Three Cocconeis Ehrenberg species, epiphytic on the leaves of Posidonia oceanica (L.) Delile, were studied by light and electron microscopy. A new species, C. pseudonotata, is described which is closely related to C. notata Petit and C. diruptoides Hustedt. Gross morphology and the presence of a sigmoid raphe and sternum are the characters shared by the three species, whereas the central areas of sternum and raphe-sternum valves, and the stria and areola densities are the main distinctive characters for the species when observed by light microscopy. Scanning and transmission electron microscopic observations revealed that the fine structure of areolae must be also considered in order to identify the species. Comparison among the three species has been carried out by observations on material from our samples, as well as on the type material of C. diruptoides and cleaned samples from the Hustedt collection. The study also provides new information on the geographical distribution of these species.

Eight Cocconeis Ehrenberg taxa (species and varieties) epiphytic on the leaves of Posidonia oceanica (L.) Delile were studied by scanning and transmission electron microscopy. Samples of Posidonia oceanica were collected throughout the year from seagrass meadows around the island of Ischia (Gulf of Naples, Italy). On the basis of some peculiar ultrastructural characteristics of the valve areolation, a new species, Cocconeis multiperforata sp. nov., and two new varieties, C. neothumensis var. marina var. nov., and C. scutellum var. posidoniae var. nov., are described. Ultrastructural analysis also revealed new morphological characteristics in C. britannica Naegeli in Kützing and C. maxima (Grunow) Peragallo & Peragallo, and confirmed observations of other authors for C. molesta var. crucifera Grunow, C. pellucida Grunow in Rabenhorst and C. stauroneiformis (Rabenhorst) Okuno. Our observations on the abundance of the different species provide new information on their distribution and ecology.