Taxonomy & naming
Phenacogaster yari was described by Romero-Figueroa & Carvalho in 2024 in a paper published in Ichthyology & Herpetology (volume 112, issue 4, pages 619–644) that diagnosed four new Phenacogaster species from the Colombian Amazon basin. The type locality is the Yarí River (Río Yarí), a blackwater tributary of the Amazon drainage in Colombia. The valid name as established in that work is Phenacogaster yari Romero-Figueroa & Carvalho, 2024.
Phenacogaster is a genus of small-bodied tetras in the family Characidae with a broad Neotropical distribution. Like many members of the genus, P. yari is distinguished by a combination of ray counts, squamation, and the presence of bony hooks on male pelvic-fin rays — a secondary sexual character shared with numerous characids and used in diagnosis of the species and assessment of sexual dimorphism. The genus name Phenacogaster combines Greek roots indicating 'deceiver' or 'phantom' with a reference to the belly, alluding to the superficial resemblance of some species to other small characids.
The specific epithet yari is a toponym honouring the Yarí River, the type locality from which the species was collected.
Morphology
Phenacogaster yari is a very small tetra, with adult males reaching approximately 1 in SL and a maximum recorded standard length of 1 in. The body plan is the typical small-characid form: laterally compressed, with a moderately deep profile and relatively large eyes. Meristic data from the original description record 8–9 dorsal-fin soft rays, 29–32 anal-fin soft rays, and 34 vertebrae. The lateral line is incomplete, a feature shared with other Phenacogaster.
A posterior humeral spot — a dark mark behind the upper edge of the operculum — is present, as is characteristic of the genus. Males bear bony hooks on the rays of the pelvic fins, a sexually dimorphic feature common across many Neotropical characids; hooks are absent from the pectoral-fin rays of males in this species, a character used in its differential diagnosis. Colour details from field-preserved material are not extensively described in available sources, and live coloration has not been reported in the hobby literature given the species' very recent description.
Habitat
The Yarí River is a blackwater tributary within the Amazon basin in Colombia, draining lowland Amazonian forest in a region characterised by high annual rainfall with a unimodal wet season from approximately March through November. Conditions recorded at collection sites indicate ambient humidity of 86–89% and water temperatures of 77–79 °F. As a blackwater system, the Yarí River is expected to carry soft, acidic water stained with tannins and humic acids from decomposing organic material, low in dissolved minerals.
Phenacogaster yari, as a benthopelagic freshwater species, likely inhabits the middle and lower water column among submerged roots, leaf litter, and vegetation along stream margins — the typical microhabitat of small Phenacogaster in blackwater drainages. The broader region is part of the Colombian Amazon, an area with limited ichthyological survey coverage, which helps explain why four new Phenacogaster species were discovered in a single revision.
Feeding
No specific feeding study of Phenacogaster yari exists in the available literature. Based on genus-level knowledge, Phenacogaster species are micropredators that consume small live foods — insect larvae, microcrustaceans, and other small invertebrates — in the wild. The very small adult size of P. yari (under 1 in SL) constrains prey size accordingly.
In an aquarium context, members of the genus typically accept small prepared foods, micro-pellets, and frozen fare such as Cyclops, micro-daphnia, and newly hatched brine shrimp alongside dry foods sized to match the fish's mouth. Given the species' obscurity, hobbyist feeding data are absent, and inference from closely related small Phenacogaster is the best available guide.
Mating
No dedicated breeding observations of Phenacogaster yari appear in the available literature. As a Phenacogaster and a small characid, the species almost certainly follows the egg-scattering, open-water spawning strategy typical of the group: adults scatter small adhesive eggs among fine-leaved vegetation or over substrate, without nest-building or parental care. The bony pelvic-fin hooks present on males are consistent with pelvic-fin clasping behaviour observed during spawning in other hook-bearing characids, where males and females briefly associate at the moment of egg and milt release.
Sexual dimorphism based on the hook character is reliably diagnosable in mature adults. Whether the species shows any courtship display behaviour or spawning-site preference beyond what is generic for small characids has not been reported.
Breeding
Captive breeding of Phenacogaster yari has not been documented in the hobby or scientific literature, reflecting the species' very recent description (2024) and its absence from the ornamental trade. Based on genus-level inference, breeding would follow the egg-scattering pattern characteristic of small Neotropical characids: soft, acidic, warm water (matching the blackwater type locality, around 77–79 °F), fine-leaved plants or spawning mops as egg deposition sites, and no parental care, with adults likely to consume eggs if left in the breeding container.
Fry would be expected to require very small first foods given the tiny adult size. All details remain inferential until captive specimens are maintained and bred.
In the aquarium
Phenacogaster yari is not established in the ornamental fish hobby as of its 2024 description, and no husbandry accounts exist. As a blackwater species from the Colombian Amazon, it would logically require soft, acidic, warm water — low pH, minimal hardness, temperatures around 77–79 °F — and a dimly lit, tannin-enriched aquarium with fine-leaved plants and leaf litter to feel secure.
Its very small size (under 1 in SL) means it is suitable only for a species or nano setup or a community of similarly sized, non-predatory fishes. If the species enters the trade it would most likely be collected alongside other small Colombian blackwater tetras. Aquarists accustomed to keeping demanding small blackwater characids — Paracheirodon axelrodi and relatives from similar habitats — would find the water chemistry requirements familiar. Data sparse; all recommendations are genus-level inference.
Conservation
Phenacogaster yari has not been assessed by the IUCN Red List as of its 2024 description — the species was formally named too recently for an evaluation to have been completed. No conservation data or population estimates appear in the available literature.
The Yarí River drainage in the Colombian Amazon is a remote region with limited human infrastructure, which may offer some de facto protection from habitat loss, but the area is also subject to broader pressures on Amazonian forests. The species' restricted known distribution — currently only the Yarí River drainage — means it could be genuinely range-restricted or simply under-surveyed; further collecting in adjacent drainages will clarify its actual range. Until an IUCN assessment is completed, no formal threat category applies.