Taxonomy & naming
Serrasalmus gibbus was described by the French naturalist Francis de Laporte de Castelnau in 1855, based on material collected during his South American expeditions; the holotype is held at the Muséum national d'Histoire naturelle in Paris (MNHN A-8648). It belongs to the family Serrasalmidae — the piranhas, pacus and silver dollars — within the order Characiformes, and to the genus Serrasalmus, the most species-rich of the true piranha genera, whose members are distinguished from the shoaling Pygocentrus by generally more solitary, territorial habits and, in many species, a body shape variously described as more "humped" or compressed.
The Catalog of Fishes (Eschmeyer, California Academy of Sciences) treats Serrasalmus gibbus as a valid species; the genus is taxonomically difficult, with many species historically confused on the basis of colour and body-shape variation that turns out to be individual or population-level rather than diagnostic, so identification of aquarium-trade specimens sold under this name should be treated with some caution. Its range overlaps that of the black piranha, Serrasalmus rhombeus, with which it has been studied as a sympatric, niche-partitioning species pair in Amazonian waters.
Morphology
Serrasalmus gibbus shows the classic piranha build: a deep, laterally compressed body, a blunt head with an underslung, powerful lower jaw, and a single row of interlocking, triangular, razor-edged teeth in both jaws. FishBase records a maximum size of 10 in standard length, making this a comparatively modest piranha rather than one of the larger Serrasalmus; aquarium literature notes that males are typically fully grown under 10 in SL, with hobby-observed sizes commonly cited in the 8–10 in range.
A resource-partitioning study of Amazonian populations found clear sexual size dimorphism, with females averaging notably larger and heavier than males — mean female length around 5 in versus roughly 3.5–4 in in males, and female weight nearly 50% greater than male weight on average. Colouration in the trade includes a golden-flanked form (the source of the "gold piranha" name), though as with other Serrasalmus, shade and pattern vary with mood, age and individual.
Habitat
The species is native to Brazil, documented from the Tocantins River basin (the type locality) and also recorded in the wider Amazon basin, where it occurs alongside Serrasalmus rhombeus. Within these systems it occupies typical lowland Neotropical river habitat — main channels, tributaries and associated floodplain waters of a large tropical drainage.
Aquarium-hobby sources describe water conditions in the mild-to-moderate range for a piranha: temperatures around 72–79 °F, pH roughly 6.5-7.5, and moderate hardness (about 6-8 °dH). This sits toward the cooler, closer-to-neutral end of the range seen across Serrasalmidae generally, and the cache does not support characterising this species' native water as extreme blackwater; treat the aquarium figures as a practical, trade-sourced guide rather than a site-specific limnological survey.
Feeding
Ecological work on Amazonian S. gibbus places it as a mid-level predator (trophic level around 3.6) that takes fish and invertebrates, consistent with the broader Serrasalmus pattern of opportunistic carnivory and fin-and-scale nipping rather than large-prey predation. As with its relatives, the fearsome reputation of the genus outruns the everyday reality: much of the diet is made up of smaller prey items, insects, crustaceans and scavenged carrion, with fin- and scale-biting of other fish supplementing rather than dominating the diet.
In the aquarium, hobby sources report that Serrasalmus gibbus readily takes fish flesh and will accept frozen or previously dead food rather than requiring live prey. A varied diet of fish fillet, prawn and other meaty foods is appropriate; live "feeder" fish should be avoided on general disease-and-nutrition grounds common to the whole family, and any tankmate small enough to be seen as prey, or slow enough to have its fins nipped, is at risk.
Mating
Specific courtship behaviour for Serrasalmus gibbus is not documented in the cache. Aquarium accounts note that this species is bred in captivity by raising a group of juveniles together and allowing them to pair off naturally as they mature, a practice consistent with the aggressive, one-fish-per-tank adult temperament typical of solitary Serrasalmus species, where forcing an unbonded adult pair together risks serious injury or death.
No external sexual dimorphism in colour or fin shape is reported; the resource-partitioning study's finding that females run larger and heavier than males is a population-level average from wild sampling rather than a field mark useful for sexing an individual fish.
Breeding
As a true piranha, Serrasalmus gibbus follows the substrate-spawning pattern documented for the genus: a bonded pair clears and defends a nest site among plants or root structure, the female deposits adhesive eggs that are fertilised by the male, and the eggs are guarded and fanned at the site, most often primarily by the male, until they hatch within a few days. It is not a mouthbrooder or a bubble-nest builder.
Practical Fishkeeping notes that the species is bred in the aquarium hobby, with breeders typically growing out a juvenile group and letting pairs form on their own rather than attempting to introduce two mature, unbonded adults directly — the latter carries a real risk of aggression given the solitary, territorial temperament of adult Serrasalmus. Detailed clutch size and hatch-time figures specific to this species are not available in the cache; data sparse beyond the general substrate-spawning pattern.
In the aquarium
Serrasalmus gibbus is a solitary, territorial predator, not a shoaling fish, and hobby sources are consistent on this point: it should be kept as a single specimen in a species-only tank, since it will attack tankmates — including conspecifics it has not been raised with — through fin-biting and predation. Practical Fishkeeping recommends a minimum of 75 US gal or a tank at least 59 in long, and The Aquarium Wiki emphasises that an extremely large tank with open swimming space is required as the fish matures; excellent filtration is essential given the fish's messy, meaty diet and bioload.
Because its adult size tops out at a comparatively modest 10 in SL, this is a more tractable piranha for a dedicated single-species setup than some of its larger congeners, but the bite and territoriality are real regardless of size — fish should always be moved by container rather than net. As with all piranhas, keepers must check state and national law before acquiring one, since Serrasalmus species are restricted or banned in many U.S. states and other jurisdictions, and a piranha should never be released into open water.
Conservation
The IUCN Red List assessed Serrasalmus gibbus as Least Concern on 7 November 2018, reflecting its occurrence across a substantial portion of the Tocantins and Amazon basin drainages in Brazil without evidence of significant population decline. FishBase and GBIF records corroborate a distribution centred on these major Brazilian river systems.
No species-specific threats or trade-pressure data are recorded in the cache; as with many mid-sized, non-flagship Serrasalmidae, the species is present in the aquarium trade but does not appear to face the large-scale habitat pressures (major damming, extensive deforestation-linked sedimentation) documented for some other Amazonian fishes at this time. Data sparse beyond the LC assessment; no updated reassessment is noted.