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Abstract “Surf diatoms” can form high biomass in the surf zone of sandy beaches around the world,AsterionellopsisandAnaulusbeing the main genera of this group in Brazil.Asterionellopsis glacialiswas considered a cosmopolitan species, and taxonomic studies using molecular and morphological tools showed thatA. glacialisis a complex with cryptic and semicryptic species. So, it would be plausible to suppose thatAnaulus australis, another surf zone patch-forming diatom with wide latitudinal occurrence could also be part of a species complex. We collected and identifiedAnaulusandAsterionellopsisstrains from tropical, subtropical and warm temperate sandy beaches on the east coast of South America (Brazil) based on genetic divergence, phylogeny, single-locus automated species delimitation methods (both genera), and frustule ultrastructure (Anaulus).AnaulusandAsterionellopsisshowed contrasting diversity patterns and spatial distribution: a single species ofAnaulus australiswas registered in tropical and subtropical beaches, while at least three species ofAsterionellopsis:A. tropicalis,A. thurstoniiandA. guyunusaewere observed at different latitudes, indicating thatAsterionellopsisspecies have distinct ecological requirements.Asterionellopsis thurstoniiwas previously reported in Europe and it is documented here for the first time in the Southern Hemisphere. The different diversity patterns between these two surf diatom genera suggest that they are likely to have distinct ages, and dispersion and/or speciation processes.