Xiphophorus

Xiphophorus is a genus of euryhaline and freshwater fishes in the family Poeciliidae of order Cyprinodontiformes, native to Mexico and northern Central America. The many Xiphophorus species are all called either platyfish (or platies) or swordtails. The type species is X. hellerii, the green swordtail. Platyfish and swordtails are live-bearers, meaning that they reproduce via internal fertilization. The name Xiphophorus derives from the Greek words ξίφος (dagger) and φόρος (bearer), referring to the gonopodium. All are small fishes, which reach maximum lengths of 3.5–16 cm (1.4–6.3 in) depending on the exact species. The various Xiphophorus species range from the southern Rio Grande basin in Mexico, through eastern drainages in the country (river basins draining into the Gulf of Mexico), to northern Guatemala, Belize and northern Honduras. Three species and their hybrids are common in the aquarium trade: the green swordtail (X. hellerii), the southern platyfish (X. maculatus) and the variable platyfish (X. variatus). These three are the only species that have large native ranges. They have also been introduced outside their native range (both in Mexico, Central America and other continents) where they sometimes represent a serious risk to native species, including other, more localized Xiphophorus. All other species of Xiphophorus are highly localized and mostly endemic to Mexico. Only three of the localized species, the Chiapas swordtail (X. alvarezi), X. mayae and X. signum, range outside Mexico and the last two are the only Xiphophorus species not found in Mexico at all. In many locations there are two sympatric species, but the localized species are mostly (though not entirely) separated from each other, even when they are restricted to the same river basin. This includes three restricted species in the Rio Grande basin (all fully separated), nine restricted species in the Pánuco River basin (mostly separated) and three restricted species in the Coatzacoalcos River basin (mostly separated). IUCN lists the marbled swordtail (X. meyeri) and the northern platyfish (X. gordoni) as Endangered, while the Monterrey platyfish (X. couchianus) is listed as Critically Endangered. In addition to those three, Mexican authorities recognize the yellow swordtail (X. clemenciae) and Catemaco platyfish (Xiphophorus milleri) as threatened. Almost all the Xiphophorus, including the rare species, have captive populations that are maintained as an 'insurance' at breeding centers and by dedicated private aquarists. Xiphophorus species are regularly used in genetic studies, and scientists have developed many interspecific hybrids. The Xiphophorus Genetic stock center, founded by Myron Gordon in 1939, is an important source of these fish for research. In addition, several species are commonly kept by aquarium hobbyists, especially the green swordtail (X. helleri), southern platyfish (X. maculatus), and variable platyfish (X. variatus). In fact, these are a key category in fishkeeping, a group of extremely hardy livebearing fish, along with the molly and guppy, that adapt to almost any water conditions, from warm temperate to tropical, freshwater to fully marine. Unlike some species, these are available almost completely as captive-raised, because of the ease with which they breed. There are currently 28 recognized species in this genus, according to FishBase. Two of these species, X. clemenciae and X. monticolus, are likely the result of natural hybrid speciation (ancestors of both are a platy species and a swordtail species). Two other proposed species, X. kosszanderi and X. roseni, are recognized by FishBase, but not by all other authorities, as the first appears to be a hybrid between X. variatus and X. xiphidium, and the second between X. variatus and X. couchianus (X. kosszanderi and X. roseni have not undergone speciation as in X. clemenciae and X. monticolus). Otherwise hybridization in the wild is uncommon in this genus and only known from three or four locations, despite the fact that many sites have two sympatric species.