Taxonomy & naming
Astyanax utiariti was described by Vinicius Abilhoa Bertaco and Valdenir Garutti in 2007 from material collected in the upper rio Tapajós drainage, Mato Grosso, Brazil. The type locality lies within the rio Papagaio system, a tributary of the upper Tapajós. Catalog of Fishes records the valid name as Astyanax utiariti Bertaco & Garutti, 2007.
The species was placed within Astyanax, one of the largest and most species-rich genera in Characidae. It was differentiated from members of the A. bimaculatus group by a combination of morphological characters: body depth 33.3–39.9% SL, predorsal distance 51.7–55.5% SL, caudal peduncle depth 12.0–13.4% SL, and head length 23.9–26.4% SL, along with distinctive pigmentation features. The original diagnosis noted a toothless maxilla and proportionally larger dentary teeth, fin-ray counts of Dorsal ii,9; Anal iii–iv,22–25; Pectoral i,11–14; Pelvic i,7.
Morphology
Astyanax utiariti reaches a maximum recorded length of 3 in standard length. It is a laterally compressed, moderately deep-bodied characid with the typical Astyanax body plan: a rounded snout, relatively large eye, and forked caudal fin.
The species is most readily identified by its pigmentation pattern. A horizontally oval black humeral spot sits behind the opercle, and a lozenge-shaped caudal-peduncle spot extends forward onto the middle caudal-fin rays, a combination that distinguishes it within the A. bimaculatus group. Scales display a reticulate pattern, and a dark longitudinal stripe runs along the midlateral surface. The maxilla is toothless, which is unusual for Astyanax and forms part of the original diagnosis.
Habitat
The species is known from the rio Papagaio of the upper rio Tapajós drainage in Mato Grosso, Brazil — a river system characterized by waterfalls, crystalline water, and swift rapids, with a substrate of stones, rocks, and sand, and bordered by riparian vegetation. Specimens have been taken in semilentic stretches along the left bank, suggesting the species exploits calmer microhabitats within an otherwise energetic riverine environment.
The Tapajós drainage is a clear-water river system of the Brazilian Shield, generally with low conductivity and slightly acidic to near-neutral chemistry. The upper drainage where A. utiariti occurs is associated with the Cerrado-Amazon transition zone in Mato Grosso, with a pronounced dry season. Water conditions in this region are typically soft and warm, consistent with other Astyanax from Tapajós tributaries.
Feeding
No detailed dietary study of Astyanax utiariti is available in the cache. Like other small characids in the genus Astyanax, it is presumed to be an omnivore, taking small invertebrates, insect larvae, and plant or algal material in the water column and at the surface.
In an aquarium setting, fish of this size and build generally accept dried flake and micro-pellet foods, as well as small live and frozen invertebrates such as brine shrimp nauplii, Daphnia, and micro-worms. A varied diet supports condition and colour in the aquarium, though specific dietary preferences for this species are undocumented.
Mating
No published mating behaviour study exists for Astyanax utiariti specifically. The genus is uniformly egg-scattering with no parental care, and males typically display to females with fin-flaring and lateral posturing. Spawning in Astyanax species generally occurs in open water or over fine-leaved vegetation, with eggs scattered and left to settle or adhere lightly among plants.
Given the species' riverine origin with seasonal rainfall patterns in Mato Grosso, spawning in the wild is likely tied to the wet season when temperature and water level rise. Whether any courtship colouration or behaviour distinguishes A. utiariti from related species is not documented in available sources.
Breeding
Astyanax utiariti has not been documented in captive breeding in sources available in the cache. The genus as a whole is egg-scattering and non-guarding: pairs broadcast adhesive or semi-adhesive eggs among fine-leaved aquatic plants, with no parental care and active predation of eggs by adults.
Breeders working with related Astyanax species typically condition adults on live and frozen foods, then move a small group into a separate breeding tank with fine-leaved plants or spawning mops, slightly cooler water, and a gentle current to simulate seasonal conditions. Eggs hatch in 24–48 hours at tropical temperatures, and the fry are free-swimming within a few days, initially taking infusoria or powdered fry food before accepting newly hatched brine shrimp. Whether A. utiariti has been kept or bred in the hobby is not confirmed.
In the aquarium
Astyanax utiariti is not established in the ornamental trade and does not appear to be available in the hobby. Data on its maintenance in captivity are absent from available sources. Based on its natural environment — crystalline, well-oxygenated upper Tapajós rapids with a rocky substrate — aquarists would likely need to replicate soft, near-neutral to mildly acidic water with good flow and high dissolved oxygen.
Being a small species at under 3 in SL, a group could be maintained in a medium-sized aquarium with open swimming space, minimal decor, and robust filtration providing current. Like other schooling characids it would benefit from being kept in groups of six or more. Should wild-caught specimens become available, the species would presumably adapt to standard characid care, though its specific requirements remain untested.
Conservation
Astyanax utiariti was assessed as Least Concern (LC) by the IUCN in 2018. The assessment reflects adequate population size and range within its known habitat in the upper rio Tapajós drainage of Mato Grosso, Brazil. No major range-wide threats were identified sufficient to elevate the species above Least Concern at the time of assessment.
Nevertheless, the species occupies a restricted range in the Brazilian Cerrado-Amazon transition, a biome under increasing pressure from agricultural expansion, soy cultivation, and associated deforestation in Mato Grosso state. Changes in water quality, sedimentation, and hydrological regime in Tapajós tributaries resulting from land use change represent potential future threats. Long-term monitoring of the species in its native habitat would be warranted given the pace of change in the Cerrado and upper Tapajós basin.