Background and aims – After its original description, Cocconeis grata A.W.F.Schmidt and Cocconeis pseudograta Hust. have been rarely quoted and illustrated. Due to the morphological resemblance between both taxa and because of their rare quotations, a certain degree of taxonomic confusion existed between both diatoms. In order to clarify the taxonomic history of these two taxa, we studied their type materials kept at the Hustedt Collection (Bremerhaven, Germany), and specimens of C. pseudograta recently collected in coastal waters of the Tahiti Island (Society Archipelago, southern tropical Pacific Ocean). Methods – Light and electron microscopy observations as well as morphometric data of frustules are presented, and an amended description for each taxon is provided. Key results – In addition to differences in stria and areola density, following frustule features allow to distinguish between both taxa: sternum valves (SV) areolae outline and occurrence of external depressions on the SV of C. grata and granules on the external valve face and marginal row of small processes on the SV mantle of C. pseudograta . Conclusions – Based on the frustule morphology and morphometric data, we conclude that C. grata and C. pseudograta are independent species. While the former can still be considered 'American' in its distribution, the occurrence of C. pseudograta in coastal waters of the Tahiti Island invalids its previous designation as 'European form'.

BENTHIC MARINE DIATOMS; BIOGEOGRAPHY; COCCONEIS; TROPICAL PACIFIC; TYPIFICATION