Taxonomy & naming
Poecilia marconii was described in 2026 by Luisa Maria Sarmento-Soares, Carlos Augusto Figueiredo and Alexandre Clistenes de A. Santos, in the journal Fishes (volume 11, issue 2, article 79). Eschmeyer's Catalog of Fishes cites the authority as Sarmento-Soares, Figueiredo & Clistenes, 2026 and treats Poecilia marconii as the valid, currently accepted combination.
The species was described in the subgenus Pamphorichthys — the full name used throughout the original paper is Poecilia (Pamphorichthys) marconii — and our atlas follows the site convention of dropping the subgenus parenthetical from the display name. Pamphorichthys species are small viviparous poeciliids known regionally in northeastern Brazil as 'parivivas'; the subgenus is diagnosed within the tribe Poeciliini by a strongly and diffusely pigmented gonopodium in mature males and an equally strongly pigmented urogenital region in females, along with conspicuous lateral processes on the subdistal portion of the second pelvic-fin ray in males. Long treated as a separate genus, Pamphorichthys is now recognised as a subgenus of Poecilia on the strength of molecular phylogenetic evidence, a classification the describing authors adopted for nomenclatural stability.
The holotype is a 0.9 in SL male (MNRJ 56133), collected on the Rio Cachoeirinha along road BA-142 near Wagner, Bahia, on 16 November 2005; paratypes (105 specimens total) are held at MNRJ. The genus-level diagnosis of Pamphorichthys within Poeciliini was refined over the past two decades: the classical concept, built on P. araguaiensis, P. hasemani, P. hollandi, P. minor and P. scalpridens, rested on an elaborate, ornamented pelvic-fin morphology shared by all of them. The more recent descriptions of P. pertapeh and P. akroa, however, revealed species with a simplified gonopodium bearing reduced ornamentation — a very different condition. Poecilia marconii is unusual in combining both: it retains the classical group's prominent pelvic-fin processes while also carrying a reduced, simplified gonopodium, a combination not seen in any other known Pamphorichthys. Two autapomorphies further distinguish it from all congeners: an ossified lateral process on the distal portion of the first pelvic-fin ray, and a unique connection of that process to the subdistal lateral processes of the second pelvic-fin ray via a series of fleshy appendages containing cartilaginous traces.
Morphology
Poecilia marconii is a very small poeciliid. The holotype male measures 0.9 in SL, and the paratype series spans roughly 0.67–0.97 in SL, including specimens cleared and stained for osteological study. Fin-ray counts include 5 pelvic-fin rays, 9 anal-fin rays in females (iii,9), 10–11 pectoral-fin rays, and 12–13 branched caudal-fin rays. The body carries 29–31 scales in longitudinal series and 16 scales around the caudal peduncle.
Both sexes show a reticulate colour pattern formed by dark chromatophores concentrated on the posterior margins of the scales, over a cream-to-pale-gold ground colour; in males this reticulation is overlaid with faint, diffuse vertical bars on the flank, appearing in life as olive-green to dusky bands. The single most conspicuous sexually dimorphic trait, though, is pigmentation of the reproductive structures themselves: males carry strong, diffuse pigmentation across the entire gonopodium — including a particularly densely pigmented gonopodial palp — while females carry an equally striking, sharply defined field of dark pigment surrounding the urogenital papilla. Females attain a larger maximum size than males, the typical livebearer pattern.
The gonopodium itself tells the species' most distinctive morphological story. It bears a well-developed, elongate gonopodial palp; ray 3 is robust with pronounced lateral projections that diminish posteriorly; ray 4 is the longest of the fin, formed of two elements (4a and 4p) joined distally by a cartilaginous tip, with ray 4p bearing small retrorse serrae; and ray 5 forms a channel in cross-section. This overall ornamentation pattern is reduced compared with the classical Pamphorichthys species, resembling instead the simplified condition of P. pertapeh and P. akroa — even though the species simultaneously retains those classical species' hallmark pelvic-fin processes, a combination the describing authors single out as evolutionarily significant. The pelvic fin in mature males carries an elongated second ray with a subdistal third ray, and the autapomorphic ossified process on the first pelvic-fin ray, connected to the second ray's processes by fleshy, cartilage-flecked appendages found in no other Pamphorichthys species.
