Taxonomy & naming
Poeciliopsis hnilickai was described by Manfred Meyer and D. Vogel in 1981, in a paper titled "Ein neuer Poeciliopsis aus Chiapas, Mexico (Pisces: Poeciliidae)" published in Senckenbergiana biologica. The type material came from the Grijalva drainage of Chiapas in southern Mexico, and the species is placed in the genus Poeciliopsis within the family Poeciliidae — the New World livebearers, the group that also contains the guppies, mollies, swordtails and mosquitofishes. Eschmeyer's Catalog of Fishes treats Poeciliopsis hnilickai Meyer & Vogel, 1981 as the valid name.
The genus name Poeciliopsis combines the Greek poikilos, "many-coloured" or "varied" — the same root behind the related genus Poecilia — with opsis, "appearance," so the name reads roughly as "resembling Poecilia." The species epithet hnilickai is an eponym honouring Erich Hnilicka of Puebla, a Mexican aquarist, collector and amateur limnologist who was associated with the gathering of the fish. The English common name in general use, "Upper Grijalva livebearer," simply pins the species to its home drainage.
Poeciliopsis is a moderately large genus of small American livebearers distributed mainly through Mexico and Central America, several of whose members are notable in biology for unusual reproductive modes. P. hnilickai itself is an ordinary, sexually reproducing, bisexual species, and within the genus it is one of the less-studied forms — its taxonomy has been stable since description, but detailed published work on the species remains sparse.
Morphology
Poeciliopsis hnilickai is a very small fish, typical of the genus. Males reach only about 1.5 in in total length, while females grow somewhat larger, to roughly 2 in. As in other poeciliids the sexes differ clearly: males are smaller and slimmer and carry the gonopodium — a rod-like intromittent organ formed from modified anal-fin rays — while females are larger, deeper-bodied and, when carrying young, show the dark gravid patch at the rear of the abdomen characteristic of pregnant livebearers.
Like most species of Poeciliopsis, P. hnilickai is a comparatively plain fish rather than a brightly coloured one; the genus as a whole tends toward subdued silvery, olive or grey body tones with limited patterning, in contrast to the gaudy males of the guppy. FishBase records a length–weight relationship for the species (Bayesian estimates a ≈ 0.00955, b ≈ 3.18), consistent with a small, slender-bodied stream fish. Beyond these basic measurements and the standard poeciliid body plan, finely detailed morphological description of the living fish in the popular literature is sparse, and FishBase carries only a drawing rather than a photograph.
Habitat
The species is endemic to the upper Grijalva drainage of Chiapas, in southern Mexico, where it is recorded from the Río Salado (Río de Salina) and associated waters of the Mezcalapa–Grijalva basin. The type locality and verified collections place it in the Río Salado of the Grijalva system. It occupies a range of small freshwater habitats within this basin — rivers, creeks, floodplains and marshes, and standing river pools — the kinds of warm, lowland and mid-elevation waters typical of small Mexican livebearers.
The water it inhabits is warm and close to neutral. FishBase characterises the species as a tropical, benthopelagic freshwater fish living at around 75–81 °F, with a pH of roughly 6.8–7.2 and a hardness span of about 6–18 dH (after Wischnath's Atlas of Livebearers). These figures describe a fish adapted to warm, soft-to-moderately-hard, near-neutral water rather than to extremes of chemistry.
Because the species is confined to one drainage, its fortunes are tied closely to the condition of the upper Grijalva basin. It is reported to be locally abundant where it occurs, but the upper basin is also subject to deforestation and dam construction, pressures that bear on the habitats of all the small endemic fishes of the system.
Feeding
Detailed dietary studies of Poeciliopsis hnilickai specifically are not available in the popular literature, but its feeding ecology can be inferred from its body plan, habitat and close relatives. Small stream-dwelling poeciliids of this kind are generalist micro-omnivores, grazing on algae, diatoms and organic detritus and taking small aquatic invertebrates, insect larvae and other tiny drifting or benthic items as opportunity allows. A fish of warm rivers, creeks and floodplain pools would feed across the surface, water column and substrate margins on whatever small food is locally abundant.
