Taxonomy & naming
Serrasalmus auriventris was described by the German-Argentine naturalist Hermann Burmeister in 1861, placing it in the genus Serrasalmus within the family Serrasalmidae — the piranhas, pacus and silver dollars — inside the order Characiformes. The type locality is the Río Paraná in Argentina's Entre Ríos province, west of the city of Paraná, and according to the taxonomic reference Peces Criollos the species was subsequently documented by Koerber & Litz in 2008 but has not been reliably reported since; it is known essentially only from its original description and type material.
Serrasalmidae has been heavily revised in recent decades — silver dollars have been split out of the old genus Myleus into Myloplus, Tometes and Ossubtus, and the pacus separated from Colossoma — but Serrasalmus itself remains a large, still-being-revised genus of true, solitary piranhas distinguished from the shoaling Pygocentrus by dentition and body proportions. No name change is indicated for this species; it is retained here as Serrasalmus auriventris (Burmeister, 1861) pending any future Catalog of Fishes revision.
Morphology
No species-specific description of size or body proportions was found in the available sources. For context, Serrasalmus is a genus of comparatively modest-sized true piranhas: congeners such as S. maculatus reach around 10 in and S. compressus runs 8–10 in, and most species in the genus stay under 8 in, well short of the largest Serrasalmidae. Absent a documented figure for S. auriventris, its size is best treated as unknown but plausibly in that same modest range typical of the genus; this article does not assign a maximum length.
Like other Serrasalmus, it can be expected to show the genus's characteristic deep, laterally compressed body, an underslung lower jaw and a single row of interlocking, blade-like tricuspid teeth used for scavenging and for nipping fins and scales from other fish — but no photograph or morphological description specific to this species was available to confirm colouration or fin pattern.
Habitat
Serrasalmus auriventris is known only from the Río Paraná in Entre Ríos province, Argentina, west of the city of Paraná — a subtropical stretch of one of South America's largest river systems, well south of the Amazon basin. The USFWS ecological risk assessment for the species states it is known only from its type locality and has never been reported established outside its native range.
No water-chemistry data specific to this species was found. The Paraná basin in this reach runs subtropical rather than tropical, and river conditions there are typically cooler on average than the warm Amazonian rivers most piranhas are associated with; beyond that general basin context, this article does not assign specific temperature or pH figures for the species.
Feeding
No diet study specific to Serrasalmus auriventris was found. Serrasalmus as a genus are opportunistic carnivores and scavengers — taking small fish, fins and scales nipped from other fish, insects, crustaceans and carrion, with occasional plant or seed matter — rather than dedicated large-prey hunters, and there is no reason from the available sources to expect this species to differ. In an aquarium setting, genus-typical practice is to offer meaty foods such as fish fillet, prawn and quality pellets rather than live "feeder" fish, which carry disease and nutritional risk; this is a general genus guideline, not a documented account for S. auriventris specifically, since the species has essentially no aquarium history on record.
Mating
No courtship or spawning behaviour has been documented for Serrasalmus auriventris. Across the genus, Serrasalmus species are solitary and territorial rather than shoaling, and a conditioned pair is reported to isolate from other fish, clear a patch of substrate or vegetation, and become markedly more defensive of that site as spawning approaches — but this is genus-level information drawn from better-studied congeners such as S. maculatus, not an observation of this species. No sexual dimorphism has been described for S. auriventris.
Breeding
There is no record of Serrasalmus auriventris being bred in captivity, and given that the species is known only from a single historical Argentine locality with no confirmed modern specimens, that is unsurprising. For the genus generally, Serrasalmus are substrate egg-layers: a pair clears and defends a nest site among vegetation or cleared sediment, the female releases adhesive eggs that are fertilised externally, and the male typically guards and fans the eggs, which hatch in roughly two to three days at warm temperatures. This is a genus-typical mode reported for relatives such as S. maculatus, not a documented account for this species, and it should not be read as confirmation that S. auriventris breeds the same way in the wild or has ever done so in an aquarium.
In the aquarium
Serrasalmus auriventris has no documented aquarium-keeping history in the sources available, and it does not appear to be an established fish in the hobby. Prospective keepers should treat any fish offered under this name with caution given how poorly documented the species is, and should default to genus-typical care for Serrasalmus: a solitary predator kept one to a tank rather than in a group, since unlike the shoaling Pygocentrus, Serrasalmus species are territorial toward their own kind. A single fish needs generous swimming space, dense cover from wood or roots, and secure tankmates, if any, chosen for size and speed rather than for compatibility with a fin-nipper.
As with all true piranhas, the jaws are capable of a serious bite and the fish should be moved by container rather than net. Piranhas of this genus are restricted or banned outright in many U.S. states and a number of countries, so a keeper must check local and state law before acquiring one, and must never release a piranha into open water.
Conservation
Serrasalmus auriventris has not been formally assessed by the IUCN Red List and carries no status; it is treated here as Not Evaluated. The species is known only from a single type locality on the Río Paraná in Entre Ríos, Argentina, and the taxonomic reference Peces Criollos notes that no modern specimens are known to exist, which — while not itself an extinction risk assessment — underscores how little is known about its current status, range or abundance.
The USFWS ecological risk screening summary for the species found no evidence it has ever been introduced or established outside its native range. Given the near-total absence of recent records, any future conservation assessment would need fresh field survey work in the Paraná basin to establish whether the species persists and where.