Taxonomy & naming
Tetragonopterus juruena was formally described by Silva, Melo, Oliveira & Benine in 2016. The type specimen — holotype LIRP 14033, measuring 3 in standard length — was collected from the Rio Juruena in the upper Rio Tapajós drainage, Mato Grosso, Brazil (approximately 12°50'59"S, 58°55'36"W). The species is currently known from the Rio Juruena and Rio Teles Pires river systems, both major tributaries of the Tapajós, which itself drains into the Amazon.
The genus Tetragonopterus belongs to the family Characidae. The genus name, from the Greek for 'four-angled fin', alludes to the characteristic square or rhomboidal body outline of its members. Tetragonopterus juruena is distinguished from close relatives within the genus by a combination of meristic features: 4–6 maxillary teeth, 9 dorsal soft rays, 24–28 anal soft rays, and 31 vertebrae. Catalog of Fishes records the valid name as Tetragonopterus juruena Silva, Melo, Oliveira & Benine, 2016.
Morphology
Tetragonopterus juruena is a moderately deep-bodied, laterally compressed characid with the rhomboidal body silhouette typical of the genus. The maximum recorded standard length is 3 in. The species carries the standard Tetragonopterus colour scheme: a generally silvery body marked by two distinct vertical humeral blotches behind the opercle and a dark blotch on the caudal peduncle.
Key diagnostic counts include 9 dorsal soft rays, 24–28 anal soft rays, and 31 vertebrae. The maxilla bears 4–6 teeth. Sexual dimorphism is expressed primarily through the anal fin: males develop bony hooks on the first eight branched anal-fin rays, a modification seen across many characids and used in species-level diagnosis within Tetragonopterus. Females lack these hooks and, as is typical in characids, may become more robust-bodied when gravid.
Habitat
This species inhabits the Rio Juruena and Rio Teles Pires river systems within the upper Tapajós basin in the state of Mato Grosso, central-western Brazil. The Tapajós drains the southern Brazilian Shield and is characterised by clear, sometimes tea-coloured waters over sandy or rocky substrate, generally with lower conductivity and softer water chemistry than white-water Amazonian rivers.
FishBase classifies Tetragonopterus juruena as a freshwater pelagic species, suggesting it occupies the open water column rather than being strictly benthic. The Juruena and Teles Pires systems flow through a mixture of cerrado (savanna) and transitional forest landscapes, with pronounced seasonal flood regimes that influence fish community structure and access to floodplain resources. Specific water-chemistry parameters for the collection sites are not documented in available sources.
Feeding
No detailed dietary study for Tetragonopterus juruena is available in the literature consulted. Congeners within Tetragonopterus are broadly omnivorous, taking small invertebrates, plant material, and sometimes smaller fish, consistent with the robust, toothed jaws characteristic of the genus.
In captivity, close relatives and similar-sized characids typically accept a range of prepared and live foods, including flake, small pellets, and frozen invertebrates such as Daphnia, brine shrimp, and bloodworm. Until a specific dietary account exists for this species, the genus-level omnivore model is the best available guide.
Mating
No published observations of courtship behaviour in Tetragonopterus juruena are available. The presence of bony hooks on the anal-fin rays of males — a widespread modification in characids — is thought to facilitate contact or clasping during spawning in at least some species, though its precise function varies across the family.
As with other open-water characids, spawning is expected to involve the male closely following the female as she releases eggs, with fertilisation occurring in the water column or over fine substrates. No spawning site fidelity or mate-guarding behaviour has been described for this or closely related species.
Breeding
Tetragonopterus juruena has not been bred in captivity in any documented account available at the time of writing. Based on the general biology of Tetragonopterus and small characids of the Tapajós drainage, the species is presumed to be an egg-scatterer with no parental care — the standard mode for most open-water characids. Eggs would likely be broadcast over fine-leaved vegetation or sandy substrate and left to develop and hatch without adult attention.
In the wild, breeding presumably tracks seasonal flood cycles, as is typical for characids of the central Brazilian Shield, with rising waters providing access to flooded vegetation and invertebrate prey that bring fish into spawning condition. Specific information on clutch size, incubation period, or fry development is not available.
In the aquarium
Tetragonopterus juruena is essentially unknown in the aquarium hobby and is not in commercial trade at the time of writing. Based on its size (max 3 in SL) and the general husbandry requirements of Tetragonopterus, it would likely suit a spacious aquarium of at least 39.5 in in length, ideally kept as a small group, with open swimming space and moderate current to reflect its pelagic habit in clear Tapajós-system rivers.
Soft, slightly acidic to neutral water — consistent with the Brazilian Shield's typically low-conductivity river chemistry — would be appropriate, though precise parameters for the collection sites have not been published. Should specimens become available, care would broadly follow that of other medium-sized Tetragonopterus: a peaceful but active fish best kept with similarly sized tankmates, fed a varied diet of prepared and live or frozen foods.
Conservation
Tetragonopterus juruena has not been assessed by the IUCN Red List; no species-specific evaluation is currently available. The species is described from the Rio Juruena and Rio Teles Pires systems within the upper Tapajós basin — a region that has experienced significant pressure from soy agriculture, cattle ranching, and hydropower development in Mato Grosso state.
The Rio Teles Pires in particular has been subject to a cascade of large dam projects, and the associated changes in hydrology, sediment transport, and fish passage represent potential threats to endemic ichthyofauna. Without a formal range estimate or population data, the species' vulnerability cannot be quantified; however, its apparently restricted known distribution warrants attention in future biodiversity assessments of the upper Tapajós.