Most species are found in the Río de la Plata, eastern Amazon (west to Tapajós basin), Tocantins–Araguaia and São Francisco basins in Brazil. A few members of this genus range west into Bolivia, and into Paraguay and Argentina.

    There are 60 species in the genus and more being added regularly as new areas of the Amazon and Sao Francisco River basins are explored.

References:

1.  Costa, W.J.E.M.; P.F. Amorim; R.C. Rizzieri (2016). “Molecular phylogeny and biogeography of the South American savanna killifish genus Melanorivulus (Teleostei: Aplocheilidae)”. Vertebrate Zoology. 66: 267–273. doi:10.3897/vz.66.e31565. S2CID 257198386.
2. Huber, J.H. (2012). “Reappraisal of the Phylogeny of Rivulus and its Allied focused on External Characters”. Killi-Data Series. 2012: 9–25
3. Eschmeyer, William N.; Fricke, Ron & van der Laan, Richard (eds.). “Melanorivulus“. Catalog of Fishes. California Academy of Sciences.

Melanospilus nova species
Photo located at
img.kil.palo-alto.ca.us