Taxonomy & naming
Tetragonopterus manaos was formally described by Urbanski, Melo, Silva & Benine in 2018. The type locality is the Solimões River and its associated drainage, within the broader Amazon basin of Brazil, in the vicinity of the city of Manaus in Amazonas state. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Van der Laan) is the governing authority for the valid name.
The genus Tetragonopterus Cuvier, 1816 is one of the oldest-established characid genera, but its membership has been repeatedly revised as molecular and morphological tools have sharpened. The genus gives its name to the informal grouping sometimes called tetragonopterines, though the broader assemblage has been shown to be non-monophyletic, and the genus itself has historically been a repository for deep-bodied, rhomboid characids. T. manaos is one of several species recognised in recent years as molecular and comparative morphological work has resolved finer-scale species boundaries within the group.
No widely adopted common name is recorded for the species in the scientific or hobby literature to date.
Morphology
Tetragonopterus manaos reaches a maximum recorded standard length of 3 in, placing it among the smaller to mid-sized members of its genus. Like all Tetragonopterus, the body form is strongly compressed laterally and noticeably deep, giving the fish its characteristic rhomboid or diamond outline in lateral view — a shape that distinguishes the genus from the more fusiform tetras encountered in the same river systems.
The genus is characterised by a prominent, well-defined dark lateral stripe running from behind the head to the caudal peduncle, terminating in a dark caudal spot that extends into the base of the caudal fin rays. Scales are relatively large, silvery, and refractive. The mouth is terminal and moderately sized, with teeth adapted for opportunistic feeding. Specific meristic and morphometric details distinguishing T. manaos from its congeners are documented in the 2018 original description; published data for living coloration in preserved or aquarium specimens are sparse.
Habitat
The species is recorded from the Solimões River drainage of the Amazon basin in the state of Amazonas, Brazil — the upper Amazon mainstem as it is known in Brazilian nomenclature before its confluence with the Rio Negro near Manaus. FishBase notes the species inhabits clear-water and black-water streams associated with the broader drainage, environments that include both the várzea floodplain and terra firme forest streams.
Like most Amazonian characids, T. manaos likely encounters a wide range of dissolved oxygen levels, temperatures, and pH values across the seasonal flood cycle. The Solimões carries sediment-rich white water, while adjacent drainage streams and lakes can be clear or highly tannic; the species' tolerance of these contrasting chemistries is not characterised in depth in the available literature. Water temperatures in this part of the Amazon lowlands typically fall between 75 °F and 86 °F through the year.
Feeding
Detailed dietary studies for Tetragonopterus manaos have not been published to date. The genus as a whole is regarded as omnivorous and opportunistic, consuming small invertebrates, insects, plant material, and occasionally smaller fishes. Deep-bodied characids with the Tetragonopterus body plan are often capable of taking prey items larger in relation to their body size than more slender tetras.
In an aquarium context, T. manaos would be expected to accept a wide range of foods typical for medium-sized tetras: quality dried foods (flake and small pellets), frozen invertebrates such as bloodworm, brine shrimp, and daphnia, and live foods when available. A varied diet supports condition and coloration. Given the body form and jaw structure of the genus, care should be taken when housing T. manaos with smaller, finnage-rich tankmates.
Mating
No specific observations on the mating behaviour of Tetragonopterus manaos have been published. Tetragonopterus is a characid genus, and members of this family are broadly egg-scattering, open-water or plant-spawning fishes without parental care. Males likely engage in competitive displays and courtship pursuit of females prior to spawning, consistent with the pattern seen across the family.
Adhesive or semi-adhesive eggs are probable, scattered among fine-leaved vegetation or deposited over substrate in shallow, vegetated areas. Seasonal flooding in the Amazon basin drives reproductive cycles for many species, with spawning activity often correlating with the onset of the rising-water period when food becomes abundant and conditions for larval survival improve.
Breeding
Captive breeding records for Tetragonopterus manaos are not documented in the available literature, and the species is not established in the ornamental trade. Based on the reproductive mode typical for Characidae, breeding would involve egg scattering over fine-leaved plants or a similar substrate, with no parental care and a need to separate eggs or fry from adults to prevent predation.
Eggs in Tetragonopterus and related genera are generally small and hatch within a few days at Amazonian temperatures; fry are typically minute and require very small live foods at first — infusoria or commercial fry preparations — before graduating to larger items as they grow. Detailed breeding protocols for the genus are lacking in the hobby literature.
In the aquarium
Tetragonopterus manaos is essentially unknown in the aquarium hobby, and no dedicated care guides exist. Specimens are unlikely to be available through normal trade channels; the species was described only in 2018 and does not appear in commercial availability lists. Any husbandry notes below are therefore extrapolated from the ecology of the species and the general care requirements of the genus.
As a medium-sized, deep-bodied characid from the Amazon lowlands, T. manaos would require a spacious tank with soft to moderately hard, slightly acidic to neutral water in the 75–82 °F range, subdued lighting, and areas of cover among plants and driftwood. Given the genus's reputation as a capable predator of small fishes, tankmates should be of similar or larger size. A group of several individuals would allow natural social behaviour. Water quality and filtration should be maintained carefully, as all Amazonian stream fishes are sensitive to accumulated waste.
Conservation
Tetragonopterus manaos has not been assessed by the IUCN Red List; no evaluation has been published since the species' description in 2018. Its conservation status is therefore Not Evaluated (NE). The species is endemic to the Solimões River drainage of the Brazilian Amazon, a system under broad anthropogenic pressure including deforestation, sedimentation, and urban and agricultural runoff in the Manaus region.
As a recently described species with limited published ecological data, population size and trends for T. manaos are unknown. The broader Amazon basin supports extraordinary freshwater biodiversity and is a global conservation priority, but individual species within it — particularly those described recently and absent from the aquarium trade — receive little targeted attention. Ongoing ichthyological surveys and the continued commitment of Brazilian research institutions to Amazonian fish biodiversity are the primary tools likely to shed further light on this species' status.