Taxonomy and distribution of Cocconeis species epiphytic on the leaves of Posidonia oceanica from different Mediterranean meadows

TAXONOMY AND DISTRIBUTION OF COCCONEIS SPECIES EPIPHYTIC ON THE LEAVES OF POSIDONIA OCEANICA FROM DIFFERENT MEDITERRANEAN MEADOWS Mario De Stefano Stazione Zoologica ‘A. Dohrn’, villa Comunale, 80121, Naples (Italy) The microalgal communities epiphytic on the most diffused Mediterranean seagrasses, Posidonia oceanica, Cymodocea nodosa and Zostera noltii, are largely dominated by diatoms, both motile and prostrate species. Cocconeis species dominate the diatom populations in terms of cell density, constituting an almost continuous layer of prostate cells. The highest species diversity has been observed on P. oceanica where almost all the Cocconeis taxa recorded for the Mediterranean Sea may be found. In spite of the important ecological role of the dominant epiphytic diatom species in the functioning of the Posidonia oceanica ecosystem, detailed studies on the taxonomy and distribution of Cocconeis species are still lacking. The main objectives of this study were to describe the Cocconeis populations on P. oceanica at different sampling sites of the Mediterranean Sea in order to identify possible geographical differences, as well as to revise the taxonomy of the different species and infra-specific taxa. P. oceanica leaves were collected by SCUBA diving from eight meadows located in different areas of both the western and eastern Mediterranean sub-basins. At all sampling sites, forty shoots were collected in the springtime when the epiphytic diatom community reached its maximal abundance and diversity. The results obtained so far show that species diversity was high at all the sampled meadows. The same species were present throughout the entire Mediterranean basin and the differences among the different sites were only due to the relative abundance of different species. More than twenty Cocconeis taxa have been recorded so far and their morphology has been studied in detail by electron microscopy. The new peculiar characters observed have prompted us to introduce two new species and three new varieties. Morphological characters of all the species were compared with those described in the taxonomical literature, as well as with the type materials we examined at the Hustedt collection in Bremerhaven.