Abstract
The morphology of the genus Chaetoceros, one of the most abundant and diverse planktonic diatom groups, was investigated using material collected in the eastern Adriatic Sea from 2006 to 2012. Twenty-seven morphologically distinct species have been identified from both field samples and cultivated strains. Two species, C. bacteriastroides and C. pseudodichaeta, are reported for the first time for the area. Morphometric data, general morphology and ultrastructural characters are presented for each species, based on light and electron microscopy observations, with a special emphasis on species-specific distinctive features. Valve ultrastructure appeared to be characteristic for single species or group of closely related species, especially inside the subgenus Hyalochaete. A collection of appropriate micrographs will facilitate future identification of Chaetoceros species and morphological comparison with material from other geographic areas.