mattes) in southern Sicily, where the alien appeared particularly abundant, at invaded and non invaded sites. Polychaetes were more abundant at the impacted sites, while diversity was higher at control sites. Both univariate and multivariate analyses indicated that the composition and structure of the polychaete assemblages in impacted sites was significantly different from those in the control ones. Tolerant capitellids Peresiella clymenoides and Pseudoleiocapitella fauveli and other tolerant species such as Euclymene oerstedi, Glycera tridactyla, Tharyx marioni, Myriochele oculata and Sabellaria spp characterized the impacted location. Differences in Eh and RPD in the mat-associated sediment might be responsible for the observed pattern. Our results raise serious concern about the possible negative effects of the new alien invader on the native macrobenthic fauna.