Taxonomy & naming
Phenacogaster jancupa was described by Luiz Roberto Malabarba and Zilda Margarete Lucena in 1995 from specimens collected at Jangada, on the Ribeirão Espinheiro, a tributary within the Rio Cuiabá drainage of the upper Rio Paraguai basin in Mato Grosso, Brazil. The type locality thus places the species firmly in the Paraguay River system, one of the major South American drainages of the southern interior.
The genus Phenacogaster belongs to the family Characidae and comprises small, superficially similar characins distributed across tropical and subtropical South America. Malabarba and Lucena distinguished P. jancupa from its congeners principally by meristic characters: it carries 37–38 perforated scales in the longitudinal series (mean 37.4), and its humeral spot is not surrounded by a pale, unpigmented zone — a feature that separates it cleanly from P. franciscoensis, which has fewer perforated scales (34–37) and does display that clear halo. Eschmeyer's Catalog of Fishes records Phenacogaster jancupa Malabarba & Lucena, 1995 as a valid species.
Morphology
P. jancupa is a small, slender characid of typical Phenacogaster proportions: laterally compressed, with a moderately pointed snout, large eye, and the forked tail characteristic of the family. Precise maximum size data are sparse for this species specifically, but members of the genus typically reach 1.5–2.5 in standard length, placing them firmly in the micro- to small-tetra size range.
The key external features used to identify the species are the dark humeral spot — a pigmented blotch behind the operculum — which in P. jancupa lacks the surrounding pale area seen in related species, and the scale count of 37–38 perforated scales in the lateral series. A caudal peduncle stripe extending into the middle caudal-fin rays is also present, as in much of the genus. Coloration in life is not fully documented in published sources for this species; by analogy with congeners it is likely a silvery fish with a faint lateral stripe and the characteristic dark spots, rather than a boldly coloured tetra.
Habitat
The type locality of P. jancupa is the Ribeirão Espinheiro at Jangada, in the Rio Cuiabá sub-basin of the upper Rio Paraguai, Mato Grosso, Brazil. The Rio Cuiabá drains part of the Cerrado and pantanal-edge region of west-central Brazil, and its smaller tributaries — ribeirões such as Espinheiro — typically flow through gallery forest, carrying warm, moderately soft to slightly hard water influenced by the surrounding savanna landscape.
Detailed habitat observations specific to P. jancupa at its type locality are not available in published literature accessible at the time of this writing. Given the species' assignment to the genus Phenacogaster and its drainage context, it likely inhabits slower-flowing to moderately flowing, clear or slightly tannin-stained waters with submerged vegetation or leaf-litter substrate, as is characteristic of small Cerrado-fringe streams.
Feeding
No species-specific dietary studies of Phenacogaster jancupa have been published. Members of the genus are understood to be micro-predators and generalist insectivores in the manner of most small characids, targeting small invertebrates — aquatic insect larvae, microcrustaceans, and terrestrial insects that fall to the water surface — supplemented opportunistically by other small invertebrate prey.
In an aquarium context, guidance from closely related congeners such as P. tegatus suggests that Phenacogaster species do well on live and frozen small invertebrates including copepods, brine shrimp, and grindal worms, with prepared micro-pellets and fine flake accepted as staples. A varied diet with a live or frozen component is recommended to maintain condition.
Mating
No breeding observations specific to Phenacogaster jancupa appear in the published literature or in the hobby record. By analogy with other small characids of similar ecology, the genus is presumed to employ egg-scattering open spawning with no parental care — the default reproductive mode across the majority of characin lineages.
Males in breeding condition likely display to females with intensified coloration and fin spreading, as is typical for small tetras. Spawning would be expected to occur among fine-leaved aquatic vegetation or over substrate, with the parents taking no further interest in the eggs once released. Specific triggers for spawning in captivity are undocumented for this species.
Breeding
Phenacogaster jancupa has not been bred in captivity under documented conditions in the available hobby literature. Based on the genus-level pattern seen in other small South American characids from similar drainages, breeding in an aquarium would likely follow the typical egg-scattering model: small, semi-adhesive eggs deposited among fine-leaved plants or spawning mops, hatching after several days at tropical temperatures, with fry requiring very small first foods such as infusoria or commercial fry preparations before graduating to newly hatched brine shrimp.
Parental care is not expected, and adults would need to be separated from the eggs to prevent predation. Specific temperature and water-chemistry preferences for successful breeding in this species remain undocumented.
In the aquarium
Phenacogaster jancupa is not an established aquarium species and is essentially absent from the commercial hobby trade. It is known primarily from its original description and from systematic surveys of the Rio Paraguai drainage. No dedicated care literature exists for this species in the hobby press.
Hobbyists working with related Phenacogaster species such as P. tegatus report that the genus does well in a small aquarium of 60 × 12 in or larger, kept in groups of eight or more individuals, at temperatures of 68–82 °F and pH 6.0–7.5. Clear water, dense vegetation, some driftwood or leaf litter, and a diet of small live and frozen invertebrates are recommended. Being small and peaceful, Phenacogaster species are appropriate tankmates for other small, non-aggressive characids and similarly sized fish. Should P. jancupa become available in the hobby, these parameters would be a reasonable starting point.
Conservation
Phenacogaster jancupa has not been assessed by the IUCN Red List. It is a narrowly described species known from its type locality in the Rio Cuiabá drainage of Mato Grosso, Brazil, and its range extent and population status have not been formally evaluated.
The upper Rio Paraguai basin, including the Cerrado streams of Mato Grosso, faces ongoing pressure from agricultural expansion, deforestation of gallery forest, and altered hydrology associated with soybean and cattle farming in the region. Whether P. jancupa remains common or has declined in its native streams is unknown without current survey data. Its restricted documented range and the general vulnerability of small Cerrado-stream fishes to habitat modification make it a candidate for future systematic assessment.