Taxonomy & naming
Astyanax nobre was described by Fernando Dagosta and Manoela Marinho in 2022 from the rio Salobra, a tributary of the rio Cuiabá within the upper rio Paraguai basin, in the municipality of Nobres, Mato Grosso State, Brazil. Eschmeyer's Catalog of Fishes treats the name as valid; the authority is given without parentheses because the species was described directly in Astyanax, where it remains.
Astyanax is one of the largest and most species-rich genera in the family Characidae, with the greatest diversity concentrated in the Neotropics. The genus was named for Astyanax, the son of the Trojan hero Hector in Greek mythology. At the time of its description, A. nobre was notable for reaching the smallest maximum standard length recorded for any species in the genus. The species is known only from its type locality, making it an endemic restricted to a single drainage.
Morphology
Astyanax nobre is a diminutive characid, with mature individuals measuring approximately 1 in standard length and the maximum recorded size reaching 1 in SL — making it the smallest known Astyanax at the time of description. Like most members of the genus, it has the typical characid body plan: a moderately compressed, fusiform profile, a single dorsal fin, an adipose fin, and the terminal mouth of an open-water generalist.
Detailed meristic and morphometric data were provided in the original description. The species' small adult size and restricted known range distinguish it readily from most other Astyanax in the rio Paraguai basin, where multiple congeners occur.
Habitat
Astyanax nobre inhabits the rio Salobra, a clear-water river in the upper rio Paraguai drainage (rio Cuiabá sub-basin) within Mato Grosso State, Brazil. The type locality is characterised by clear water, moderate flow, and a substrate of rock and sand — conditions typical of highland streams in this part of the Brazilian interior.
Despite its restricted known range, the species was recorded as one of the most abundant fishes at the type locality at the time of collection, suggesting it is well adapted to the local conditions and locally common within its narrow footprint. Its occurrence within the rio Paraguai basin places it in the upper Pantanal hydrological system, a region that experiences marked seasonal variation in rainfall and river level.
Feeding
No detailed dietary studies specific to Astyanax nobre have been published. As with most small Astyanax, the diet is inferred to be omnivorous, likely encompassing small invertebrates, algae, and organic matter taken from the water column and substrate — the typical feeding strategy of generalist characids in clear-water Neotropical streams.
In the aquarium, small Astyanax species generally accept a wide variety of foods including good-quality flake and micro-pellet foods, frozen or live small invertebrates such as brine shrimp nauplii and micro-worm, and finely ground prepared foods. Given the species' very small adult size, appropriately scaled food items are important from the start.
Mating
No breeding observations specific to Astyanax nobre have been published. Small Astyanax species are generally egg-scattering open spawners following the typical characid pattern: males pursue females and spawning takes place in open water or over fine-leaved vegetation, with adhesive or semi-adhesive eggs scattered among plants or left to fall to the substrate. No parental care is provided; adults may consume their own eggs if left with them.
Given the species' very small body size, spawning groups would presumably be kept in a well-planted aquarium with fine-leaved plants or a spawning mop, removing adults after spawning to protect the eggs.
Breeding
Captive breeding of Astyanax nobre has not been documented in the available literature. By analogy with other small Astyanax and with characids generally, the species would be expected to scatter adhesive eggs among fine-leaved vegetation with no parental care; eggs and fry would need to be separated from adults to prevent predation.
At the species' diminutive size — maximum 1 in SL — fry would be very small at hatching and would likely require infusoria or similar microscopic first foods before graduating to newly hatched brine shrimp. Data on incubation times, clutch size, and larval development remain unpublished.
In the aquarium
Astyanax nobre is not established in the ornamental fishkeeping trade and is unlikely to be encountered outside of specialist or scientific collections. Its very small adult size (maximum 1 in SL) would make it suitable for a nano or species tank if it were to become available, and its origins in clear, moderate-flow water suggest it would appreciate well-oxygenated conditions and a substrate of sand or fine gravel with some rocky cover.
As a schooling characid from a single restricted locality, any future hobbyist interest in the species would best be served by captive-bred stock. Water conditions matching its type locality — clear, well-oxygenated, neutral to slightly acid or alkaline depending on the local geochemistry of the rio Salobra — would be a reasonable starting point. No husbandry reports from aquarists are available.
Conservation
Astyanax nobre is assessed as Least Concern (LC) on the IUCN Red List. Despite being known only from a single type locality — the rio Salobra in Nobres municipality, Mato Grosso — the species was found to be one of the most abundant fishes at that site, which informed the Least Concern assessment rather than an automatic elevation to a threatened category on the grounds of restricted range alone.
Nonetheless, its extremely limited known distribution means any significant degradation of the rio Salobra — through agricultural runoff, deforestation of riparian zones, or hydrological modification — could affect the entire known population. The rio Paraguai basin and its upper tributaries face ongoing land-use pressures from agricultural expansion in Mato Grosso, and continued monitoring of the species and its habitat would be prudent given how recently it was described.