Taxonomy & naming
Hyphessobrycon pastanai was described in 2026 by Thiago C. Faria, Willian M. Ohara, Isabela L. P. Monteiro and Claudio Oliveira in the Journal of Fish Biology, from a single stream tributary of the Igarapé do Tamanduá, Rio Jatuarana, lower río Aripuanã basin, río Madeira basin, in Apuí, Amazonas State, Brazil. The holotype, LBP 34081, is 1.16 in standard length. Eschmeyer's Catalog of Fishes lists Hyphessobrycon pastanai Faria, Ohara, Monteiro & Oliveira, 2026 as the valid name, placed by the Catalog in Acestrorhamphidae, subfamily Hyphessobryconinae; this atlas follows the traditional and still widely used placement of the genus in Characidae.
The authors place the species in the Hyphessobrycon agulha lineage, one of four principal clades within Hyphessobryconinae recovered by a 2024 molecular phylogeny, and corroborate the placement using DNA barcoding (cytochrome c oxidase subunit I). In their gene tree, H. pastanai groups with three genetically distinct populations of Hyphessobrycon ericae and with Hyphessobrycon ribeiroi, despite the three species having strikingly different colour patterns; H. pastanai is most similar in overall appearance to Hyphessobrycon agulha itself, though genetic evidence indicates the two are not closely related within the lineage.
Morphology
Hyphessobrycon pastanai reaches at least 1.26 in standard length among the paratype series; females (up to 1.25 in SL) grow slightly larger than males (up to 1.16 in SL, the size of the holotype). It is distinguished from most congeners by the combination of a midlateral dark stripe together with a midlateral iridescent stripe in life, a horizontally elongated humeral blotch with a diffuse ventral expansion, bony hooks confined to the pelvic fin of mature males (not the anal fin), a strongly marked caudal-peduncle blotch, and just 7–9 pored lateral-line scales.
In life the body is olive overall, with a silvery lower head and abdomen, a red upper iris, and an orange snout and ventral head. A tricoloured longitudinal pattern runs from behind the operculum to the base of the caudal fin: a narrow, interrupted red stripe dorsally (becoming a solid, thickened red blotch on the caudal peduncle), a continuous iridescent greenish stripe in the middle, and a ventral dark pattern formed by the humeral blotch and a thin midlateral dark stripe. Most fin bases are orange, with the exception of the anal-fin lobe, which is also orange. In alcohol the body is beige with a dark, reticulated pattern on the predorsal and preadipose scales, and a diffuse but distinctly delimited caudal-peduncle blotch that extends onto the middle caudal-fin rays.
Habitat
Hyphessobrycon pastanai is known only from its type locality, a small stream affluent of the Rio Jatuarana in the lower río Aripuanã basin, río Madeira basin, near Apuí, Amazonas State, Brazil. The describing authors characterise the stream as moderate-flowing and slightly blackwater-tinged, running through forest-covered terrain over a rocky bottom; specimens were collected swimming in open water rather than sheltering in leaf litter or margins. The stream passes through a large culvert beneath the BR-230 highway followed by a small, vegetation-cleared lake of unclear function, but forest cover shading the stream both upstream and downstream of these disturbed points was intact at the time of collection. Syntopic species included Bryconops sp., Moenkhausia oligolepis, Saxatilia sp. and Moenkhausia sp.
Feeding
No stomach-content or dietary study was reported for Hyphessobrycon pastanai. As a genus-level generality, small Hyphessobrycon of this size are typically micropredators and opportunistic omnivores, taking small invertebrates, algae and fine organic matter, but this has not been directly documented for this species.
Mating
No courtship behaviour was described for Hyphessobrycon pastanai, but the describing authors report that females carrying eggs were collected in December, indicating that reproduction occurs during (or is triggered by) the regional rainy season. Sexual dimorphism includes a slightly more pointed, better-developed anal-fin lobe in females (formed by longer last-unbranched and anteriormost branched rays) and tiny bony hooks running the full length of the pelvic-fin rays in males. As a genus-level generality, Hyphessobrycon species typically scatter adhesive eggs among fine vegetation with no parental care, but specific courtship behaviour has not been documented for this species.
Breeding
No captive-breeding data exist for Hyphessobrycon pastanai, which is not known to be kept in aquaria. The single field observation of egg-carrying females in December suggests a wet-season spawning period in the wild. Based on the genus-level pattern documented for related Hyphessobrycon of the agulha lineage — egg-scattering with no parental care — a similar reproductive mode is plausible, but nothing has been confirmed by direct observation of spawning.
In the aquarium
Hyphessobrycon pastanai does not appear to be established in the aquarium trade; the original description makes no mention of the species being kept or sold, in contrast to some other recently described members of the genus. Its natural habitat — a moderate-flowing, forest-shaded, slightly blackwater stream with a rocky bottom — suggests it would suit a well-oxygenated, softly lit aquarium with driftwood or leaf litter and modest current, kept as a shoal in line with general tetra husbandry practice; this is inferred from habitat and family-level biology rather than any documented aquarium experience with the species.
Conservation
Hyphessobrycon pastanai has not been formally assessed by the IUCN Red List; its status is Not Evaluated (NE). The describing authors recommend a status of Least Concern, noting that despite four expeditions to the Apuí region since 2016, the species has been recorded only at its type locality — a stream close to the BR-230 highway but still shaded by intact forest, located about 5 mi south of Floresta Nacional do Aripuanã, which protects nearly 21 mi of the Rio Jatuarana's main channel. The authors caution, however, that Apuí has been among the most heavily deforested regions of Brazil in recent years, and that political pressure has previously targeted the surrounding mosaic of federal protected areas for downsizing, making ongoing habitat protection an open concern for this narrowly known species.