Taxonomy & naming
Girardinichthys viviparus was first described by Miguel Bustamante y Septién in 1837, in the Mexican periodical El Mosaico Mexicano, under the name Cyprinus viviparus — a placement in the carps that reflected how little the early naturalists understood of the fish beyond its startling reproductive habit. The epithet was deliberate: livebearing had been observed in this species (the Enlightenment scholar José Antonio Alzate had documented it around 1769), and Bustamante named the fish for the trait, making it the first cyprinodontiform ever recognised as viviparous. The species later passed through several names — Girardinichthys innominatus Bleeker, 1860; Limnurgus variegatus Günther, 1866; Characodon geddesi Regan, 1904 — before settling into the genus Girardinichthys, which Bleeker erected in 1860.
The genus name honours Charles Frédéric Girard (1822–1895), the Franco-American ichthyologist who worked as assistant to Louis Agassiz, combined with the Greek ichthys, "fish." Eschmeyer's Catalog of Fishes treats Girardinichthys viviparus (Bustamante, 1837) as the valid name, the parentheses recording the move out of the original genus Cyprinus. The species sits in the family Goodeidae, subfamily Goodeinae — the splitfins — a radiation of livebearing toothcarps endemic to the highlands of central Mexico and distinguished from the New World poeciliids by a different mode of internal gestation and a different male copulatory structure.
Goodeids are not poeciliids, and the distinction matters for understanding this fish. Where guppies and their relatives nourish their broods chiefly from yolk, goodeines feed their developing young through trophotaeniae — ribbon-like placental appendages — and the male's intromittent organ is not a true gonopodium but a spermatopodium, formed from a notched and shortened front section of the anal fin rather than a fully fused tube. Girardinichthys viviparus is the type species of its genus and forms no recognised subpopulations or evolutionarily significant units.
Morphology
The Chapultepec splitfin is a small, rather plain fish for most of the year. Maximum recorded size is about 2.5 in total length in males, with females reaching roughly 2 in; standard length tops out near 2.5 in (Miller et al., 2005). The body is moderately deep and laterally compressed, the head bearing a small, near-vertical mouth cleft set with small pointed teeth — the apparatus of a surface-feeding micro-carnivore rather than a grazer.
The ground colour is an unassuming grey to silvery grey, and at rest the sexes are not dramatically different in pattern. Females keep this grey-silver tone year-round and develop a gravidity spot at the rear of the abdomen when carrying young. The drama is reserved for breeding males, which transform: in courtship a male turns a uniform deep black over the whole body, throwing his enlarged dorsal and anal fins into stark relief against the pale female.
The diagnostic male feature is the goodeine spermatopodium — the first roughly seven rays of the anal fin are shortened and separated by a notch, forming a structure that delivers sperm internally. Combined with the larger median fins, this gives mature males a noticeably different silhouette from the females even outside the breeding flush, and is the surest way to sex the species in the tank.
Habitat
Girardinichthys viviparus is endemic to the endorheic Valley of Mexico — the closed high-altitude basin that once held the great interconnected lake system of Texcoco, Xochimilco, Xaltocan, Zumpango and Chalco. It is the only goodeid native to Mexico City itself, and historically it was exceedingly common throughout these waters, abundant enough to support a small market fishery. The draining of the basin's lakes over centuries, accelerated by the explosive growth of the modern megacity, has shattered that range. The species now persists in remnants: its stronghold is the artificial Lago Mayor in the Bosque de Chapultepec in Mexico City, with smaller numbers surviving in Xochimilco and Zumpango, plus an introduced population discovered in 2008 in Laguna Tecocomulco, in the neighbouring Pánuco basin.
The fish favours quiet water — the still margins of lakes, ponds, canals and ditches — over muddy bottoms, in water that ranges from clear to murky and is typically shallow, on the order of 1–2 ft deep. Dense aquatic vegetation is characteristic of its haunts: green algae, Potamogeton, water hyacinth, Lemna and Chara provide cover and foraging structure.
Water chemistry reflects the alkaline, mineral-rich character of the basin. Recorded pH runs from about 7.1 to 8.7 in the Chapultepec ponds and nearer neutral elsewhere, with conductivity from roughly 310 to 1600 µS/cm. The species tolerates surprisingly low dissolved oxygen — below 7 mg/L — and a wide annual temperature swing, from around 77 °F in summer down to under 59 °F in the highland winter, a seasonality that turns out to govern its breeding.
Feeding
Girardinichthys viviparus is a carnivore, and its anatomy advertises the fact. The intestine is short and uncoiled — only about as long as the body — which is the gut of an animal-eater rather than a plant-processing grazer, and the small pointed teeth in a near-vertical, upturned mouth are built for seizing prey at or near the surface. FishBase places the species at a trophic level of about 2.2, consistent with a diet dominated by small animal prey.
In the wild it takes mosquito and other insect larvae, copepods and small invertebrates, working the surface film and the upper water column where its mouth orientation gives it an advantage. The dense vegetation of its habitats concentrates this invertebrate prey, and the fish hunts among the plants and along the open margins.
