Taxonomy & naming
Poecilia pertapeh was described by C. A. Figueiredo in 2008, originally as Pamphorichthys pertapeh, in a paper naming a new poeciliin from central Brazil (Zootaxa 1918: 59–68). It belongs to the tribe Poeciliini within the subfamily Poeciliinae of the family Poeciliidae, the New World livebearing toothcarps. Eschmeyer's Catalog of Fishes governs the valid name; the species is now placed in the genus Poecilia, the large and repeatedly revised genus that also contains the guppy and the mollies, and which has absorbed several formerly distinct poeciliin genera.
The small genus Pamphorichthys, in which the species was first placed, comprises diminutive poeciliins of eastern and central South America, and the reshuffling of these fishes into and out of Poecilia reflects the unsettled state of poeciliin classification rather than any doubt about the species itself. Figueiredo diagnosed P. pertapeh by two unique derived characters not seen elsewhere in the subfamily: bilobed teeth on the dentary and on the external tooth row of the premaxilla, and the distal-most segments of the anterior branch of the fourth gonopodial ray and of the third gonopodial ray each independently fused into elongated segments. These osteological details of jaw and gonopodium are what set the species apart from its relatives.
The specific epithet pertapeh is taken directly from the name of the type locality, Lagoa Perta-pé — literally "Squeeze-Foot Lake" — so named for the dense aquatic vegetation that mats the lake everywhere but its shore. The common name recorded in the literature, "Jungle toothcarp," likewise evokes the densely overgrown habitat in which the fish was found.
Morphology
Poecilia pertapeh is a very small fish even by poeciliin standards: the maximum recorded length is about 0.5 in standard length. Its body proportions and fin counts are typical of a small livebearing toothcarp — dorsal fin with i/ii and 7–8 branched rays, the female anal fin with iii,9 rays, five pelvic rays, and 10–11 pectoral rays. The length–weight relationship reported for the species (a ≈ 0.00851, b ≈ 3.15) is unremarkable for a fish of this size and form.
Several pigment and structural features distinguish the species. Females carry a dark longitudinal stripe along the caudal peduncle and a heavy concentration of dark pigment around the urogenital region, together with a specialised pair of anal scales positioned just in front of the urogenital opening. In males the third pelvic-fin ray is curved distally. The diagnostic characters of greatest taxonomic weight, however, are not external colour but the bilobed jaw teeth and the modified, elongated terminal segments of the third and fourth gonopodial rays — the latter being part of the male intromittent organ.
Beyond these described characters, detailed information on live coloration, behaviour and individual variation is sparse: the species is known from a small series of preserved specimens from a single lake, and has not been the subject of the kind of field or aquarium study that fleshes out the natural history of better-known livebearers.
Habitat
Poecilia pertapeh is known only from its type locality: Lagoa Perta-pé, a marginal lake on the left side of the Bezerra river, itself a tributary of the Rio Negro, which feeds the Paracatu and ultimately the upper São Francisco river basin. The site lies near Formosa, in Goiás, central Brazil, at roughly 15–16°S and 47–48°W. The fish is a freshwater, tropical species occupying a wetland (inland-waters) environment.
The lake is small — about 0 mi² — and is densely covered by aquatic vegetation across most of its surface, leaving open water only at the shore; it is this near-total weed cover that gives the lake (and the fish) its name. The specimens on which the species is based were collected at the lake margins, among the dense marginal vegetation, which appears to be the species' core microhabitat.
Because the species has never been recorded anywhere else, its entire known global range is this single body of water. Both its extent of occurrence and area of occupancy are estimated at just 2 mi², making it one of the most geographically restricted of all poeciliid fishes — an extreme point-endemic confined to a few hectares of weed-filled lake.
Feeding
No direct diet study has been published for Poecilia pertapeh, and the species' feeding habits in the wild are not documented in detail. FishBase places it at a trophic level of about 3.0, consistent with a small omnivorous to micro-carnivorous toothcarp rather than a strict herbivore or a higher-level predator.
