Xenotoca is a genus of fish in the family Goodeidae from Mexico, where various species are found in a wide range of habitats, from rivers and creeks to pools and lakes, in the Lerma–Grande de Santiago, Panuco, Cuitzeo and other basins of the Mesa Central. While no goodeid is a very common aquarium fish, the redtail splitfin (X. eiseni), is one of the most common aquarium goodeids; its bright colors offset its reputation for being aggressive towards and occasionally even killing tankmates.[1] Similarly to X. eiseni, two species described in 2016 have males with red-orange tails, but this feature is not shared by the remaining members of the genus.[2] All Xenotoca species are relatively small, reaching up to in standard length.[3]
There are currently five recognized species in this genus,[2] but two possibly undescribed species, tentatively referred to as Xenotoca cf. melanosoma and Xenotoca cf. variata, are known.[4] [5]
Genetic work has shown that the genus, as currently defined, is not monophyletic: The type species X. variata is distantly related to the remaining, which could eventually be reallocated to their own genus.[2] Phylogenic research by Shane Webb showed that the closest relative of X. variata is the butterfly goodeid, Ameca splendens.[6]