ABSTRACT: Detailed inventory of filtered samples taken across the southern Indian Ocean in May-June 2003 yields quantifiable morphological groundwork for species analysis. Two samples were remarkable by their high content in Discosphaera tubifera, Rhabdosphaera clavigera, R. stylifera and Palusphaera vandelii, providing us with the opportunity to quantify intraspecific variability in these taxa. Despite its extreme geographic and ecologic restriction, this study is the first to provide an integrated description of rhabdoliths and rhabdospheres based on a large number of specimens. We show that the amplitude of variations varies broadly among Rhabdosphaeraceae species and that R. clavigera and R. stylifera are separate taxa. We discuss the implications of our results with regard to species definition, characterization of dimorphism, and inferences of living communities from fossil assemblages. We also report for the first time on malformations in these taxa.