Taxonomy & naming
Moenkhausia phaeonota was described by William L. Fink in 1979, with the type locality recorded as the shores of an island below the Rio dos Peixes, Rio Arinos, Mato Grosso, Brazil. The holotype is catalogued as MZUSP 13793. Eschmeyer's Catalog of Fishes lists the valid name as Moenkhausia phaeonota Fink, 1979 — the authority is unparenthesised because the species was placed directly in Moenkhausia and remains there.
Moenkhausia is one of the largest genera in the family Characidae and encompasses well over a hundred described species distributed across tropical South America. Relationships within the genus have long been difficult to resolve owing to the prevalence of shared, possibly convergent characters; a number of species complexes remain poorly delimited. M. phaeonota has been noted as the only species in the genus carrying its particular combination of colour markings, which is unusual enough to make identification straightforward despite the genus being taxonomically sprawling.
Morphology
Moenkhausia phaeonota is a small tetra reaching approximately 1.5 in standard length. Like other members of the genus it is laterally compressed with the typical characid body plan: a relatively deep body, a small adipose fin between the dorsal and caudal fins, and a forked tail. The dentition and scale counts typical of Moenkhausia are present, though the species' original description provides the formal meristic data.
The species is distinguished within Moenkhausia by a colour pattern unlike any other in the genus. The exact appearance is described in the original description and is the diagnostic character; Aquarium Glaser have noted that it is the only Moenkhausia with this particular pattern, suggesting the markings are immediately useful for field and aquarium identification. In preservation and under stress, colour patterns in small characids can fade considerably, so freshly collected or well-conditioned aquarium specimens show the pattern to best advantage.
Habitat
The species is known from the upper Rio Tapajós basin in Mato Grosso, Brazil. The type locality is in the Rio Arinos, a major southern tributary of the Tapajós, where the holotype was taken from the shores of a river island. The upper Tapajós system drains the Mato Grosso plateau and comprises a mix of clear and blackwater rivers flowing through cerrado and transitional forest habitats.
Broader habitat notes indicate a preference for slow-moving rivers, swamp forests, and areas of flooded vegetation — the sheltered marginal habitats typical of many small characids in this part of South America. Water conditions in the upper Tapajós tributaries are typically soft and slightly to moderately acidic. Reported parameters for the species include temperatures of 75–82 °F and slightly acidic water around pH 5.8, consistent with the chemistry of Mato Grosso headwater streams. The flooded vegetation and swamp-forest margins provide both cover and a rich supply of small invertebrates and plant material.
Feeding
Moenkhausia phaeonota is an omnivore. Available notes indicate a diet of aquatic plants, algae, and small invertebrates — the broad opportunistic feeding mode typical of the genus and of small characids generally in the seasonally dynamic habitats of central Brazil.
In the aquarium, omnivorous Moenkhausia species accept a wide range of prepared and live foods. A diet combining quality flake or small pellets with occasional frozen or live invertebrate foods such as Daphnia or brine shrimp is appropriate. Including some vegetable matter — spirulina-based foods or blanched plant material — supports the omnivorous component of the natural diet. Because it is rarely encountered in the trade, specific aquarium feeding notes are limited.
Mating
Aquarium Glaser have noted that M. phaeonota displays behaviour reminiscent of the croaking tetras (subfamily Stevardiinae), which is unusual for a Moenkhausia. Courtship in egg-scattering characids typically involves males displaying to females with intensified colour and active chasing. In species with notable courtship colour, males may circle females and display their markings before spawning.
As with other open-water egg-scatterers, no pair bond is formed and no parental care is provided. Spawning in captivity is most likely to occur when well-conditioned adults are kept in appropriate, quiet conditions with fine-leaved vegetation or equivalent spawning substrate.
Breeding
Moenkhausia phaeonota is an egg-scattering open spawner in the typical Moenkhausia mode. Eggs are scattered among fine-leaved vegetation or over the substrate, are adhesive, and receive no parental attention; adults will consume eggs and fry if given the opportunity, so a separate breeding or egg-collection vessel is advisable for deliberate breeding attempts.
Specific data on clutch size, incubation period, or fry requirements for this species are not documented in available sources. By analogy with similarly sized Moenkhausia and small characids from comparable habitats, incubation at 79–82 °F would be expected to produce larvae within 24–48 hours, with the fry becoming free-swimming and seeking small live foods — infusoria, rotifers — within a few days. The species is rarely bred in captivity owing to its scarcity in the trade.
In the aquarium
Moenkhausia phaeonota is rarely available in the aquarium trade and is essentially a specialist fish encountered mainly in the collections of characid enthusiasts. Aquarium Glaser have noted its scarcity. When kept, it should be treated as a small, soft-water characid requiring warm, slightly acidic, well-filtered water — parameters consistent with the upper Tapajós basin conditions: 75–82 °F, pH around 5.5–6.5, and low to moderate hardness.
As a small schooling tetra it is best kept in groups of six or more in a well-planted aquarium. Dark substrate and subdued lighting will encourage natural behaviour and bring out the colour pattern. Tankmates should be similarly small and peaceful — other soft-water tetras, small Corydoras, or diminutive rasboras are suitable companions. Diet should include a mix of small prepared foods and regular live or frozen invertebrate items. Its behaviour — including the Stevardiinae-like displays noted by importers — may make it of particular interest to observers of characid behaviour.
Conservation
Moenkhausia phaeonota is assessed as Least Concern on the IUCN Red List, with populations considered stable at the time of assessment. The upper Tapajós basin, while relatively remote, faces ongoing pressure from agricultural expansion — particularly soy cultivation and cattle ranching on the Mato Grosso plateau — as well as hydroelectric development and associated habitat alteration in the Tapajós river system.
The species' restricted range within the upper Tapajós and its apparent scarcity in collections suggest that continued habitat monitoring is warranted. Its rarity in the trade means collecting pressure is not a significant current concern, but habitat loss through land-use change in the cerrado-forest transition zone of Mato Grosso represents the principal long-term threat to small, range-restricted fishes such as this one.