Taxonomy & naming
Xenotoca variata was first described by Tarleton Hoffman Bean in 1887, on the basis of material from the central Mexican highlands. It is placed in the family Goodeidae, the splitfins, within the order Cyprinodontiformes; the egg-bearing freshwater goodeids of the Mexican plateau form the subfamily Goodeinae. Eschmeyer's Catalog of Fishes governs the valid name, and the species is carried under the combination Xenotoca variata (Bean, 1887), the parentheses indicating that Bean originally placed it in a different genus.
The genus name Xenotoca is built from the Greek xenos, "strange," and tokos, "birth" — a reference to the unusual reproductive biology of these fishes, which nourish their embryos internally rather than provisioning them with yolk. The species epithet variata derives from the Latin variare, "to vary," and fits a fish whose coloration and patterning differ markedly between populations across its range.
Within the species, genetic work has revealed considerable structure. An analysis by Domínguez-Domínguez and colleagues (2010) recognised five evolutionarily significant units (ESUs) within X. variata; one of these, the broadly distributed lineage informally labelled Xenva1, is widespread and secure, while the more geographically restricted units are comparatively vulnerable. This hidden diversity is one reason the conservation-minded hobby keeps populations separate by collection locality.
Morphology
Xenotoca variata is a small, deep-bodied splitfin. FishBase records a maximum total length of about 2.5 in in males and 3 in in females, with first maturity around 2.5 in — a robust, chunky build typical of the highland goodeids rather than the slim profile of a guppy. The common English name, jeweled splitfin, captures the look of a well-coloured adult: the flanks carry scattered iridescent, jewel-like spots over a darker ground, and dominant males in particular can show a striking sheen.
Coloration is highly variable, which is the trait that gave the fish its epithet. Different populations across central Mexico differ in the intensity and arrangement of the metallic spotting and in body tone, and individual males brighten markedly when in condition and displaying. As in other goodeids, there is no gonopodium of the poeciliid type; instead male goodeids possess a foreshortened, notched front portion of the anal fin (the andropodium or spermatopodium) used in internal fertilisation, a structure that is subtle compared with the long intromittent organ of guppies and mollies.
Sexual dimorphism is present but moderate. Females grow somewhat larger and are heavier-bodied, especially when gravid; males tend to be the more vividly coloured sex and carry the modified anal fin. Beyond these features the sexes are broadly similar in shape, and colour rather than fin development is usually the clearest field cue.
Habitat
The species is endemic to the central highlands of Mexico, between roughly 19° and 21° north, spanning the states of San Luis Potosí, Guanajuato, Jalisco, Michoacán and Zacatecas. Its range is built around the Río Lerma drainage — including the ríos Laja, Guanajuato and Turbio and Laguna Yuriria — together with the Río Angulo drainage (including Lago Zacapu), the Río Grande de Santiago and Laguna de Chapala, the headwaters of the ríos Santa María (Pánuco drainage) and Verde, and isolated springs in the endorheic Río Aguanaval system of Zacatecas.
Within these waters X. variata occupies clear to murky rivers and lakes with moderate to absent current, generally in shallow water less than a metre deep. It is found over substrates ranging from deep mud to rock, and is strongly associated with abundant aquatic vegetation — Potamogeton, Eichhornia, Nasturtium and Myriophyllum, along with filamentous green algae. Goodeid Working Group surveys in 2017 found the fish mainly in shallow creeks, sheltering between rocks and mats of filamentous algae.
Water conditions are warm-temperate to subtropical: FishBase gives a range of roughly 68–81 °F, pH 6.5–7.6, and a wide hardness span of about 5–25 dH. The species is notably tolerant of pollution and habitat modification — along with Goodea atripinnis it persists in degraded reaches where most other goodeids have already disappeared — although it too is in decline, having been lost from the most heavily polluted stretches of the Santiago and Lerma and from lakes and reservoirs invaded by introduced largemouth bass.
Feeding
Xenotoca variata is an omnivore with a leaning toward animal food. Its gut is short — at most about one and a half times the body length — which is consistent with a diet richer in protein than in bulky plant matter; FishBase places it at a low trophic level of about 2.0. In the wild, adults take crustaceans, worms and insect larvae alongside algae and the general aufwuchs of vegetated shallows, shifting with what each habitat offers.
In the aquarium the fish is undemanding and readily accepts prepared foods — flake, granules and tablets — as well as freeze-dried and frozen items such as brine shrimp. A varied diet that includes both meaty foods and an algal or vegetable component keeps colour and condition up, and matches the omnivorous-with-carnivorous-tendency feeding of wild fish. As with most active small livebearers, modest, frequent feedings suit it better than occasional large meals.
