Xiphophorus is a
genus of
euryhaline and freshwater fishes in the family
Poeciliidae of
orderCyprinodontiformes, native to Mexico and northern Central America. Xiphophorus species can be divided into 3 groups based on their evolutionary relationships: platyfish (or platies), northern swordtails, and southern swordtails. Platyfish formerly were classified in another genus, Platypoecilus, which is now obsolete. The
type species is X. hellerii, the
green swordtail. Like most other new world Poeciliids, platies and swordtails are
live-bearers that use internal fertilization and give birth to live young instead of laying eggs like the bulk of the world's fishes. The name Xiphophorus derives from the
Greek words ξίφος (dagger) and φόρος (bearer), referring to the
gonopodium on the males. All are relatively small fishes, which reach a maximum length of 3.5–16 cm (1.4–6.3 in) depending on the exact species involved.[3]
Distribution and conservation status
The
Monterrey platyfish is one of three Xiphophorus species that is restricted to the southern
Rio Grande basin and threatened
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.[4][5]
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.[4] They have also been
introduced outside their native range (in Mexico, Central America, and other continents) where they sometimes become invasive and outcompete and endanger native species, including other, more localized members of Xiphophorus.[6]
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.[4][5] In many locations there are two or more
sympatric species,[5] but the localized species are mostly (though not entirely) separated from each other, even when they are restricted to the same river basin.[4] 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).[4]
Xiphophorus species are regularly used in
genetic studies, and scientists have developed many interspecific
hybrids,[9]. Xiphophorus have proved a useful model to understand the consequences of hybridization, especially in the context of melanoma research since the 1920s.[10] The Xiphophorus
Genetic stock center, founded by
Myron Gordon in 1939, is an important source of these fish for research.[7]
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 three species comprise one of the most prominent groups of aquarium species, being part of a group of extremely hardy
livebearing fish, alongside the
molly and
guppy, that can adjust to a wide range of conditions within the aquarium. Unlike some species, these are almost always offered as captive bred individuals due to the ease of breeding these livebearers.
In captivity, they will coexist with many other fish species, although in an aquarium with too many males and not enough females, fighting can ensue between males of the same species. They can also easily jump out of an inadequately covered aquarium.[11]
Species and taxonomy
Males of the
Montezuma swordtail have the proportionally longest tail "sword" among the swordtails, but as in all species the females lack itDespite its common name, the
marbled swordtail is part of the platy group based on its geneticsOne of the typical captive forms of the
variable platyfish
There are currently 28 recognized species in this genus, according to
FishBase.[3] 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).[4][5] Two other proposed species, X. kosszanderi and X. roseni, are recognized by FishBase,[3] 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).[4][12] Contemporary hybridization in the wild was thought to be uncommon in this genus[13] until recently.[14][15]
Although traditionally divided into swordtails and platies, this separation is not supported by
phylogenetic studies, which have shown that the swordtails are
paraphyletic compared with the platies. These studies suggest that the genus can be divided into three
monophyletic groups: the northern swordtails (of the
Pánuco River basin, marked with a star* in the list), southern swordtails (southern Mexico to Honduras) and the platies.[4] The common names given to individual species in this genus do not always reflect their actual relationships; for example, the marbled swordtail (X. meyeri) is actually in the platy group based on its genetics while the short-sword platyfish (X. continens) is closer to the swordtails.[4]
Hybridization:Xiphophorus is a leading animal model systems in the study of
hybridization between species. In particular, studies in Xiphophorus provided some early evidence that recombination controlled patterns of ancestry in hybrid genomes.[16] An interesting consequence of hybridization is the origin of spontaneous melanoma in hybrids.[17] For example, in the well-studied Gordon-Kosswig cross between
X. maculatus and
X. hellerii, hybrids develop spontaneous melanoma due to an interaction between the
oncogene xmrk and a repressor locus on a distinct chromosome.[18]
Pigmentation:Xiphophorus pigmentation has been investigated since the 1920s using classical genetics techniques such as crosses.[19] As a result, inheritance patterns of many traits have been described. More recently, researchers have identified specific genes and even mutations causing pigment pattern variation (for instance, a spot allowing males to mimic females[20]) and investigated selective mechanisms on these traits.
Sexual selection: Researchers since
Darwin have studied Xiphophorus species for their sexually selected ornaments (e.g. caudal fin "swords").[21] More recent studies have focused on male ornamentation, courtship strategies, and female mate choice.[22]
^
abcCeballos, G.; E.D. Pardo; L.M Estévez; H.E. Pérez, eds. (2016). Los peces dulceacuícolas de México en peligro de extinción. Fondo de Cultura Económic. pp. 48, 279–284.
ISBN978-607-16-4087-1.
^
abcMeyer M (1983). "Xiphophorus-Hybriden aus Nord-Mexiko, mit einer Revision der Taxa X. kosszanderi und IIIX. roseni". Zoologische Abhendlungen Steatliche Naturhistorische Sommlungen Dresden. 38: 258–291.
^Kallman, K.D.; S. Kazianis (2006). "The genus Xiphophorus in Mexico and Central America". Zebrafish. 3 (3): 271–285.
doi:
10.1089/zeb.2006.3.271.
PMID18377209.