Habitat
Poecilia marconii is known only from its type locality, the Rio Cachoeirinha, a third-order tributary of the Rio Utinga within the upper Rio Paraguaçu basin, in the Chapada Diamantina region of Bahia, Brazil — a biogeographically complex plateau known for high endemism and isolated drainages. At the time of collection in November 2005, the type locality held dark, tea-coloured water with a slow current (0–3 ft/s), a depth of 13.8–27.6 in, and a water temperature of 75 °F, at an elevation of 1877 ft. The substrate was pebbles, sand and decomposing plant material, and the channel was densely covered by emergent grasses and ferns along with floating macrophytes.
Twenty years later, that same stream had collapsed. In March 2025 the Rio Cachoeirinha — along with the wider Rio Utinga, of which it is a tributary — was found completely dry, a consequence the describing authors trace to unsustainable water extraction for agriculture across the Utinga basin. Repeated survey attempts at multiple other sites within the Utinga drainage over the preceding decade (Rio do Riachão, Rio da Soltinha, sites along the Rio Utinga itself) failed to locate any population of the species.
Feeding
No dietary study has been published for Poecilia marconii. As general context for the genus and subgenus, rather than a documented finding for this species: Poecilia (Pamphorichthys) species of small Neotropical streams are generally presumed omnivorous, but no diet data specific to P. marconii are available in the source material we have access to.
Mating
Poecilia marconii reproduces by internal fertilisation, using the male's gonopodium — the modified anal fin characteristic of all poeciliids — to transfer sperm to the female. The species' gonopodium is distinctive even within its own subgenus: it combines the classical Pamphorichthys pelvic-fin processes with a simplified, reduced-ornamentation gonopodial structure otherwise documented only in P. pertapeh and P. akroa, a combination the describing authors interpret as evidence that the evolution of these complex reproductive structures may be more decoupled, and subject to more repeated convergent loss or simplification, than previously assumed for the group. Beyond this structural description, no courtship behaviour specific to P. marconii has been documented.
Breeding
As a poeciliid, Poecilia marconii is presumed to be a viviparous livebearer, following the reproductive pattern of internal fertilisation and live birth shared across the family. No brood size, gestation length, or other quantitative reproductive data specific to this species have been published, and given how recently it was described — and how quickly its only known habitat has since dried up — such data may never be gathered from the wild.
In the aquarium
Poecilia marconii is not an aquarium-trade species and has no husbandry history; its type series was collected for scientific purposes in 2005 and the species was not seen alive again by the describing team despite repeated survey efforts. The describing authors themselves raise the possibility of an ex-situ captive breeding programme as a conservation safeguard, given the proven adaptability of many poeciliid species to captivity generally — but this is a stated conservation goal rather than an existing aquarium practice, and no captive population of P. marconii is known to exist.
Conservation
Poecilia marconii has not been formally assessed on the IUCN Red List, as is typical for a species described in 2026. The describing authors, however, frame the description explicitly as an urgent conservation call rather than a routine taxonomic record: twenty years after the type series was collected, the Rio Utinga basin shows severe environmental degradation linked to unsustainable water extraction for agriculture, and the Rio Cachoeirinha — the species' only known habitat — was found completely dry in March 2025. The species has not been recorded in field surveys over the past decade and may already be at the point of disappearance.
Using IUCN Red List threat classification, the authors identify the primary driver as unsustainable abstraction and impoundment of surface water for irrigation, describing this as an ongoing and planned intervention rather than a hypothetical risk: state technical reports formally designate the sub-basin for a 'Hydroagricultural Development' project intended to irrigate roughly 400 hectares. Compounding pressures include in-stream sand mining and domestic-waste pollution, worsened by increasingly frequent and intense droughts linked to climate change. The authors call for regulation and monitoring of water extraction in the basin, a habitat restoration plan for the Rio Cachoeirinha, formal conservation assessment to support legal protection, and exploration of an ex-situ captive breeding programme as an insurance population — framing the fate of P. marconii as a test case for the wider, threatened aquatic biota of the Chapada Diamantina.