Cocconeis intermedia Peragallo & Heribaud (Bacillariophyta): new information on morphology, distribution and ecology of a poorly known diatom species

Cocconeis intermedia was originally described by Peragallo & Heribaud in Heribaud (1893) as a fossil diatom species from samples collected in the Puy-de-Dome department, Auvergne-Rhone-Alpes region (France). After the first brief description, the species was mostly mentioned as C. placentula var. intermedia in classic floristic books and, over time, it was reported living in a wide variety of aquatic environments. Despite this, the morphological features of C. intermedia remained poorly known: detailed information and micrographs are not available in the literature and the fine structure was never investigated. A Cocconeis species found in the thermomineral spring San Saturnino in Sardinia (Italy) was identified as Cocconeis intermedia Peragallo & Heribaud after an extensive literature search and a comparison with specimens from slides of Tempere & Peragallo collection housed in the Natural History Museum (London, United Kingdom). In this study we provide an amended description of the species with new morphological details as observed in light and scanning electron microscopy. We also provide additional information on the seasonal distribution and occurrence of the species on a variety of substrata in its habitat in Sardinia.