Alternative genusname Melanorivulus. This study includes the first phylogenetic analysis for the R. zygonectes species group, a clade of small rivulid killifishes, endemic to an area comprising the transitional zone between the Amazon rain forest and the Cerrado savannah, in central and northeastern Brazil. The phylogenetic analysis integrating 59 morphological characters and DNA sequences highly supports monophyly of the group. Fourteen species are considered valid, including eleven previously described, of which most had their distribution areas and diagnoses revised on the basis of recent new collections, and three new species: R. kayabi from the Rio Tapajós and middle Rio Xingu drainages; R. rubroreticulatus from the lower Rio Xingu drainage; R. modestus from the upper Rio Tapajós drainage; R. megaroni from the upper and middle Rio Xingu drainage; R. salmonicaudus, R. spixi, new species, R. rubromarginatus, R. crixas, R. javaeh, R. karaja, and R. zygonectes, R. petrisecundi, new species, and R. wallacei, new species, from the Araguaia-Tocantins drainage system; and R. parnaibensis from the Rio Parnaíba basin. All clades contained in the R. zygonectes group have at least part of their members occurring in the Araguaia-Tocantins drainage system, a distribution pattern that is interpreted as evidence of a possible past distribution restricted to that area. This study once again supports the importance of using colour pattern characters of live specimens in systematics of aplocheiloid killifishes, since they were useful to diagnose species and sub-clades of the R. zygonectes group, besides increasing clade support when added to molecular data.

Keeping and breeding is easy but a tight fitting cover on the aquarium is necessary because their ability to jump, even through the smallest opening, is unbelievable.

This species can, if there is enough space for them, be housed in a tank with more than one male and several females. If separated before and brought together again however, they can be very aggressive towards each other and fight to become the dominant fish. The best way to produce numbers of fry is to put a pair or a trio in a small tank of about 10 litres with a small filter in it or with some aeration. On the bottom of the tank place some peat moss or dark gravel and a floating "mop" to give space for laying their eggs. The fish will, if they are fed a rich variety of live food, spawn during their whole adult life and produce daily between 5 to 15 eggs.

The eggs should be collected by hand and stored for about 14 days in a small container. Eggs are 1.6 mm and amber coloured. It is wise to add an anti fungus to the water you store the eggs in, to prevent fungal attacks.

After hatching the young fry can eat fresh artemia nauplii without any problem. It takes 4 to 5 months to raise them to maturity. Often the males far out number females. For some species this can become a major problem for the breeder, as he has to raise many fry to enable distribution of a few pairs to other hobbyists. Recent personal tests found that lower pH levels during the time of hatching gave a higher percentage of females. To achieve this I used some peat moss that I added to the container with the eggs just before they were ready to hatch. This peat moss also is a good way to prevent eggs from being attacked by fungus.

Their lifespan can be up to 3 years in captivity.

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