Taxonomy & naming
Poeciliopsis catemaco was described by the American ichthyologist Robert Rush Miller in 1975, in his paper "Five new species of Mexican Poeciliid fishes of the genera Poecilia, Gambusia and Poeciliopsis," published in the Occasional Papers of the Museum of Zoology at the University of Michigan. The specific epithet refers directly to the type locality, Laguna de Catemaco in Veracruz, the lake to which the species is endemic. Eschmeyer's Catalog of Fishes and ITIS both treat Poeciliopsis catemaco Miller, 1975 as the valid name, and the species carries the common names Catemaco livebearer in English and guatopote blanco in Spanish.
The genus Poeciliopsis was erected by Regan in 1913 and contains the desert topminnows and their relatives, a group of small live-bearing toothcarps distributed from the south-western United States through Mexico and into Central America. The genus sits within the subfamily Poeciliinae of the family Poeciliidae — the New World livebearers — alongside the guppies, mollies, swordtails, platies and mosquitofishes. Several Poeciliopsis are well known to biologists for their unusual reproductive systems, but P. catemaco is a straightforward sexually reproducing, internally fertilising species with no documented hybrid or all-female complexes.
Morphology
Poeciliopsis catemaco is a small fish with marked sexual size dimorphism, the females being much larger than the males — a pattern typical of the genus. FishBase records a maximum length of about 1.5 in total length for males, while females reach roughly 3 in, so a mature female may be twice the length of an adult male. The body is the slender, slightly compressed shape characteristic of Poeciliopsis, built for life in open lake water and its inlets.
As in all poeciliids, the most diagnostic feature is the male's gonopodium — a slender intromittent organ formed from modified rays of the anal fin and used to transfer sperm to the female internally. Females lack this structure and present the plainer, deeper-bodied form, developing a darkened gravid region when carrying young. Detailed pigmentation and meristic notes for this rarely-photographed endemic are data sparse in the hobby literature; the Spanish name guatopote blanco ("white topminnow") points to a generally pale, unshowy fish rather than the bright patterning seen in many of its poeciliid relatives.
Habitat
Poeciliopsis catemaco is a single-lake endemic. It is restricted to Laguna de Catemaco, a permanent freshwater lake at the centre of the Sierra de Los Tuxtlas in south-central Veracruz, east-central Mexico, and it also occurs in one of the lake's inlets. The IUCN places its extent of occurrence at roughly 155 mi², essentially the lake and its immediate catchment, distributed across about three locations. The laguna drains southward via the Rio Grande de Catemaco, and its water level is now controlled by several dams that replaced the historic waterfalls on the outflow.
The habitat is warm tropical freshwater. FishBase gives a temperature range of about 75–82 °F, a pH of 7.0 to 7.8, and water hardness of around 10 to 25 dH — neutral to mildly alkaline, moderately hard lake water. As a benthopelagic species it ranges through the open water column and near the bottom of this large volcanic lake. The Sierra de Los Tuxtlas is a region of high endemism, and Laguna de Catemaco supports a distinctive fish assemblage of which this guatopote is one of the more localised members.
Feeding
No dedicated dietary study for Poeciliopsis catemaco is available in the hobby or popular literature, so its feeding ecology is inferred from its family and habitat rather than from species-specific observation — this is genuinely data sparse. Like other small lake-dwelling poeciliids, it is best understood as an opportunistic omnivore and micro-predator, taking algae, detritus and aufwuchs together with small aquatic invertebrates, insect larvae and other small drifting items from the water column and surface.
In the rare event it is kept in captivity, a Poeciliopsis of this size would be expected to accept the standard livebearer fare: good-quality flake and micro-pellets supplemented with small live and frozen foods such as Daphnia, brine shrimp and finely chopped bloodworm, with a share of algal or vegetable matter. As with most small, near-constant grazers, several modest feedings suit it better than one large meal.
Mating
Reproduction in Poeciliopsis catemaco is internal, the defining trait of the family Poeciliidae. The male transfers sperm to the female using the gonopodium, the rod-like organ formed from the modified anterior rays of the anal fin; fertilisation occurs inside the female's body rather than among scattered eggs. Males of small poeciliids generally pursue females persistently and may combine brief courtship with sneak mating attempts, though the specific courtship behaviour of this little-studied endemic has not been documented in detail.
Unlike some of its congeners — Poeciliopsis is famous in biology for species with all-female unisexual-hybrid lineages and for forms showing superfetation, the simultaneous carrying of broods at different developmental stages — P. catemaco shows no documented superfetation and is a conventional, sexually reproducing, lecithotrophic livebearer. The young are nourished by the yolk of the egg rather than through a placental connection, and there is no evidence of the matrotrophic provisioning seen in some other members of the genus.
Breeding
Poeciliopsis catemaco is viviparous: 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 days, after which the female delivers a brood of roughly 20 to 50 young — a substantial output for a small fish, and consistent with the high resilience the species shows (a population doubling time of under fifteen months).
The newborn fry are fully formed miniature fish, able to swim and feed from birth. As is the rule among poeciliids, no parental care follows birth, and adults — including the mother — will readily eat fry given the chance, so in nature survival depends on cover and on the fry's immediate independence. Captive breeding records for this species are essentially absent given how rarely it is kept; the figures here rest on FishBase's reproductive data and on the standard biology of livebearing toothcarps rather than on a documented aquarium spawning.
In the aquarium
Poeciliopsis catemaco is a rarely-kept Mexican endemic, and there is no established body of hobby husbandry for it — no dedicated American Livebearer Association species page was located, and care guidance is genuinely data sparse. What can be said is drawn from its measured water parameters and from general livebearer practice rather than from a tradition of keeping this particular fish.
The target water is warm, neutral to mildly alkaline and moderately hard: roughly 75–82 °F, pH 7.0 to 7.8 and 10 to 25 dH, matching its native Laguna de Catemaco. Given the marked size difference between the sexes — females reaching about 3 in against the males' 1.5 in — a keeper would want a tank with open swimming space and structure for fry to shelter in, and would expect the species to breed readily under good conditions like other livebearers. Because this is a threatened single-lake endemic, the most responsible aquarium role for it is conservation-oriented maintenance of documented stock rather than casual trade, and any fish should be sourced and recorded carefully.
Conservation
Poeciliopsis catemaco is assessed as Endangered on the IUCN Red List, under criterion B1ab(iii), in an assessment dated 12 March 2018 and published in 2019 (citation e.T191758A2002469). The listing rests on the species' extremely restricted range — it is endemic to a single lake, Laguna de Catemaco, with an extent of occurrence of about 155 mi² across only three locations — combined with a continuing decline in the quality of its habitat. Its population size is data deficient and its trend unknown, but the small, single-system distribution leaves it inherently vulnerable.
The principal threats are biological invasions and water degradation. Introduced predators and competitors — the largemouth bass Micropterus salmoides and the African blue tilapia Oreochromis aureus — now share the lake, alongside over-fishing and pollution from fertiliser, herbicide and pesticide runoff and sedimentation from neighbouring farms and nurseries. The species is covered by Mexican regulation under NORMA Oficial Mexicana (NOM-059-SEMARNAT and the fisheries norm NOM-041-PESC-2004), but Laguna de Catemaco does not lie within a formally protected area. For a fish that exists nowhere else, the health of this one volcanic lake is the whole of its conservation story.