Taxonomy & naming
Serrasalmus nigricans was described in 1829 by Johann Baptist von Spix and Louis Agassiz, part of the wave of piranha descriptions that followed Spix's Brazilian collecting expedition of 1817–1820 and Agassiz's subsequent work on the resulting fish material. It belongs to the family Serrasalmidae — piranhas, pacus and silver dollars — within the order Characiformes, and to the large genus Serrasalmus, which the Catalog of Fishes (Eschmeyer, California Academy of Sciences) recognises as the most species-rich piranha genus, distinguished from the shoaling Pygocentrus by generally more solitary, territorial habits and by dentition and body-shape details used to separate the many similar-looking species.
Serrasalmus as a genus has a long history of taxonomic confusion, with numerous species described from limited material and later synonymised or re-split; S. nigricans is one of the earlier-named forms in the genus. It is native to and apparently restricted to the Amazon River basin in Brazil. The data available for this species is comparatively sparse next to better-known relatives such as S. rhombeus or S. maculatus, and no dedicated IUCN Red List assessment exists for it.
Morphology
Detailed morphological data specific to Serrasalmus nigricans is sparse in the available record, and no maximum length figure could be confirmed from FishBase or other sources consulted. Like other members of the genus, it can be expected to show the general Serrasalmus body plan: a laterally compressed, deep-bodied silhouette, a strongly underslung lower jaw, and a single row of sharp, interlocking tricuspid teeth in both jaws adapted for shearing flesh and fins.
Given the genus's difficulty, positive field identification of S. nigricans against other Amazonian Serrasalmus typically requires close attention to body depth, fin-ray counts and dentition rather than colour pattern alone, and this species is easily confused with congeners in the same range. Data on adult colouration and precise proportions is not confirmed here; treat any online photos labelled S. nigricans with caution given the genus's history of misidentification.
Habitat
Serrasalmus nigricans is native to the Amazon River basin in Brazil. Within that basin, Serrasalmus species as a genus typically occupy a range of lowland freshwater habitats — river channels, floodplain lakes, flooded forest and backwaters — with individual species often showing narrower habitat or tributary preferences than the wide-ranging Pygocentrus. No specific water-chemistry data (temperature, pH, hardness) for this species was found in the sources consulted.
As a benthopelagic, tropical freshwater fish from the Amazon basin, it can reasonably be expected to encounter the generally warm, soft-to-neutral conditions typical of much of that system, but precise figures are not confirmed for this species and are omitted here rather than assumed.
Feeding
No species-specific dietary study for Serrasalmus nigricans was found in the sources consulted, but the genus Serrasalmus as a whole is well documented as an opportunistic carnivore and scavenger: fish, fin and scale fragments taken from live prey, aquatic insects, crustaceans, and carrion make up the typical diet, with fin-and-scale nipping a recurring, well-documented behaviour across the genus rather than the wholesale predation of popular myth.
In captivity, Serrasalmus species are generally kept on a varied carnivorous diet — chopped prawn, mussel, squid, whitebait, earthworms and quality pellets for adults, with bloodworm, Tubifex and Artemia suitable for juveniles. Live vertebrate feeder fish and fatty mammalian or avian meats are best avoided on welfare and nutritional grounds, and there is no reason to expect S. nigricans to differ meaningfully from this genus-wide pattern.
Mating
No courtship or pairing behaviour specific to Serrasalmus nigricans was found in the sources consulted. Across Serrasalmus generally, reliable external sexual dimorphism is not well documented, and breeders typically rely on behavioural cues — pairing off and increased territoriality between a conditioned male and female — rather than visible sex differences. Given how rarely this species appears in the hobby, essentially nothing has been recorded about its mating behaviour specifically, and this account does not speculate beyond the genus-level pattern.
Breeding
Serrasalmus nigricans has not been documented as bred in the aquarium hobby, and the sources consulted note that only a few Serrasalmus species have been bred in captivity at all. True piranhas in general — including the better-studied members of this genus — are substrate nest-spawners: a pair clears and defends a site among roots or vegetation, the female releases adhesive eggs that are fertilised and guarded, most often by the male, and the eggs hatch within a few days. There is no reason from the available record to expect S. nigricans to differ from that pattern, but no direct observation of it spawning, in the wild or in captivity, was found.
Given the total absence of documented breeding accounts for this species, keepers should treat any claim of captive-bred S. nigricans with skepticism, and prospective breeders should expect to be working from genus-level inference rather than species-specific guidance.
In the aquarium
Serrasalmus nigricans is essentially absent from the mainstream aquarium trade, and the sources consulted specifically note it rarely appears in hobby circulation and is subject to legal restriction in many regions. Like other Serrasalmus, it should be treated as a solitary, territorial predator rather than a shoaling fish — genus guidance calls for keeping one individual per tank, since cohabiting Serrasalmus of similar size routinely fight, sometimes fatally. A single adult of a similarly sized congener is recommended a tank of at least roughly 120 x 60 x 23.5 in (around 120 US gal), with dense cover from driftwood and roots to give the fish a sense of security, and strong filtration for a heavy carnivorous bioload.
The jaws and teeth are genuinely capable of serious injury, so fish should always be moved by container rather than net, and hands kept clear at feeding time. Most importantly for a prospective keeper: piranhas of this genus are restricted, permit-controlled or outright banned across many U.S. states and a number of countries because of establishment risk, so local and state law must be checked before acquiring one, and a piranha must never be released into open water under any circumstances.
Conservation
Serrasalmus nigricans has not been formally assessed by the IUCN Red List and is therefore classified as Not Evaluated (NE) rather than assigned a threat category. It is restricted to the Amazon River basin in Brazil, and no population trend, abundance estimate or specific threat data was found in the sources consulted.
A U.S. Fish & Wildlife Service ecological risk screening for this species found no evidence of established populations or trade within the United States and rated its overall invasion risk as uncertain given the limited data available, while noting that possession or import of Serrasalmus piranhas is regulated or banned in numerous U.S. states. As with many single-basin, rarely traded Serrasalmus, the honest summary is that too little is known about S. nigricans specifically to say more than that it has not been evaluated and appears to face no documented trade-driven pressure.