Taxonomy & naming
Poeciliopsis fasciata was described by the American ichthyologist Seth Eugene Meek in 1904, on the basis of Mexican material; the parenthetical authority — (Meek, 1904) — indicates that the species has since been moved into a genus other than the one in which Meek originally placed it, a routine outcome of the repeated revisions that the livebearer genera have undergone. Eschmeyer's Catalog of Fishes governs the valid name, placing the species in the genus Poeciliopsis within the family Poeciliidae, the New World livebearers, in the subfamily Poeciliinae and the order Cyprinodontiformes.
The genus Poeciliopsis is a large and biologically remarkable assemblage of small Middle American livebearers, several of which are famous in evolutionary biology for unisexual, hybridogenetic and gynogenetic reproductive systems. Poeciliopsis fasciata itself is a sexual, bisexual species, but its placement in this genus situates it among relatives that have become textbook examples of unusual reproductive modes. The specific epithet fasciata refers to the banding of the body, and the species' English name, San Jeronimo livebearer, ties it to its southern-Mexican range; beyond FishBase's listing the species carries little additional vernacular nomenclature.
Morphology
Poeciliopsis fasciata is a small fish, with a maximum recorded length of about 2 in SL — a size in keeping with the modest dimensions typical of the genus. Detailed published descriptions of its colour and form are sparse, but the name fasciata points to the diagnostic feature: a series of dark vertical bars or bands along the flanks, set against the generally subdued, silvery-to-olive ground colour usual in stream-dwelling poeciliids.
As in all members of the family, the species shows the sexual dimorphism characteristic of the livebearers. Males are smaller and bear the gonopodium — a slender intromittent organ formed from modified rays of the anal fin and used to transfer sperm to the female — while females are larger and deeper-bodied, developing the dark gravid patch at the rear of the abdomen when carrying young. Because the species is so rarely kept and little photographed in the hobby, fine-scale descriptions of fin shape and male nuptial coloration are not well documented; the atlas notes this as data sparse rather than guessing at specifics.
Habitat
Poeciliopsis fasciata is endemic to the Pacific slope and adjacent drainages of southern Mexico. Its range runs from Laguna Coyuca near Acapulco, in the state of Guerrero, southward to the Río Pijijiapan in Chiapas, and reaches the Río Coatzacoalcos system in Oaxaca, spanning the lowland streams of Mexico's southern Pacific coast and the isthmian region. FishBase records it as a freshwater and brackish, benthopelagic species, indicating that it occupies both pure freshwater and the slightly saline waters near the coast.
The most distinctive note on its ecology is that it is rheophilic — it prefers rapidly flowing waters, favouring the current-swept reaches of streams rather than still pools. The waters it inhabits are tropical and warm, broadly in the range of 75–86 °F, with neutral-to-alkaline chemistry (pH about 7.0 to 8.0) and moderate hardness (up to roughly 25 dH). This combination — warm, flowing, sometimes brackish water on a tropical coastal slope — defines the habitat to which the species is adapted and which any attempt to keep it would need to reproduce.
Feeding
No dedicated dietary study of Poeciliopsis fasciata was located in the cached sources, and its precise natural diet is therefore not well documented. By the strong general pattern of small stream-dwelling poeciliids, however, the species can be expected to be an opportunistic omnivore and micro-feeder, taking a mix of algae and biofilm, detritus, diatoms, and small aquatic invertebrates and insect larvae drawn from the substrate and the water column of the swift streams it favours.
In the rare event that the fish is maintained in captivity, the husbandry that suits comparable rheophilic livebearers would apply: small, frequent feedings of good-quality flake or micro-pellets supplemented with small live and frozen foods, together with a share of algal or vegetable matter. The atlas flags the absence of species-specific feeding data rather than inventing detail; the guidance here rests on the well-established feeding ecology of the family, not on a documented account of this particular species.
Mating
Reproduction in Poeciliopsis fasciata is internal and viviparous, as in all poeciliids. Fertilisation is achieved when the male transfers sperm to the female by means of the gonopodium, the modified anal fin that is the defining reproductive structure of the family. Males of livebearing toothcarps generally do not build nests or hold territories; instead they pursue females and inseminate them directly, and in many poeciliids females are able to store viable sperm internally for an extended period, allowing successive broods to be produced from a single mating.
The finer details of courtship and mate choice specific to P. fasciata are not documented in the available sources. The genus Poeciliopsis is celebrated in reproductive biology for the unisexual and hybridogenetic systems found in some of its members, but P. fasciata is a conventional sexual species, and the atlas does not extend those exotic mechanisms to it. Where the cache is silent on this species' precise mating behaviour, the article keeps to what the family's shared biology — internal fertilisation via the gonopodium, with likely sperm storage — reliably implies.
Breeding
Poeciliopsis fasciata is a livebearer that gives birth to free-swimming young rather than laying eggs. FishBase records a gestation period of about 28 days, after which the female produces a brood of roughly 10 to 30 young — a moderate clutch size in line with the species' small body. The newborn fry emerge fully formed and able to swim and feed at once, the standard condition for poeciliid young.
As is typical of the family, parental care is essentially absent: the adults provide no protection to the fry and, as in most livebearers, may prey on them, so in captivity dense cover or separation of the young would be needed to raise good numbers. Whether this species shows superfetation — the ability to carry several broods at different stages of development simultaneously, which occurs in a number of Poeciliopsis species — is not confirmed for P. fasciata in the cached sources, and the atlas does not assert it; the documented reproductive facts are the roughly 28-day gestation and the 10-to-30 brood size.
In the aquarium
Poeciliopsis fasciata is an obscure, rarely-kept fish with no substantial body of hobby husbandry behind it. No dedicated American Livebearer Association profile or care article was located for the species, and it does not circulate in the general aquarium trade the way guppies, mollies and platies do; it is encountered, if at all, among specialist livebearer keepers rather than in shops.
What care guidance can be offered follows from its documented natural requirements rather than from kept-fish experience. The species is rheophilic, so it would need warm, well-oxygenated, briskly moving water — tropical temperatures of roughly 75–86 °F, neutral-to-alkaline chemistry around pH 7 to 8, and moderate hardness — and it tolerates slightly brackish conditions. A current-providing filter or powerhead and a mature, stable tank would suit it better than the still, planted setup that suffices for a guppy. Because reliable husbandry detail is sparse, the atlas presents these as conditions inferred from the wild habitat, not as a tested care regime.
Conservation
Poeciliopsis fasciata is assessed as Least Concern on the IUCN Red List (assessment dated 4 December 2018). The species is not listed on CITES or CMS and is not considered threatened at the species level, reflecting a range that spans multiple drainages across the southern Pacific slope of Mexico.
As a small, water-tolerant livebearer of coastal streams, the species' principal long-term vulnerabilities are the general ones facing freshwater fishes of the region — habitat alteration, water abstraction and pollution of the streams it depends on — rather than any specific, documented threat. The cached sources record no targeted conservation concern for the fish beyond its Least Concern listing, and the atlas does not speculate beyond that assessment.