Taxonomy & naming
Utiaritichthys longidorsalis was described in 1992 by Michel Jégu, Lúcia Rapp Py-Daniel (as Tito de Morais in some citations) and colleagues, in a paper that also distinguished it from its only congener, U. sennaebragai. It belongs to the family Serrasalmidae — the piranhas, pacus and silver dollars — within the order Characiformes, and to the genus Utiaritichthys, a small Amazonian lineage of silver dollars endemic to the Madeira River basin. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the governing authority for the valid name and continues to recognise Utiaritichthys longidorsalis as a distinct, valid species within Serrasalmidae; GBIF's taxonomic record confirms the same placement.
The species epithet longidorsalis is Latin for "long-dorsal," referring to the notably longer base of the dorsal fin that distinguishes this species from U. sennaebragai — a directly diagnostic, comparative name rather than a descriptive flourish. The type locality is the Rio Aripuanã, a tributary of the Madeira, and the original description was published in French. In the aquarium trade the species turns up occasionally under the vague trade name "green pacu," a label that lumps it in with the unrelated, much larger Colossoma/Piaractus pacus and should not be taken as a taxonomic statement — this fish keeps the genus Utiaritichthys, not Myleus or Colossoma.
Morphology
Utiaritichthys longidorsalis has the classic silver dollar profile: a laterally compressed, disc-like body with a deep, rounded outline built for holding station and manoeuvring in current rather than for ambush predation. FishBase records a maximum length of 8 in standard length for males and unsexed individuals, a modest size compared to many of its Serrasalmidae relatives and well within reach of a large home aquarium. Diagnostic features noted in the original description and FishBase's meristic data include 28–31 prepelvic spines and 14 postpelvic spines along the ventral keel, and 33–35 circumpeduncular scales — technical counts that separate it from U. sennaebragai alongside the longer dorsal-fin base its name references.
Colouration is not extensively documented in the available sources, but the trade name "green pacu" suggests an overall silvery body with greenish overtones, consistent with the muted, schooling colouration typical of Serrasalmidae silver dollars living in open river channels. Data on this species' fine-scale colour pattern and any age- or sex-related variation is sparse; this account does not speculate beyond what the record supports.
Habitat
Utiaritichthys longidorsalis is endemic to the Madeira River basin in Brazil, with its type locality in the Rio Aripuanã, a Madeira tributary. FishBase describes it as benthopelagic, meaning it occupies both the water column and the zone near the riverbed rather than sticking strictly to open water or the substrate, consistent with a species foraging across submerged vegetation, driftwood and soft riverbed structure typical of Amazonian tributaries. The US Fish & Wildlife Service's ecological risk screening summary likewise characterises it as a soft-substrate river dweller with a benthic/pelagic lifestyle.
Specific water chemistry data for this species is not captured in the available sources, but as a Madeira-basin Amazonian characin it can be expected to inhabit warm, generally soft and mildly acidic river water, broadly consistent with other Serrasalmidae silver dollars from the region. This account does not assert precise pH or hardness figures beyond what is documented for the species itself.
Feeding
Silver dollars of the genus Utiaritichthys are herbivorous grazers, and the general Serrasalmidae silver-dollar pattern — confirmed across the group in hobby literature — is a diet dominated by aquatic and riverine plant material, algae, and some fallen fruit and seed material typical of flooded-forest characins, with comparatively little animal protein. This species is not documented to be a predator or scavenger in the mould of true piranhas; its jaw and dentition are built for cropping vegetation rather than shearing flesh.
In the aquarium, silver dollars in general take vegetable-based foods readily and eagerly — blanched greens such as spinach, cucumber and courgette, spirulina-based flakes or pellets, and algae wafers — supplemented with a smaller portion of protein such as quality flake or pellet food. Keepers should expect a Utiaritichthys silver dollar to behave the same way: a committed plant-grazer that will treat live aquarium vegetation as a food source, not merely decor.
Mating
No species-specific courtship or spawning behaviour for Utiaritichthys longidorsalis is documented in the available sources. Drawing on the well-established pattern for Serrasalmidae silver dollars generally, courtship in this group is not associated with elaborate pairing rituals or nest construction; conditioned schooling groups are typically brought into spawning condition with warmer, softer, more acidic water, and eggs are broadcast rather than laid at a defended site. No reliable external sexual dimorphism is documented for this species — data is sparse, and this account does not assert a colour or fin difference between sexes that has not been confirmed.
Breeding
There is no documented aquarium or hatchery breeding record specific to Utiaritichthys longidorsalis in the available sources. Based on the well-documented pattern for silver dollars in the genus's broader family, the expected mode is that of an egg-scatterer: a conditioned pair or group releases adhesive eggs over fine-leaved plants or vegetation, with no nest built and no parental care of eggs or fry. This is consistent with the general Serrasalmidae silver-dollar pattern, which is occasionally achieved in the hobby in soft, acidic water, though data confirming a spawning record for this particular species is sparse — it should be treated as expected-by-relatedness rather than confirmed observation.
Given how little is published on this species specifically, prospective breeders should treat any attempt as exploratory: there is no commercial hatchery record, and hobbyist accounts of successful spawns for Utiaritichthys longidorsalis were not found in the sources consulted.
In the aquarium
Utiaritichthys longidorsalis is a moderate-sized silver dollar — FishBase's maximum of 8 in SL means a mature specimen needs meaningfully more swimming room than a typical community tetra, but it stays well short of the true giants of the family. As with all silver dollars, this is a schooling fish that should be kept in a real group rather than alone or in pairs; a lone or thinly-kept individual is a stressed, skittish fish rather than a calm one. A spacious, open-swimming aquarium with robust, hard-scaped decor — driftwood, rockwork, floating cover — suits it far better than a delicately aquascaped planted tank, because this species, like its silver-dollar relatives, will graze and shred live aquarium plants down to bare stems.
Because hard data on this species' aquarium requirements is limited, keepers should default to conservative, well-documented Serrasalmidae silver-dollar husbandry: warm water, strong filtration for a sizeable bioload, and a diet weighted heavily toward vegetable matter (blanched greens, spirulina, algae wafers) with modest protein supplementation. This is not a beginner rarity readily available in the trade — it turns up occasionally under the vague "green pacu" label — so keepers should confirm they are getting the genuine Utiaritichthys species rather than an unrelated pacu before planning a tank around it.
Conservation
The IUCN Red List assessed Utiaritichthys longidorsalis as Least Concern, with the assessment dated 7 November 2018, reflecting a species that, while endemic to a single river basin, does not currently face a documented threat sufficient to elevate its risk category. FishBase and GBIF confirm the same status and the same restricted native range: the Madeira River basin of Brazil, with no evidence in these sources of a wider distribution or of introduction outside its native range.
Endemic, single-basin species like this one warrant a watch-and-wait posture even at Least Concern, since the Madeira basin has seen substantial hydroelectric dam construction and associated habitat change in recent decades; none of the sources consulted here document a specific population-level impact from that development on Utiaritichthys longidorsalis, so this account does not assert one. The US Fish & Wildlife Service's ecological risk screening summary for the species describes an uncertain, rather than elevated, ecological risk profile.