Taxonomy & naming
Tetragonopterus carvalhoi was described by Melo, Benine, Mariguela and Oliveira in 2011, based on specimens collected from Igarapé Iratapuru in the rio Jari drainage, Amazon basin, Amapá State, northern Brazil. The original description was published in the journal Neotropical Ichthyology. The valid name as recognised by the Catalog of Fishes is Tetragonopterus carvalhoi Melo, Benine, Mariguela & Oliveira, 2011, with no parenthetical authority, as the species was described in the genus where it currently sits.
The genus Tetragonopterus belongs to the family Characidae, the large and diverse neotropical tetra family. Tetragonopterus species are stocky, lozenge-shaped characids with prominent, relatively large scales and a strong, compressed body. T. carvalhoi was distinguished from its congeners both morphologically and by molecular evidence: COI gene-sequence data showed the Jari population to be genetically distinct from other Tetragonopterus species, supporting its recognition as a new taxon.
Morphology
Tetragonopterus carvalhoi reaches a maximum of 2.5 in standard length. It is a benthopelagic freshwater fish with the deep, laterally compressed, roughly diamond-shaped body typical of the genus. The dorsal fin carries 9 soft rays; the anal fin has 29 to 34 soft rays, and the species has 28 vertebrae. There are 6 to 10 predorsal scales, and the body lacks the dark longitudinal stripes seen in some related characids.
The most diagnostic external character is the dark spot on the caudal peduncle, which is distinctly lozenge-shaped — extending from beneath the adipose fin to the midline of the caudal rays — rather than the round or square spots found in congeners. This spot, noted as the primary distinguishing mark in the original description, makes field identification relatively straightforward when specimens are in good condition.
Habitat
Tetragonopterus carvalhoi is known from the rio Jari drainage of the Amazon basin in Amapá State, northern Brazil, where the holotype was collected from Igarapé Iratapuru. The species appears to be endemic to this river system. The rio Jari is a large right-bank tributary of the lower Amazon, draining an area of lowland Amazonian forest before joining the main river near the border of Amapá and Pará states.
Detailed habitat data specific to T. carvalhoi are sparse in the published literature. Like other Tetragonopterus and Amazonian benthopelagic characids, it likely inhabits clear to gently turbid lowland forest streams and river margins, associating with submerged roots, marginal vegetation, and the substrate. Water conditions in the Jari system are typical of blackwater and clearwater drainages of the eastern Amazon: soft, acidic water with low conductivity, warm temperatures, and heavily forested catchments.
Feeding
FishBase records a trophic level of 3.4 for Tetragonopterus carvalhoi, placing it in the detritivore-to-omnivore range. This is broadly consistent with what is known of the genus and family: Tetragonopterus species are opportunistic omnivores that take small invertebrates, organic detritus, plant material, and whatever is seasonally abundant in their habitat.
No species-specific dietary study has been published for T. carvalhoi. By analogy with its congeners and other similarly sized Amazonian characids, it likely feeds on small invertebrates, aquatic insects and their larvae, algae, and detrital material. In an aquarium setting, omnivorous characids of this size accept a wide range of prepared and live foods.
Mating
No published account of mating behaviour specific to Tetragonopterus carvalhoi exists in the current literature. The species is presumed to follow the general Characidae pattern of egg-scattering open spawning, which is characteristic of the family. In related characids, males pursue females during spawning events, often among fine-leaved aquatic vegetation or over the substrate, with fertilisation occurring as eggs are scattered and fall to the bottom or catch on plant matter.
Sexual dimorphism has not been formally described for T. carvalhoi; in many Tetragonopterus species females become noticeably deeper-bodied when gravid, which is a reliable field indicator of sex in mature individuals.
Breeding
No captive breeding records for Tetragonopterus carvalhoi are documented in the hobby literature, and the species is not commonly encountered in the aquarium trade. Based on the biology of the family, it is expected to be an egg-scattering open spawner with no parental care, in keeping with the standard characid reproductive mode.
In general Characidae breeding, adults scatter small adhesive or semi-adhesive eggs among fine-leaved plants or over the substrate; the adults do not guard eggs or fry and will eat eggs given the opportunity. Typical conditioning involves separating the sexes beforehand, providing live or frozen foods, and offering a densely planted spawning area or a mop. Hatching and larval development times are governed primarily by water temperature. Hobbyists working with this species would apply these general principles, adjusting water chemistry toward the soft, acidic conditions of its Amazonian home.
In the aquarium
Tetragonopterus carvalhoi is not an established aquarium fish, and it is rarely if ever available in the trade outside specialist importations of wild-caught Amazonian species. What is known of its care requirements is extrapolated from its natural habitat and the husbandry of congeners.
As an Amazonian species from soft, acidic water, it would logically be kept in warm (roughly 75–82 °F), soft, slightly acidic conditions with good filtration and moderate current. A planted aquarium with shaded areas and subdued lighting would replicate the forested stream environments of the rio Jari. Like other Tetragonopterus, it is likely a confident, active mid-water swimmer that benefits from being kept in a small group. Its modest size — up to 2.5 in SL — means it is manageable in a medium-sized aquarium, though formal care data are essentially absent from the hobby literature.
Conservation
Tetragonopterus carvalhoi was assessed as Least Concern (LC) on the IUCN Red List in 2021, as recorded by FishBase. The Least Concern listing reflects an absence of evidence of major population decline rather than a detailed quantitative survey; the species is apparently restricted to the rio Jari drainage of Amapá, which limits the area over which its population can be assessed.
The rio Jari basin has faced historical pressures including large-scale forestry projects and associated land-use change. However, much of the upper catchment retains intact Amazonian forest cover, and no specific threat has been attributed to T. carvalhoi in the published literature. Its restricted endemic range means that any significant degradation of the Jari system could have outsized consequences for the species, a consideration that warrants continued monitoring even under a Least Concern assessment.