The bilobed teeth on the dentary and premaxilla, one of the diagnostic features of the species, suggest a jaw adapted to a particular feeding mode, though the functional significance has not been tested. By analogy with other small poeciliins of densely vegetated lakes, the fish most plausibly takes a mixed diet of small invertebrates, micro-crustaceans, insect larvae, algae and detritus gleaned from among the aquatic plants. This should be read as inference from close relatives and habitat, not as a documented account of the species' diet — for which the data are sparse.
Mating
Like all members of the Poeciliinae, Poecilia pertapeh reproduces by internal fertilisation. The male transfers sperm to the female using the gonopodium, an intromittent organ formed from modified anterior rays of the anal fin; in this species the terminal segments of the third and fourth gonopodial rays are uniquely fused into elongated segments, a feature used to diagnose the species and a reminder that the fine structure of the gonopodium is central to poeciliin taxonomy and to how these fishes mate.
No direct observations of courtship or mating behaviour in P. pertapeh have been published. In poeciliins generally, males lack any parental or nest-building role and instead pursue females and inseminate them via the gonopodium, often combining brief courtship with sneak insemination attempts. The pronounced dark pigmentation of the female urogenital region and the specialised pair of anal scales in front of the urogenital opening described for this species are reproductive-anatomical features, but their behavioural function — for instance in mating or in sperm reception — has not been studied. The species' actual mating system is therefore best described as inferred from its family rather than directly known.
Breeding
Poecilia pertapeh is a livebearer: like the rest of the Poeciliinae it is viviparous, the female retaining the fertilised eggs internally and giving birth to free-swimming young rather than laying eggs. No brood-size, gestation or reproductive data specific to this species have been recorded — FishBase holds no reproduction figures for it — so any statement about litter size or breeding frequency would be speculation.
Many small poeciliins are capable of superfetation, carrying broods at more than one stage of development simultaneously, and females of the family can typically store sperm to produce successive broods from a single insemination; whether either occurs in P. pertapeh is unknown and has not been investigated. Given the fish's very small adult size, any broods are likely to be correspondingly small, but this is an inference from body size rather than an observation.
FishBase classes the species as having high biological resilience, with an estimated minimum population doubling time of under 15 months — a reproductive potential typical of small, fast-breeding livebearers. That intrinsic resilience, however, is set against an extreme restriction of range, and offers little protection to a fish confined to a single small lake.
In the aquarium
Poecilia pertapeh is not an aquarium fish in any practical sense. It is not present in the hobby, is not bred or traded, and is known only from preserved museum material collected at its single locality. There is accordingly no body of husbandry experience to draw on, and no captive population.
Given its Critically Endangered status and point-endemic range, the appropriate stance is conservation rather than collection: this is a fish whose survival depends on the protection of one small Brazilian lake, not on its appearance in aquaria. Any role for ex-situ care would belong to a managed conservation-breeding programme, not the ornamental trade. Aquarists encountering the name should understand it as a scientific record of biodiversity rather than as a fish to keep.
Conservation
Poecilia pertapeh is assessed as Critically Endangered on the IUCN Red List (as Pamphorichthys pertapeh), under criteria B1ab(iii)+2ab(iii), version 3.1. The assessment was carried out in November 2018 and published in 2022 (citation e.T186797A1818524). The listing rests on the species' extreme geographic restriction: it is known from a single location, Lagoa Perta-pé, with both extent of occurrence and area of occupancy estimated at just 2 mi², combined with a continuing decline in the quality of its habitat.
The threats identified are characteristic of an isolated lowland wetland surrounded by agriculture: annual and perennial non-timber crop farming and other agricultural land use, dams and changes in water management, and pollution from agricultural and forestry effluents. Any one of these acting on a 0 mi² lake could degrade or eliminate the only known population, and the assessment stresses that preserving Lagoa Perta-pé — particularly its vegetated marginal zones, the fish's core microhabitat — is necessary for the species' survival, with a proposal that the area be made a formal protected unit (Unidade de Conservação).
The species is recognised in Brazilian conservation instruments: it is covered by the national São Francisco recovery action plan (PAN São Francisco, ICMBio 2015) and is listed in the Livro Vermelho da Fauna Brasileira Ameaçada de Extinção (ICMBio 2018). For a fish confined to a few hectares of a single weed-filled lake, in-situ protection of that lake is effectively the whole of its conservation.