Marine taxa of Cocconeis on leaves of Posidonia oceanica, including a new species and two new varieties

Eight Cocconeis Ehrenberg laxa (species and varieties) epiphytic on the lea ves of Posidonia oceanica (L.) Delile were studied by scanning and transmis sion electron microscopy. Samples of Posidonia oceanica were collected thro ughout the year from seagrass meadows around the island of Ischia (Gulf of Naples, Italy). On the basis of some peculiar ultrastructural characteristi cs of the valve areolation, a new species, Cocconeis multiperforata sp. nov ., and two new varieties, C. neothumensis var. marina var. nov., and C. scu tellum var. posidoniae var. nov., are described. Ultrastructural analysis a lso revealed new morphological characteristics in C. britannica Naegeli in Kutzing and C. maxima (Grunow) Peragallo & Peragallo, and confirmed observa tions of other authors for C. molesta var, crucifera Grunow, C. pellucida G runow in Rabenhorst and C. stauroneiformis (Rabenhorst) Okuno. Our observat ions on the abundance of the different species provide new information on t heir distribution and ecology.