Taxonomy & naming
Phenacogaster nukak was described by Romero-Figueroa & Carvalho in 2024, in a paper published in Ichthyology and Herpetology that diagnosed four new Phenacogaster species. The type locality is the Inírida River in the Guaviare department of Colombia, a tributary of the Orinoco basin. The valid name, as governed by the Catalog of Fishes (Eschmeyer, CAS), is Phenacogaster nukak Romero-Figueroa & Carvalho, 2024.
The genus Phenacogaster belongs to the family Characidae and comprises a set of small, laterally compressed tetras endemic to South American river systems. With more than twenty valid species, the genus is distributed across much of tropical South America, with highest diversity in Orinoco and Amazon tributaries. Phenacogaster nukak is most similar to P. yari but differs in several meristic characters: it carries 8–11 perforated lateral-line scales (versus 6–7 in P. yari), 11–12 circumpeduncular scales (versus 10), 32–34 branched anal-fin rays (versus 29–32), and 12–14 pectoral-fin rays (versus 10–11). It also has 29–30 teeth in the maxilla.
The specific epithet nukak honours the Nukak Maku, an indigenous people of the Colombian Amazon–Orinoco transition zone whose territory includes the Inírida River drainage where the species was collected. This follows a tradition within Neotropical ichthyology of naming species for the communities and peoples of the regions from which they come.
Morphology
Phenacogaster nukak is a small, slender tetra with an incomplete lateral line — a feature shared across the genus and useful for identification within Characidae. The lateral line is short, with only 8–11 perforated scales rather than extending to the caudal peduncle as in many related genera. Body depth ranges from 28.8 to 43.3 percent of standard length, giving the species a moderately deep profile for the genus.
A humeral spot is present but positioned far from the pseudotympanum, a character noted in the original description as diagnostically significant relative to congeners. Anal-fin ray count is high for the genus at 32–34 branched rays, and the maxilla carries 29–30 teeth. The genus Phenacogaster as a whole seldom exceeds around 2.5 in standard length, and P. nukak is expected to fall within that range, though precise maximum size data for this species specifically are not yet published beyond the type series.
Colouration details for preserved material are not described in available sources; live coloration in the field is undocumented in the literature accessed. As in most small Phenacogaster, the species is presumed to carry the genus's typical translucent to silvery body with a dark lateral stripe or caudal spot, but this should be regarded as inference from congeners rather than confirmed data for P. nukak specifically.
Habitat
Phenacogaster nukak is known from the Inírida River, a blackwater tributary of the Orinoco draining the Guaviare department of eastern Colombia. The Inírida is a classic Orinoco blackwater river — tea-stained with tannins and humic acids, very soft, and markedly acidic. Water conditions typical of this system include pH values in the range of approximately 4.3–4.7 and temperatures spanning roughly 77–95 °F across seasonal cycles, with blackwater wetland zones at the warmer end of that range.
The broader Orinoco basin in this region is characterised by dense gallery forest, flooded savannas (llanos), and seasonally inundated riparian zones. Tetras of the Phenacogaster genus in general occupy vegetated, slow-moving margins and backwaters with accumulated leaf litter and submerged wood, habitats that are physically sheltered and chemically extreme by mainstream standards. Whether P. nukak specifically occupies such microhabitats or is found in faster runs is not documented in the sources available; habitat data derive from the general character of the Inírida drainage and from known Orinoco tetra ecology.
The Inírida River region falls within one of the least disturbed portions of the Colombian Orinoco basin, though the broader Orinoco drainage faces increasing pressure from agriculture, mining, and infrastructure development.
Feeding
No specific dietary data have been published for Phenacogaster nukak. Small Phenacogaster species are understood from congeners to be opportunistic micro-carnivores and omnivores, feeding on tiny invertebrates including microcrustaceans, aquatic insect larvae, and chironomid pupae, along with small amounts of algae and organic detritus.
In the acidic blackwater environments these fish inhabit, invertebrate prey tends to be sparse but diverse, and Phenacogaster are adapted to exploit small, soft-bodied items throughout the water column. In captivity, any aquarist keeping this species would be expected to offer a varied diet of small live and frozen foods — Daphnia, baby brine shrimp, micro-worm, and bloodworm — supplemented with finely crushed prepared foods.
Mating
No mating observations have been published for Phenacogaster nukak. Based on the reproductive biology of the genus and the broader Characidae, the species is presumed to be an egg-scattering open spawner that deposits adhesive eggs among fine-leaved aquatic plants or submerged leaf litter, with no parental care by either sex.
Small Characidae of this type typically spawn in pairs or small groups, with males following and nudging females during brief, vigorous spawning runs. Eggs are released and fertilised in the water column or pressed against vegetation and left to develop unattended. The absence of parental investment is standard for the family; adults may consume their own eggs if encountered after spawning. Specific triggers, clutch sizes, and incubation times are not documented for this species.
Breeding
No captive breeding records for Phenacogaster nukak are available; the species was described only in 2024 and has not entered the aquarium trade in any documented way at the time of writing. General expectations for breeding small Orinoco Characidae would call for soft, acidic, warm water matching the type-locality conditions — pH around 4.5–6.0, temperature in the mid-to-upper 20s Celsius, with peat or leaf-litter filtration to mimic the blackwater chemistry.
Breeders familiar with analogous small tetras such as other Phenacogaster or comparably sized Characidae would approach conditioning with live foods, spawning over fine-leaved plants or Java moss, and removing adults after spawning to prevent egg predation. The fry of small tetras of this type are typically tiny at hatching and require infusoria or commercial liquid fry food as first foods before graduating to newly hatched brine shrimp nauplii. These details remain speculative for P. nukak until captive experience with the species is documented.
In the aquarium
Phenacogaster nukak is not established in the aquarium hobby and is unlikely to be encountered in the trade. No husbandry records have been published. Were the species to become available, the appropriate framework would draw on the care requirements of blackwater Orinoco tetras generally: a soft, acidic, warm setup with pH in the 4.5–6.5 range, temperature around 79–86 °F, low hardness, and subdued lighting over a dark substrate and leaf litter.
As a small, schooling Characid likely not exceeding 2–2.5 in standard length, it would suit a species biotope or a community of similarly sized, equally acid-tolerant fishes from the Orinoco blackwater zone. An emphasis on water quality and stability would be important, as blackwater species adapted to extreme soft-acid conditions often prove sensitive to water chemistry fluctuations. The species' current rarity in cultivation makes aquarist observation and documentation especially valuable — any keeper successfully maintaining or breeding P. nukak would contribute meaningfully to knowledge of the species.
Conservation
Phenacogaster nukak has not been assessed by the IUCN Red List as of the time of writing; the species was described in 2024 and new species typically enter the assessment queue years after formal description. Its conservation status is accordingly listed as Not Evaluated (NE).
The type locality in the Inírida River drainage falls within a region of relatively low human impact compared with more accessible parts of the Orinoco basin, which offers some incidental protection. However, the broader Orinoco and Inírida region faces long-term threats from illegal gold mining (which significantly degrades river water quality through mercury contamination and physical disturbance), agricultural expansion, and the ongoing vulnerability of the Nukak Maku territory itself to encroachment. Whether the species has a wide or narrow range within the Inírida system is unknown; until targeted surveys establish its distribution and abundance, the degree of conservation concern cannot be quantified. A formal IUCN assessment would require range data and population estimates that do not yet exist in the published literature.