In captivity the splitfin accepts a range of small live, frozen and prepared foods — Daphnia, brine shrimp, small bloodworm, copepods and good micro-foods — and does best on a varied, protein-rich menu that respects its carnivorous bent. As with most goodeids, a clean, well-oxygenated tank and frequent feeding of modest amounts suit it better than heavy single meals.
Mating
Reproduction in Girardinichthys viviparus is internal, but the mechanics differ from the familiar poeciliid pattern. The male does not carry a true tubular gonopodium; instead he uses a spermatopodium — the notched, shortened front rays of the anal fin — to transfer sperm to the female. Fertilisation is internal, and the female then gestates the brood within her body rather than laying eggs.
Courtship is visually driven and seasonal. As the breeding season opens, mature males abandon their everyday grey for a uniform jet black and display before the grey-silver females with their enlarged dorsal and anal fins spread, the colour change serving as the central signal of male readiness and quality. The black nuptial dress is energetically costly and temporary, fading outside the breeding flush.
Unlike many poeciliids, goodeids are not known to store sperm for long periods to fuel a string of broods from a single mating, and superfetation — the carrying of overlapping broods at different developmental stages — is not a documented feature of this species; broods are produced in discrete cycles tied to the warm season. Each reproductive bout therefore requires fresh courtship and mating, making the male's seasonal black display a recurring fixture of the species' biology rather than a one-time event.
Breeding
Girardinichthys viviparus is a true viviparous livebearer that nourishes its young internally during a gestation of roughly 55 days, after which the female delivers about 25 fully formed, free-swimming young. The developing embryos are fed through trophotaeniae, the placenta-like trophic ribbons characteristic of goodeines, so the brood draws on the mother well beyond the initial yolk supply — a more invested form of livebearing than the largely yolk-fed broods of guppies and mollies.
Breeding is tightly governed by temperature and so by season. In its highland home the species breeds through most of the year except the coldest months; newly born young, 0.5 in long, are collected chiefly from February through May, with the first fry sometimes appearing as early as January and reproduction continuing into about October. The winter chill of the Valley of Mexico imposes a natural rest, and this seasonality is reproduced deliberately by aquarists.
In the tank the species spawns at around 63–64 °F and ceases reproduction above roughly 75 °F, so a cool winter period — dropping the fish below 64 °F, even down toward 50 °F, for two or three months — is used to halt fry production and give the breeders a rest before the next cycle. The relatively small, well-developed broods are easy to rear on small live and prepared foods, and the chief husbandry demands are clean, cool, well-oxygenated water rather than any difficulty in the birth itself.
In the aquarium
The Chapultepec splitfin is a specialist's livebearer rather than a beginner's community fish, and its requirements follow directly from its highland-lake origins. It wants cool, hard, alkaline, oxygen-rich water and is intolerant of the warm, stale conditions a guppy will shrug off. A tank of at least about 20 US gal is recommended, and notably one that is long and low — a height of around 10 in — to maximise surface area for gas exchange, with moderate current and high dissolved oxygen, ideally 8 mg/L or above.
Water management is the heart of keeping it well: large weekly water changes on the order of 60 to 80 percent keep the conditions the fish needs, since goodeids are sensitive to accumulated waste. Crucially, the species requires a genuine cool winter rest — temperatures below 64 °F, even down toward 50 °F, for two to three months — both to mimic its natural cycle and to halt continuous fry production; kept perpetually warm it will neither thrive nor breed properly. In suitable climates it can be kept outdoors from spring through autumn, which is in fact one of the better ways to provide the seasonal temperature swing it expects.
As a conservation-sensitive endemic maintained by the Goodeid Working Group and dedicated hobbyists, Girardinichthys viviparus carries an extra dimension beyond ordinary fishkeeping: captive populations function as a hedge against losses in the wild. Aquarists who take it on are, in a real sense, helping to bank a species whose natural habitat has been all but obliterated.
Conservation
Girardinichthys viviparus is assessed as Endangered (EN) on the IUCN Red List, under criteria B1ab(i,ii,iii,iv)+2ab(i,ii,iii,iv), in an assessment dated 12 April 2018. The listing reflects a severely restricted and declining range within the Valley of Mexico, driven by the draining of the basin's historic wetlands, the relentless urban expansion of Mexico City, pollution of the surviving waters, and the impact of introduced species. Mexico's national NOM-059-SEMARNAT-2010 likewise classifies it as P — en peligro de extinción, in danger of extinction.
The specific threats are clear and ongoing. Wetland drainage and pollution have erased most of the lake system the species once filled; non-native fishes, especially common carp (Cyprinus carpio) and tilapia, compete with and prey on it; and introduced parasites such as the anchor worm Lernaea and the Asian tapeworm Bothriocephalus acheilognathi add further pressure. A 2019 status review by Lyons and colleagues judged the population endangered but, at least at its strongholds, stable rather than still crashing.
That fragile stability rests largely on a few protected and artificial refuges — above all the Lago Mayor in Chapultepec park — together with ex-situ populations maintained by conservation breeders. For a fish that was once among the commonest animals of the Mexican high plateau and that lent science the very word for livebearing, its survival now hinging on a park lake and a network of aquarists is a stark measure of how completely the Valley of Mexico has been transformed.