Effects of the invasion of macroalga Asparagopsis taxiformis on the polychaete assemblages associated to the native macroalga Cystoseira brachycarpa in the Mediterranean Sea

The establishment of non-indigenous species is one of the major threats to Mediterranean biodiversity. The tropical red macroalga Asparagopsis taxiformis is widely distributed in the Mediterranean Sea and appears invasive along the shallow rocky areas of western Sicily (Italy). Here it competes for space with the native brown macroalgae of the genus Cystoseira, in particular with C. brachycarpa, a valuable Mediterranean habitat former. This research aims to evaluate the potential effects of invasion of A. taxiformis on the polychaete assemblages associated with C. brachycarpa. Polychaetes were sorted from Cystoseira and Asparagopsis thalli collected along the Favignana Island (Sicily) in the rocky infralittoral algal community from four locations with different degree of invasion: 1) non-invaded, 2) invaded but dominated by Cystoseira, 3) invaded and dominated by Asparagopsis, 4) completely invaded. Ten replicates of each algal thalli were collected from two sites per location. Structure of polychaete assemblages changed significantly among degree of invasion. Reductions of average polychaete abundance and species richness occurred with invasion, with 58 individuals and 12 species per thallus on non-invaded location, only 4 individuals and 2 species on completely invaded location, and almost equal, intermediate values, on two partially invaded locations. Decrease of abundance was mainly caused by the decrease of sabellid Amphiglena mediterranea and syllids Syllis prolifera and Sphaerosyllis austriaca. These results raise concerns on the spread of non-indigenous habitat forming species and their potential impact on biodiversity at local scale.