Mating
Like all goodeids, Xenotoca variata reproduces by internal fertilisation, but the mechanism differs from that of the poeciliid livebearers. Male goodeids lack the elongate gonopodium of guppies and mollies; instead the first few rays of the anal fin are shortened and separated by a notch to form an andropodium, used to transfer sperm during a brief, close approach to the female rather than through a long intromittent organ. Courtship in the splitfins centres on males displaying to and pursuing females, with the most overt aggression occurring between males of similar size.
Because fertilisation is internal, mating is not a spawning event over a substrate but a direct transfer between two fish, and a receptive female is inseminated to begin a single, defined gestation. Wild reproduction is seasonal: in X. variata it runs at least from February through May, and Goodeid Working Group fieldwork in March 2017 turned up both pregnant females and young of 1 in that had clearly been born in January and February.
Unlike the poeciliids, goodeids are generally not thought to store sperm for long periods or to produce a string of broods from one mating; each gestation typically follows its own insemination. This makes the seasonal pattern of mating and birth more sharply defined than the near-continuous reproduction of a guppy, and it is mirrored in aquarium behaviour, where breeding is tied to temperature cues rather than running year-round.
Breeding
Xenotoca variata is viviparous in the full goodeid sense — a matrotrophic livebearer. After internal fertilisation the embryos develop inside the female and are nourished not by a yolk sac but through trophotaeniae, ribbon-like external extensions of the gut that act as a placenta-like organ, absorbing nutrients directly from the mother. The young are therefore born as comparatively large, well-developed fry, a hallmark that sets goodeids apart from the egg-and-yolk livebearing of the poeciliids; goodeids do not exhibit the superfetation (overlapping broods at different stages) seen in some poeciliid genera.
Reproduction is seasonal and temperature-driven. In the aquarium, breeding resumes in spring at around 68–70 °F and stops once temperatures climb much above 77 °F, echoing the February–May reproductive window observed in the wild. Newborns are large enough to take finely crushed flake and newly hatched brine shrimp from the outset and grow steadily; the March 2017 field records of 1 in young born only weeks earlier give a sense of their early growth.
As with many livebearers, adults will sometimes eat the fry, so cover or separation improves survival, though the larger size of goodeid newborns gives them better odds than the tiny fry of smaller species. Specialist breeders deliberately give the fish a cool winter rest below 68 °F for two to three months, which keeps strains reproductively healthy and primes them for a strong spring spawning season.
In the aquarium
Xenotoca variata is one of the more approachable goodeids and a mainstay of the livebearer-conservation hobby, but it is not a beginner's community fish in the way a guppy is. It is hardy and adaptable, accepting a wide range of hardness and a near-neutral to mildly alkaline pH, and it does well in roomy, well-filtered tanks with good vegetation and some open swimming space. Males can be boisterous and nippy toward one another, so groups with enough females and enough room work best.
The central husbandry requirement is clean, cool, well-oxygenated water. Like most spring- and river-dwelling goodeids the species expects large, frequent water changes — on the order of 60–80% weekly — and it should be kept below about 77 °F rather than at the higher temperatures often used for tropical community fish. Giving the fish a genuine winter rest below 68 °F for two to three months is widely recommended to keep captive strains vigorous; spawning then resumes in spring. The species also does well outdoors in ponds and tubs from spring through autumn wherever daytime water temperatures stay above roughly 59–63 °F.
Because the wild fauna is under such pressure, the hobby treats X. variata as a fish to be kept by locality: populations are maintained separately and not interbred, both to preserve the genetic structure documented within the species and to keep ark populations available should wild stocks fail. Organisations such as the Goodeid Working Group coordinate this kind of maintenance breeding.
Conservation
Xenotoca variata is assessed as Least Concern on the IUCN Red List, in an assessment dated 16 March 2018. That ranking reflects the species' wide distribution across the central Mexican highlands and its unusual tolerance of pollution and habitat modification: where most goodeids have vanished, X. variata — together with Goodea atripinnis — often persists, which keeps it off the threatened categories at the species level.
That headline status, however, masks a fish in retreat. It has been extirpated from the most heavily polluted reaches of the Río Lerma and Río Grande de Santiago, and from lakes and reservoirs where introduced largemouth bass have taken hold. Genetic work showing five evolutionarily significant units within the species means that the secure, broadly distributed lineage effectively carries the Least Concern rating, while several more localised units are considerably more vulnerable — a structure that a single species-level category cannot capture.
The goodeids as a family are among the most imperilled freshwater fishes of Mexico, hit hard by water extraction, pollution, drainage modification and invasive species. For that reason X. variata is a flagship of ex-situ conservation in the aquarium world: kept by locality, bred in coordinated programmes such as the Goodeid Working Group, and held as insurance against further losses in the wild. For the aquarist, the responsible practice is to source captive-bred fish, keep population lines pure, and never release stock into local waters.