Taxonomy & naming
Xiphophorus nezahualcoyotl was described in 1990 by Rauchenberger, Kallman and Morizot as part of a broader revision of the northern swordtails of the genus Xiphophorus. The genus, which today contains roughly two dozen described species of platyfishes and swordtails, is one of the most intensively studied groups of fishes in evolutionary genetics, and the northern swordtails of eastern Mexico — of which this species is one — have been central to that work. Eschmeyer's Catalog of Fishes treats Xiphophorus nezahualcoyotl Rauchenberger, Kallman & Morizot, 1990 as the valid name, placing it in the family Poeciliidae, subfamily Poeciliinae.
The specific epithet honours Nezahualcóyotl (1402–1472), the poet, philosopher and ruler of the Texcocan people of central Mexico — a fitting eponym for a fish endemic to Mexican highland waters. The genus name Xiphophorus is built from the Greek xiphos, a sword, and pherein, to carry, in reference to the elongated lower lobe of the male caudal fin, the "sword" for which the swordtails are named. In the hobby the species circulates under the common names Mountain swordtail, Northern Mountain Swordtail and the affectionate "Nezzie Sword," the last a nod to its occurrence at higher elevations than most of its relatives.
Morphology
The Mountain swordtail is a small, comparatively plain swordtail. FishBase records a maximum total length of about 2 in for males and unsexed fish and about 2.5 in for females, following Wischnath's Atlas of Livebearers; hobbyist accounts give males of roughly 3.5 inches including the sword and females around 2.5 inches, with sexual maturity reached at about 2 inches. The body is tan, green or grey, with each scale finely edged in black, giving a subtly reticulated look, and the flanks carry six to seven vertical bars that fade or intensify with the fish's mood.
Sexual dimorphism is pronounced, as in all swordtails. Males bear the diagnostic sword — an elongated, yellow-orange extension of the lower caudal-fin rays edged in black — together with a greenish dorsal fin, and they are altogether the more colourful sex. Females are drabber, deeper-bodied and lack both the sword and, typically, the strong vertical barring. Males also carry the gonopodium, the rod-like intromittent organ formed from modified anal-fin rays that is the structural hallmark of the male poeciliid. This is a wild-type swordtail with no fancy aquarium strains; what reaches keepers is essentially the fish as it occurs in the Mexican highlands.
Habitat
Xiphophorus nezahualcoyotl is endemic to north-eastern Mexico, where it is restricted to the Río Tamasí (Tamasopo) drainage in the state of San Luis Potosí, at roughly 21–23° N and 98–100° W. It is a high-altitude fish: FishBase describes it inhabiting fast-flowing creeks in mountainous terrain up to about 1,656 ft elevation, and hobbyist field notes place it in shallow streams of moderate current, closely associated with algae-covered rocks.
The water is cool to temperate by tropical-livebearer standards. FishBase gives a temperature floor of about 66 °F and above and a benthopelagic, freshwater habit, with pH around neutral and into the alkaline range; livebearer keepers report the species doing well in neutral to hard water at roughly 68–73 °F. This is an upland, well-oxygenated, current-loving fish rather than a still-water lowland species — a profile consistent with its restricted highland range and a reminder that, beyond the broad strokes recorded here, fine-scale habitat data for the species are sparse.
Feeding
In the wild the Mountain swordtail is principally an algal grazer, taking advantage of the algae-covered rocks of its upland streams, supplemented by the small invertebrates and detritus that such grazing fishes inevitably ingest. FishBase places it at a trophic level of about 3.1, consistent with an omnivore that leans heavily on plant and algal matter but is not strictly herbivorous.
In the aquarium the species is unfussy and accepts the standard foods of small livebearers — good-quality flake and small pellets of around a millimetre or less — and benefits from a vegetable or algal component that reflects its natural grazing habit. As with most small, near-constant feeders, several modest feedings suit it better than one heavy meal, and a varied diet improves both colour and breeding condition.
Mating
Reproduction in Xiphophorus nezahualcoyotl is internal, as in all poeciliids. The male transfers sperm to the female using the gonopodium, the intromittent organ formed from modified rays of the anal fin; fertilisation occurs inside the female, and there is no egg-laying. Males pursue and court females, displaying the sword and what colour they have, and in the confines of an aquarium the chasing can be persistent enough that keepers recommend providing hiding places and cover for the females.
As is typical of the family, females are able to store viable sperm internally and can produce successive broods from a single mating, so a female may continue to drop young after being separated from males. Detailed studies of mate choice and male competition specific to this species are limited in the popular literature, but the broad pattern — male display of the sword, female choice, and sperm storage enabling serial broods — follows the well-documented biology of the northern swordtails as a group.
Breeding
The Mountain swordtail is a viviparous livebearer: after fertilisation the female carries the developing embryos internally and gives birth to free-swimming young rather than laying eggs. FishBase reports an average gestation of about 24 days, after which the female drops some 20 to 40 young, though hobbyist accounts of aquarium broods can be considerably smaller — one documented brood was about a dozen fry — reflecting female size, condition and captive circumstances.
Like many swordtails, the species readily eats its own fry, so successful rearing depends on heavy floating cover or the use of a breeding trap to separate and protect the newborns. Keepers report holding females gravid and raising fry on newly hatched brine shrimp, with the young growing steadily under those conditions. There is no evidence of superfetation — the simultaneous carrying of broods at different developmental stages seen in some poeciliids such as Heterandria and Poeciliopsis — being a notable feature of this species; on present information it breeds in the conventional single-brood-at-a-time manner of the swordtails.
In the aquarium
Xiphophorus nezahualcoyotl is regarded by livebearer specialists as not difficult to keep, but it is essentially a hobbyist's fish: a wild-type swordtail maintained by enthusiasts and livebearer associations rather than a pet-shop species, and one unlikely to be encountered in the general trade. It suits neutral to hard water and relatively cool temperatures of around 68–74 °F (68–73 °F), in keeping with its upland origins, and it appreciates the well-oxygenated, gently flowing conditions of its native creeks.
The main husbandry points are social rather than chemical. Males chase females persistently, so the aquarium should offer ample hiding places and cover — dense or floating planting works well — to give pursued females refuge, and stocking that skews toward females helps spread male attention. Because adults eat fry, anyone hoping to raise young needs to plan for cover or separation from the outset. Kept in clean, stable, cool, hard water with cover and a varied, partly vegetable diet, the Mountain swordtail is a rewarding and undemanding species for the keeper who can find it.
Conservation
Xiphophorus nezahualcoyotl is listed as Data Deficient (DD) on the IUCN Red List, assessed in 2018 (species ID 191789). The Data Deficient category means that the available information was judged insufficient to make a direct or indirect assessment of the species' risk of extinction — it is an acknowledgement of uncertainty rather than a statement that the fish is secure or threatened.
That uncertainty is consequential for a narrowly distributed endemic. The species is restricted to a single highland drainage, the Río Tamasí system of San Luis Potosí, and small-range mountain fishes of this kind are inherently vulnerable to localised pressures on their streams. Neither CITES nor CMS lists the species. Until a fuller assessment of its population and habitat is carried out, its true conservation status remains open, and continued maintenance by livebearer hobbyists and associations serves as a modest insurance against loss of the wild form.