Regions with high levels of biodiversity, which mostly correspond to tropical regions, can be treated as treasuries of endemism, even for single-celled life forms. Learning about these areas, and the species present in them, is key to understanding the processes that govern their emergence and sustainability. This paper is a contribution towards revealing the biodiversity of marine benthic diatom species of the world. The samples used in this paper were taken from East Java (Indonesia), Nosy Be (north-west Madagascar) and the Tagus River estuary (Portugal). We describe a new diatom genus, Catenulopsis, and propose the first species of the genus from Nosy Be, named Catenulopsis catenulafalsa. We also describe two new Catenula species, one from Java (Catenula javanica) and the other from Portugal (Catenula brotasiae). The paper also discusses the characteristics of Catenula adhaerens and variations in the species across geographic space. We characterize the morphological variation of C. adhaerens in samples from equatorial West Africa, South Africa, Portugal, Japan, Madagascar and Indonesia.