TAXONOMY AND BIOGEOGRAPHY OF THE DIATOM GENUS COCCONEIS EHR. ASSOCIATED TO THE SEAGRASS POSIDONIA OCEANICA (L.) DELILE IN THE MEDITERRANEAN SEA

Posidonia oceanica is a marine phanerogam endemic of the Mediterranean Sea, where it produces dense matte on sandy bottoms with unsteady hydrodynamic conditions, from shallow waters to 40 meters depth. Posidonia oceanica leaves represent an optimal surface of colonization for a complex epiphytic community encompassing both macroalgal and microalgal species. Epiphytic microalgae on P. oceanica are mainly represented by pennate diatoms. Differences in the geographic distribution of the host plant act mainly on the species composition of the epiphytic diatom community. Indeed, in term of functional classes or growth forms, this community seems to be represented by a well-circumscribed group of diatom genera that can be considered adapted to the epiphytic life-habitus. Several genera belonging to monoraphid (e.g Cocconeis, Achnanthes, Campyloneis) and araphid (e.g Licmophora, Synedra, Fragilaria) diatoms represent the primary encrusting component of epiphytic community. To this component it is also associated a motile, casual component more variable in terms of genera and characterized mainly by biraphid diatoms (e.g. Mastogloia, Navicula, Amphora). Despite to its crucial ecological role in the trophic chain, the diversity of the diatom community epiphytic on P. oceanica leaves and on seagrasses in general is poorly studied and greatly underestimated. Within the epiphytic diatom community of P. oceanica, the genus Cocconeis can be considered one of the most relevant in terms of species diversity and quantitative abundance. In the present work, species and intraspecific taxa belonging to the genus Cocconeis have been identified and described in 19 diatom communities epiphytic on P. oceanica from selected Mediterranean areas. The fraction of Cocconeis specimens within the total diatoms, their species diversity and the relative abundance of each species have been analyzed in each sampled locality. Furthermore, the diversity of Cocconeis assemblages in the different Mediterranean basins have been compared in order to identified potential biogeography pattern in the Mediterranean specie distribution