Invasive macrophytes in a marine reserve (Columbretes Islands,NW Mediterranean): spread dynamics and interactions with the endemic scleractinian coral Cladocora caespitosa

The invasive algae Lophocladia lallemandii and Caulerpa racemosa are becoming an important threat to benthic assemblages in the Mediterranean Sea. Both species were first detected in Illa Grossa Bay (Columbretes Islands Marine Reserve, NW Mediterranean) in 2006, and their invasion was monitored until 2012. L. lallemandii showed a rapid outburst, spreading around the entire bay in just 2 years and showing the highest abundances between 5 and 10 m of depth (82.07 ± 3.53 % (±SE) in 2011). Caulerpa racemosa showed a slower but steady spread and remained in deeper areas during the first years; however, drastic changes in the depth distribution, with algae invading toward shallower areas, were noted beginning in 2010 and reached abundances of 57.76 ± 1.07 % (±SE) between 10 and 20 m of depth in 2011. Illa Grossa Bay hosts one of the most important populations of the endemic coral Cladocora caespitosa. This study is the first to quantitatively assess interactions between the coral and invasive algae. Although both invasive species L. lallemandii and C. racemosa had overlapping distributions with C. caespitosa, we did not find any lethal or sublethal effects of either invasive algal species. On the other hand, C. caespitosa exhibited toxic activity, which could explain the low overgrowth of living colony parts by C. racemosa.