Cocconeis Ehrenberg (Bacillariophyta), a genus dominating diatom communities associated with Posidonia oceanica Delile (monocotyledons) in the Mediterranean Sea

A fine-scale analysis of diatoms associated with Posidonia oceanica in selected locations was carried out along a longitudinal gradient in the Mediterranean Sea (00°21'W-27°07'E) to investigate the composition and diversity of epipthytic communities and to determine the distribution and relative importance of their components with special reference to the most common and abundant genus, Cocconeis. Posidonia oceanica was sampled twice (once per year for two years) at 19 sites. At each site, forty samples of P. oceanica shoots separated by several metres were taken. Epiphytic diatom communities were studied with SEM (scanning electron microscopy) and TEM (transmission electron microscopy). Fifty-two diatom genera were identified. In every studied case, adnate Cocconeis spp. cells dominated the community. Among 32 Cocconeis species and varieties found, recently described C. neothumensis var. marina and C. scutellum var. posidoniae were almost always the most numerous. The structure of Cocconeis assemblages changed across the various geographic regions both in terms of the species' presence-absence and their relative abundance. In general, the most abundant species appeared to be broadly distributed, while less abundant species had a more distinct biogeographic pattern. In the course of this study, several Cocconeis species described as Indo-Pacific were found in the Mediterranean Sea for the first time. Potential relationships between different Cocconeis species and genetically distinct P. oceanica populations are discussed. © 2013 Elsevier B.V.