Taxonomy & naming
Utiaritichthys esguiceroi was described by Pereira and Castro in 2014 in Neotropical Ichthyology, from specimens collected in the Serra dos Parecis region of the upper rio Juruena, part of the rio Tapajós drainage in Mato Grosso State, central Brazil. The species epithet honours André L. H. Esguícero, who collected the holotype and paratype material. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) lists it as a currently valid name within the genus Utiaritichthys, a small Neotropical genus erected by Miranda Ribeiro and placed in the family Serrasalmidae — the pacus, silver dollars and piranhas — within the order Characiformes.
Serrasalmidae taxonomy has been reshuffled considerably in recent decades, with silver dollars split out of the old catch-all genus Myleus into several narrower genera. Utiaritichthys was not affected by that particular split — it has long stood as its own distinct lineage — but it remains a poorly studied genus, with only a handful of described species, all restricted to fast-flowing habitats in the Amazon and Tapajós system. This species is distinguished from its congeners by a specific combination of meristic characters: 99–101 perforated lateral-line scales, 17–19 prepelvic spines, 20–21 postpelvic spines and 23–25 circumpeduncular scales.
Morphology
Utiaritichthys esguiceroi is a modestly sized, laterally compressed serrasalmid with the deep, disc-like body plan typical of silver dollars, built for holding position and manoeuvring in moving water rather than cruising open lake margins. FishBase records a maximum length of 10 in standard length, making it a relatively small member of the pacu-and-silver-dollar group compared to the true pacus, which can approach a metre.
The species is identified from its close relatives chiefly by scale and spine counts rather than by dramatic external colour pattern — 99–101 perforated lateral-line scales, 17–19 prepelvic spines, 20–21 postpelvic spines and 23–25 circumpeduncular scales, per the original description. Detailed published colouration and dimorphism data are sparse; no reliable sexual dimorphism has been documented for this species.
Habitat
The species is known from the rio Juruena, a major headwater tributary of the rio Tapajós in the Serra dos Parecis region of Mato Grosso State, Brazil — part of the southern Amazon basin. FishBase and GBIF occurrence records place it specifically in rapidly flowing water within this drainage, distinguishing its habitat from the quiet floodplain lakes and flooded forest more typical of many other serrasalmids.
Utiaritichthys as a genus is associated with rocky, fast-current stretches where its members graze algae and aquatic plants directly off submerged rock — a niche closer to that of a rheophilic grazing loach than a typical still-water silver dollar. Detailed water chemistry data for this specific locality has not been captured in the sources reviewed; as a Mato Grosso headwater stream draining ancient crystalline shield country, conditions are likely soft and circumneutral to mildly acidic, but that inference should be treated as provisional pending direct measurements.
Feeding
Utiaritichthys esguiceroi is herbivorous, and FishBase specifically documents its diet as aquatic plants of the family Podostemaceae together with filamentous algae, both grazed from submerged rock in the fast-flowing sections of its home river. Podostemaceae are a distinctive family of river-weeds adapted to cling to rock in whitewater rapids, and a fish specialised to graze them is adapted to a genuinely demanding current habitat.
This diet places the species firmly in the silver-dollar mould: a grazer of plant and algal material rather than an opportunistic omnivore or predator. In an aquarium setting this profile implies a need for vegetable-forward feeding — blanched leafy greens, spirulina-based foods and algae wafers — with any live plants at serious risk of being stripped, exactly as with other silver dollars.
Mating
No courtship behaviour has been documented for Utiaritichthys esguiceroi in the sources reviewed. The species was only formally described in 2014 from a small number of museum specimens, and no aquarium or field breeding observations appear to have been published. Data on pairing behaviour, spawning triggers or any external sexual dimorphism is simply not available; it would be irresponsible to speculate beyond what the genus-level pattern for silver dollars suggests (see Breeding).
Breeding
As with other silver dollars in Serrasalmidae, the expected reproductive mode for Utiaritichthys esguiceroi is egg-scattering over fine-leaved vegetation, with no parental care of eggs or fry — but this is inferred from the broader silver-dollar group rather than documented directly for this species. No aquarium or field breeding records for U. esguiceroi were found in the cache; it does not appear to be an established aquarium fish, let alone one bred in captivity.
Given its habitat specialisation in fast, rocky current and its status as a recently described, poorly known species, spawning in this genus is more plausibly tied to seasonal flow and temperature cues in its native rapids than to anything replicable in a typical home aquarium. Data on this species' breeding is sparse to non-existent, and any keeper attempting to breed it would essentially be working from genus-level inference alone.
In the aquarium
Utiaritichthys esguiceroi is not an established aquarium fish — it does not appear in the hobby literature reviewed, and no husbandry accounts, tank-size recommendations or trade history were found. What can be said comes from its biology: at a modest maximum of 10 in SL it is smaller than the large pacus that famously outgrow home tanks, but as a grazing silver dollar it should still be assumed to be a determined plant-shredder that would strip a planted aquascape, and as a rheophilic species from fast, well-oxygenated rock-strewn current, it would need strong water movement and high dissolved oxygen to thrive — conditions closer to a hillstream or riffle setup than a standard still-water community tank.
Silver dollars as a group are shoaling fish that do poorly kept singly, and a fast-current specialist like this one would likely need both companions and a robust, hardscaped tank rather than a delicate planted one. Given the complete absence of documented husbandry, any prospective keeper should treat this as an essentially untested aquarium subject and proceed cautiously, prioritising strong flow, open swimming space and vegetable-forward feeding based on its known wild diet.
Conservation
Utiaritichthys esguiceroi is assessed by the IUCN Red List as Data Deficient, with the assessment dated November 7, 2018 — reflecting how little is known about the species' population size, trend or specific threats since its description in 2014. It is known only from a restricted range within the upper rio Juruena and Serra dos Parecis region of the rio Tapajós drainage, Mato Grosso State, Brazil.
As a single-headwater endemic in a specialised rocky, fast-flowing habitat, the species would plausibly be vulnerable to the pressures affecting the wider Tapajós system — damming, mining-related sedimentation and agricultural runoff associated with Mato Grosso's expanding soy and cattle frontier — but no formal threat assessment has been published, and the sources reviewed do not document specific population data. Data Deficient status here genuinely reflects an absence of research rather than evidence of security.