Taxonomy & naming
Serrasalmus scapularis was described by the Austrian ichthyologist Franz Steindachner in 1908. It has also been placed historically in the genus Serrasalmus, and Practical Fishkeeping's piranha guide notes the species' reclassification into Pristobrycon as part of the broader taxonomic revision of the family Serrasalmidae — the piranhas, pacus and silver dollars, within the order Characiformes. FishBase lists P. scapularis (Günther, 1864) as a synonym, which explains one of the vernacular names still used for the species in the trade, "scapularis piranha."
The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the governing authority for the currently valid genus placement; this article keeps Pristobrycon as the valid genus per that standard and per the cached sources. No type locality is specified in the available records. The species is rarely captured and rarely discussed even within specialist piranha literature, so taxonomic and distributional detail beyond the above is thin.
This fish was long listed as Pristobrycon striolatus; Eschmeyer's Catalog of Fishes now treats that name as a synonym of Serrasalmus scapularis, the valid name followed here.
Morphology
Serrasalmus scapularis has the compact, laterally compressed, diamond-shaped body typical of small serrasalmids, with the deep body, blunt head and strongly underslung, tooth-lined lower jaw shared across the piranha lineages. FishBase records a maximum length of 7.9 in total length, and other sources place typical size somewhat smaller, around 5.9–7.9 in TL — a modest size within the piranha group as a whole.
Sexual dimorphism is reported in the anal fin: males have a hooked anal-fin margin, while females have a straight border, a pattern noted independently by both OPEFE and Aquarium Glaser. Breeding condition is said to intensify body coloration, with more pronounced orange tints appearing on conditioned fish, though detailed colour description is not available in the cached sources.
Habitat
The species occurs in the Amazon and Orinoco River basins, including the northern and eastern rivers of the Guiana Shield in South America. Multiple sources agree it favours the upper reaches of main rivers and acidic, blackwater tributaries rather than the large lowland channels where more familiar piranhas are commonly collected, which likely explains why it is rarely captured and rarely seen in the trade.
The Aquarium Wiki gives a working water-chemistry range for captive care of pH 6.5–7.5 and 75–82 °F, though given the species' documented preference for acidic blackwater in the wild, water on the softer and more acidic end of that range — or somewhat lower in pH — is plausibly closer to natural conditions. Data on specific tributaries, seasonal flooding patterns and precise habitat structure is sparse.
Feeding
OPEFE describes Serrasalmus scapularis as an opportunistic carnivore whose diet shifts with the seasons: it feeds on fish during the dry season and switches toward fruits when the forest floods, a pattern consistent with the flood-pulse feeding ecology documented across many Amazonian serrasalmids. It also takes fins, terrestrial insects, crustaceans, scales and crushed seeds, and OPEFE specifically flags it as a dedicated fin-biter.
In captivity, Aquarium Glaser reports that the species feeds readily on standard piranha foods. As with other piranhas, live "feeder" fish should be avoided on disease and nutritional grounds even though the wild diet includes fish; meaty foods such as fish fillet, prawn and quality carnivore pellets are the safer captive equivalent, though no source gives species-specific feeding guidance beyond general acceptance of prepared piranha diets.
Mating
No courtship behaviour has been documented for Serrasalmus scapularis in the cached sources. OPEFE notes only that fish in breeding condition show more intense coloration, with pronounced orange tints, and that the anal fin provides the only reported external sexual dimorphism — hooked in males, straight-edged in females. Aquarium Glaser states plainly that "almost nothing is known for sure about this rare species," which applies as much to its mating behaviour as to any other aspect of its biology.
Given how rarely the species is kept or bred, any extrapolation from better-studied relatives such as Pygocentrus would be speculation rather than documented fact for this species, so none is offered here.
Breeding
Breeding biology for Serrasalmus scapularis is essentially undocumented. Aquarium Glaser states directly that spawning behaviour is largely unknown for this species. Based on its close relationship to other true piranhas (Serrasalmus and Pygocentrus), which are substrate-spawning egg-layers where a pair clears and guards a nest site and the male fans the adhesive eggs, a similar mode is plausible — but this has not been confirmed for S. scapularis specifically in any available source, and it should not be presented as established fact.
The species is not reported as bred in the hobby or in commercial aquaculture; it reaches the trade, when it does, as wild-caught stock supplied to wholesalers rather than as captive-bred fish. A prospective keeper should not expect to find captive-bred specimens or documented spawning reports for this fish.
In the aquarium
Serrasalmus scapularis is extremely rare in the aquarium trade — Aquarium Glaser notes it is supplied almost exclusively to the wholesale trade rather than sold as a routine retail species. The Aquarium Wiki gives a minimum tank size of around 55 US gal, water at pH 6.5–7.5 and 75–82 °F, and a reported captive lifespan of 8–12 years.
Both OPEFE and Aquarium Glaser agree the species should be kept solitary: it is a dedicated fin-biter, relatively peaceful as a juvenile but developing aggression rapidly with maturity, and its fin-eating habit makes it unsuitable for company even by piranha standards. As with all piranhas, handle only by container rather than net, and check local and state law before acquiring one — piranhas are restricted or banned outright in many U.S. states and countries, and this fish should never be released into open water under any circumstances.
Conservation
Serrasalmus scapularis has not been evaluated by the IUCN Red List (status: Not Evaluated), which is common among small, rarely-collected characins from remote blackwater tributaries where population data is scarce. Its preference for upper-river, acidic headwater habitat away from major population centres likely limits both direct fishing pressure and the kind of habitat disturbance affecting more accessible lowland rivers, but no formal assessment exists to confirm a trend either way.
Because the species is rarely captured and its distribution across the Amazon, Orinoco and Guiana Shield river systems is broad but poorly documented in fine detail, there is no specific evidence of population decline in the cached sources. Given the general pressures on Amazonian and Guiana Shield freshwater systems — damming, deforestation-linked sedimentation and mining runoff in some tributaries — ongoing monitoring would be useful, but that is a general regional concern rather than a documented species-specific one.