Rhizosolenia setigera was described by T. Brightwell in 1858. Generally accepted synonyms are Rhizosolenia japonica Castracane 1886 and Rhizosolenia hensenii Schuett 1900. Rhizosolenia pungens was described by Cleve-Euler in 1937. Rhizosolenia crassispina Schroeder shares many features with both var. setigera and var. pungens, and may also be synonymous. The resting spores were originally described as Pyxilla baltica Grunow in Van Heurck 1881. According to Hustedt (1930), R. setigera var. kariana Henckel 1925 is also subsumed into var. setigera.  Sundstrom (1986) postulated that R. setigera is not a "true" Rhizosolenia species, a conclusion supported by Hernandez-Becerril (1995), who also maintains separate species identities for R. setigera, R. pungens, and R. crassispina. The inclusion of R. pungens into R. setigera was first mde by Brunel (1970) as Rhizosolenia setigera Brightwell forma pungens (Cleve-Euler) Brunel.   Rhizosolenia setigera and R. pungens have alternately been considered separate species, varieties, or forms. The primary distinctions are the shape of the spine (gradually tapering in setigera; basally thickened, then abruptly hairlike distally in pungens) and the presence of resting spores (setigera only). These differences are clear, but when many cells are present, e.g., bloom conditions, one may easily find cells that are indeterminate between R. setigera and R. pungens, and sometimes cells with setigera spine structure on one theca, and pungens on the other theca can be seen. One may thus question separation at the species level. Whether the ternary names are appropriately 'subspecies', variety' or 'form' becomes a matter of personal conviction. In this discussion, 'variety' is used for convenience. Certainly the ability to form resting spores in some but not others implies a genetic differentiation at some level, as yet to be defined.