Margaritum terebro is an estuarine centric diatom recorded along the southeast coast of Brazil, the Atlantic coast of Africa, and in regions close to the Galapagos Islands. Previous works only give a very limited information on the morphology and taxonomy of the genus based on light microscope. In this work, the species is described with scanning electron microscope. Results show a valvar surface with specific structures such as external projections of the rimoportulae bearing a central pore and convex sides. The internal openings are sessile. The term rimoportulae "papilliformis" is proposed for this new structure. The margin of the valve is undulated and presents a ring of sessile rimoportulae and spines irregularly arranged. Comparisions between the monospecific genus Margaritum and other related genera (Podosira and Hyalodiscus), and its systematic positioning are made.
Margaritum; Diatom; Hyalodiscaceae; Taxonomy; Estuarine; Periphyton; Southern Brazil