Ceratium rhomvoides Hickel (Dinophyceae): Morphology, taxonomic implications, and expanded distribution

Ceratium rhomvoides Hickel is a freshwater photosynthetic dinoflagellate characterised by a robust theca having a blunt-tipped apical horn, one sharply pointed antapical horn and a short, but sharply pointed, postcingular horn. Its Kofoidian plate formula is 4', 6'', 6c, 6''', 2'''', with the fourth apical plate shortened, not reaching the apex. This species was found in freshwater samples from the Limpopo Province (South Africa). Although the species is reported frequently in the rest of the world, it was found only once in Africa, dating from more than a century ago. This study provides a detailed morphological and taxonomical description, distinguishing C. rhomvoides from closely related species, and correcting several historical taxonomical inaccuracies. High-resolution light and scanning electron microscope images of the species are presented, together with an updated record of its geographical distribution. The paper also presents the first record of its presence in southern Africa.