Simpsonichthys nielseni is a species of killifish in the family Rivulidae.Costa, W.J.E.M., 2005. Simpsonichthys nielseni sp. n. (Teleostei: Cyprinodontiformes: Rivulidae): a new annual killifish from the São Francisco River basin, Brazil. Zootaxa 1039:57-64. It is found in the São Francisco River basin, Brazil.
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