In an aquarium setting the species should be straightforward to feed in the manner of other small livebearers, accepting good-quality flake and micro-pellets alongside small live and frozen foods such as Daphnia, newly hatched brine shrimp and microworm, with some vegetable or algal content benefiting the fish. As with all small, frequently grazing livebearers, several modest feedings suit it better than a single large meal. This account is given as data-sparse: it reflects general poeciliid husbandry rather than published observations of this particular species.
Mating
Poeciliopsis hnilickai reproduces by internal fertilisation, the defining feature of the poeciliid livebearers. The male uses the gonopodium — a slender intromittent organ formed from modified rays of the anal fin — to transfer sperm directly to the female, so that fertilisation and the early development of the young take place inside the female's body rather than in external eggs. The species has been described as polygynandrous in its mating system, meaning both sexes mate with multiple partners rather than forming pairs.
As in livebearers generally, males do not hold territories or build any structure; instead they pursue females and attempt insemination through courtship and, as is common across the family, opportunistic mating attempts. Females of poeciliids commonly retain viable sperm internally for some time after mating, which allows successive broods to be produced from a single insemination, though species-specific data on sperm storage in P. hnilickai are not available. Superfetation — the simultaneous carrying of broods at different developmental stages, which occurs in some Poeciliopsis — has not been documented for this species in the sources at hand, and is noted here only as a genus-level possibility rather than a confirmed trait.
Breeding
Poeciliopsis hnilickai is a viviparous livebearer: after internal fertilisation the female carries the developing embryos and gives birth to free-swimming young rather than laying eggs. FishBase reports a gestation of about 28–32 days and a brood of roughly 10–35 young, with sexual maturity reached at around ten weeks of age — figures that describe a small, fast-maturing fish capable of building up numbers quickly under good conditions, consistent with its rated high biological resilience (population doubling time under fifteen months).
The newborn fry emerge as fully formed miniature fish, able to swim and feed immediately. As with other small livebearers, the young are vulnerable to predation — including from adults of their own kind — so in a breeding aquarium dense planting, floating cover or the separation of gravid females would be the usual means of raising good numbers of fry. Newly born young can be started on infusoria, finely powdered foods and newly hatched brine shrimp, growing on quickly toward maturity. Because this species is rarely kept, much of this breeding guidance is extrapolated from general livebearer practice rather than from documented aquarium spawnings of P. hnilickai itself.
In the aquarium
Poeciliopsis hnilickai is an obscure fish in the hobby, kept mainly by livebearer specialists rather than offered in the general aquarium trade, and there is little published husbandry written specifically for it. What guidance can be offered follows from its biology: a small, warm-water, soft-to-moderately-hard, near-neutral-water livebearer suited to a modestly sized, well-planted aquarium kept in the range of about 75–81 °F with a pH near neutral.
Like most small Poeciliopsis it is best regarded as a peaceful, active species for a group rather than a showpiece, and would do well in a quiet community or species setup with gentle tankmates and plenty of plant cover. Its small size and prolific live-bearing reproduction mean that, as with other livebearers, managing the population becomes the practical husbandry task once a colony is established. A keeper of this species should treat it as a conservation-minded project fish — maintaining a healthy line-bred colony of a localised Mexican endemic — rather than as a casual community addition.
Conservation
Poeciliopsis hnilickai is assessed as Least Concern on the IUCN Red List, in an assessment dated 22 March 2018 (assessor Wilfredo Matamoros; published 2019; assessment e.T191760A2002476). The species is reported to be locally abundant within its range, and no major threats have been confirmed to directly and significantly impact it at present, which underpins the Least Concern listing.
The assessment nonetheless flags the species as potentially threatened by anthropogenic pressures within the Mezcalapa–Grijalva River system, notably deforestation in the upper basin and dam construction. As a fish restricted to a single drainage, P. hnilickai has no refuge outside the upper Grijalva, so degradation of that basin's rivers, creeks and floodplains would bear directly on it even though its current status is secure.
For an aquarist the relevant point is that this is a localised endemic of conservation interest rather than a mass-market fish. Where it is kept, maintaining genetically healthy captive lines and avoiding the release of any livebearer into local waters are the responsible practices — the standard caution for the easily naturalised poeciliids applies here as to all members of the family.