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Abstract
The morphology of Chaetoceros tenuissimus Meunier, a little-known species, has been studied for the first time with electron microscopy. C. tenuissimus has been placed as a synomyn of C. simplex Ostenfeld since approximately 1930, and has been misidentified or neglected in routine phytoplankton counts. The cells are very small and solitary, square to rectangular in girdle view. The setae are narrow and straight, arising from the two poles of the valve at an angle of 45° to its apical axis. The valve face possesses an annulus, delimited by a hyaline rim. Costae and small areolae radiate from the annulus. A single rimoportula occurs within the annulus. The rimoportula opens externally in a flatened tube and internally by a simple slit. The setae exhibit spirals of minute pores and an intricate pattern of spiral and perpendicular costae; spines are regularly present. Similarities with closely related species in Section Simplicia are discussed.