Introduction of species is a major driver of global change and loss of biodiversity in ecosystems. The Mediterranean Sea is the most heavily invaded region in the world by introduced seaweeds. The fan mussel Pinna nobilis is the largest Mediterranean bivalve, and a protected species, present at depths from 0.5 m to 60 m. In Cabrera National Park marine protected area (MPA) (Balearic Islands, north-western Mediterranean) the population of P. nobilis is affected by the invasive seaweeds Lophocladia lallemandii and Caulerpa racemosa; however the extent of this colonization on the bivalve is unknown. Therefore, the main aims of the present study were to: (1) determine the extent of invasion of L. lallemandii and C. racemosa on the P. nobilis population; and (2) test if this invasion showed differences with depth. This is the first study assessing invasion by seaweeds on a large bivalve in a high density P. nobilis population. Pinna nobilis was distributed in all seagrass meadows surveyed within the MPA; we found a total of 872 live P. nobilis individuals, 449 and 423 of which were found at 10 and 20 m depth, respectively. High cover of L. lallemandii and low presence of C. racemosa were detected in the P. nobilis population, being quantified in 49.37% and 1.38% of the population of P. nobilis, respectively. Bathymetric differences on Lophocladia epizoism over P. nobilis has been recorded in this study; with increasing values of L. lallemandii cover index, biomass and volume at deeper depths. Pinna nobilis could be a preferential substratum for L. lallemandii in Posidonia oceanica meadows in deeper waters, acting as a stepping-stone for invasive seaweeds and facilitating the invasion throughout seagrass meadows.

The coenocytic green alga Caulerpa racemosa colonized the Mediterranean some time after the opening of the Suez Canal in 1869. For most of the past century it has remained confined to the southeastern Mediterranean, but over the past several years the species has been reported in abundance along the western coasts of Italy, Sardinia and the Marseille area of southern France. Whether this constitutes a natural range expansion or a possible new introduction is under discussion. The purpose of this study was to establish the ITS signatures of populations inside and outside the Mediterranean. ITS1 sequences were compared among 78 clones from 21 individuals representing 11 populations of C. racemosa, collected from the Western Mediterranean, the Canary Islands, Panama and Western Australia. Intra-individual polymorphism of ITS1 was found to be as high or higher than inter-individual polymorphism across all samples tested. Polymorphism parsimony analysis, which takes into account intra-individual variation, revealed strong separation between individuals from Panama, Fuerteventura (Canary Is.) and the Mediterranean. In contrast, none of the six Mediterranean populations could be distinguished from one another. In addition, individuals and their clones collected from Perth (Australia) and Gran Canaria were spread throughout the Mediterranean clade. Possible reasons for the lack of homogenization of ITS arrays in C. racemosa under the mechanisms of concerted evolution and implications for phylogeographic interpretation are discussed and compared with Caulerpa taxifolia.