Chaetoceros Ehrenberg, one of the most abundant and diverse diatom genera, is ecologically relevant due to its worldwide distribution and to the fact that different species may reach bloom abundances. Due to their morphological features (long setae and formation of long chains) or mucilage production, some species can be harmful, causing physiological damage and even death of fish by clogging their gills. In the present study we show a preliminary exploration of the diversity of this genus from planktonic samples obtained at different Catalan coastal zones (NW Mediterranean). The investigation has been based on the establishment of cultures for different Chaetoceros strains and their posterior morphological (light and scanning electron microscopy) and molecular (LSU and SSU rDNA sequences) characterization. Twenty one strains were established and 18 different morphotypes were detected. For most of them morphological and molecular information allowed specific identification. In some cases, the sequences represent the first available molecular information for the morphospecies. The morphotypes detected include four potentially harmful species (C. socialis, C. danicus, C. peruvianus and C. curvisetus). To our knowledge, we report the resting spore of C. tortissimus for the first time. However, the LSU rDNA sequences obtained for this species and for C. costatus did not have enough taxonomic resolution to discriminate them. For some strains disagreements were found among the morphology-based identification and the molecular-based taxonomic assignment retrieved from public databases, highlighting the need to combine detailed morphological and molecular information, as well as the difficulties to identify close Chaetoceros species

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