Below you see the grouping of species that were found to be related to Simpsonichthys after re-shuffeling by Costa in 2007 .

Type species of Simpsonichthys is Simpsonichthys  boitonei Carvalho, 1959 in the image to the right. This species from Brazil lacks ventral fins, which was the main reason that A.L. de Carvalho erected the genus.
Nowadays it is generally not accepted that a single character is sufficient grounds to create a genus.

Dr. Wilson Costa (1996) transferred 21 species (which  lacked branched filamentous or extended dorsal and anal fins) from Cynolebias to Simpsonichthys, and made Pterolebias elegans a junior synonym of Simpsonichthys whitei. Thereby Simpsonichthys came to comprise 23 species.

During the next decade, collecting activities continued, and descriptions of new species were published. In 2006 Costa published a revision of Simpsonichthys, creating five subgenera. In 2007, Costa further revised this genus, and distributed 50 species among five subgenera thus: 

Xenurolebias, from the coastal plains of eastern Brazil, includes four species, S. myersi, S. cricariensis, S. izecksohni and S. pataxo.

Ophthalmolebias, from the coastal river basins of eastern and southeastern Brazil, comprises S. bokermanni, S. constanciae, S. ilheusensis, S. perpendicularis, S. rosaceus, S. suzarti and the recently described O. ilheusensis (Costa 2011)

Simpsonichthys, from the central Brazilian plateau, in the upper Paraná, upper Araguaia and upper São Francisco river basins, comprises S. boitonei, S. espinhaensis, S. cholopteryx, S. parallelus, S. nigromaculatus, S. punctulatus, S. santanae, S. zonatus 2012) and S. margaritatus (Costa 2012).

Spectrolebias, from the Middle Tocantins, middle Araguaia, Xingu, Mamoré, and Paraguay river basins, in Brazil, Bolivia and Paraguay, includes S. reticulatus, S. costai, S. chacoensis, S. filamentosus, and S. semiocellatus; and the recently described S. ineaquipinnatus (Costa 2008) and S. brousseaui. (Nielsen 2013)

Hypsolebias, from the São Francisco, Tocantins, Middle Jequitinhonha and lower Jaguaribe river basins and isolated coastal basins of northeastern Brazil, comprises S. ocellatus, S. rufus, S. stellatus, S. nielseni, S. notatus, S. radiosus, S. similis, S. gibberatus, S. virgulatus, S. trilineatus, S.auratus, S.hellneri, S.adornatus, S.fulminantis, S.carlettoi, S.magnificus, S. picturatus, S.antenori, S. macaubensis, S.mediopapillatus, S. ghisolfii, S. igneus, S. flavicaudatus, S.flagellatus, S. janaubensis, S.marginatus, S. delucai, S.alternatus, S. fasciatus, S. multiradiatus, S. flammeus, and S. brunoi, H. guanambi and H. nudiorbitatus. More species followed after 2006, for excample the additional described S. lopesi (Nielsen, Shibatta, Susart & Faour, 2010).

Detailed descriptions and keys to identification for all species of Simpsonichthys are provided.

In his work "Historical Biogeography of Cynolebiasine annual killifishes inferred from dispersal-vicariance analysis, 2010" Costa gave all above subgenera the full genus status which is not followed by Huber and his "Killi Data" website.

Behaviour:

All Simpsonichthys  spp. are, like Cynolebias, typical divers that live an annual life. After displaying its courtship dance, the male stands head-down to the bottom, waiting for the female to join him. If contact is made by the female they dive together to spawn a single egg into the soft bottom layer. Egg incubation time depends on the species, and the seasonal cycle in their natural habitat. This can vary between 2 and 10 months. Often eggs need to be stored at high temperatures, to ensure development and hatching within a reasonable time.