Taxonomy & naming
Serrasalmus brandtii was described by the Danish zoologist Christian Frederik Lütken in 1875, based on material from Lagoa Santa in Minas Gerais, Brazil — the same karst lake region made famous by the Danish naturalist Peter Wilhelm Lund's fossil collections a few decades earlier, with which Lütken had close ties. The species sits in the family Serrasalmidae (piranhas, pacus and silver dollars) within the order Characiformes, in the genus Serrasalmus, whose members are the classic solitary, blade-toothed "true piranhas" distinguished from the shoaling genus Pygocentrus by behaviour as much as by fine anatomical differences in the teeth and humeral spot.
The species is endemic to the rio São Francisco drainage, and historical museum series — including juvenile-to-subadult specimens collected by Carl H. Eigenmann in the early twentieth century — show it was recognized in that basin long before modern taxonomic revision of the family. As with much of Serrasalmidae, the genus has undergone repeated revision at the Catalog of Fishes; this account follows the currently accepted combination Serrasalmus brandtii and does not attempt to "correct" it.
Morphology
Serrasalmus brandtii has the deep-bodied, laterally compressed, saw-bellied silhouette typical of true piranhas, with a blunt head, a strongly underslung lower jaw and a single row of interlocking, triangular, razor-edged teeth built for shearing rather than crushing. SeriouslyFish gives a maximum size of 8–10 in standard length, while FishBase records a maximum published length of 11.5 in SL and a maximum published weight of 978 g, making it a mid-sized member of its genus — smaller than the largest Serrasalmus species but still a substantial, powerfully built predator by the time it reaches adult size.
Colouration follows the common "silver piranha" pattern that gives the species its common name: a bright, flat-silver body, often with fine dark speckling across the flanks, and comparatively little of the vivid red or orange seen in Pygocentrus species. As in other Serrasalmus, colour and pattern shift somewhat with age, mood and water conditions, and there is no reliably documented external difference between the sexes.
Habitat
Serrasalmus brandtii is endemic to the rio São Francisco basin of eastern Brazil, one of the country's major river systems, running largely through Minas Gerais and downstream states. Within that basin it has been recorded from the das Velhas and Grande tributaries and from the upper reaches around Urucuia, and it has also become established in the man-made Pedra do Cavalo reservoir following introduction. Its type locality, Lagoa Santa, is a small lake system in the karst country of Minas Gerais that has long been a focus of Brazilian natural history collecting.
Water across its range is warm and covers a fairly broad chemistry window: SeriouslyFish reports 68–79 °F, pH 5.0–8.0 and hardness of roughly 36–357 ppm, indicating a fish that tolerates real variation in mineral content rather than one restricted to soft blackwater. This breadth is consistent with a species that persists in both river channels and reservoir habitat across a large drainage basin.
Feeding
Serrasalmus brandtii is an opportunistic carnivore rather than an indiscriminate flesh-stripper: documented diet centres on small fishes and fish flesh, taken both by active predation and scavenging. Juveniles are reported to accept bloodworm and earthworm readily, while adults take fish strips and larger invertebrates; notably, sources describe no mammalian or avian meat in its diet, distinguishing its feeding ecology from the broader, carrion-inclusive diet reported for some other piranhas.
In the aquarium it generally accepts fish flesh once acclimated, though newly imported or wild-caught individuals may initially take only live fish before converting to prepared foods. As with other Serrasalmus, keepers are advised against feeding fatty mammalian meat and against relying on live "feeder" fish, which carry disease risk and offer poor nutrition over time.
Mating
Detailed courtship behaviour for Serrasalmus brandtii specifically has not been documented in the sources available, but the species reproduces year-round in the wild with reproductive activity peaking during the wet season, matching the seasonal flood-triggered spawning pattern reported broadly across Serrasalmidae. As a solitary, highly aggressive and territorial species, pairing for spawning in the wild is presumed to be a brief, tightly choreographed event between otherwise separate individuals rather than a shoal-wide event.
No reliable external sexual dimorphism has been documented for this species; as in related Serrasalmus, sexes are not reliably distinguished by colour or fin shape, and any pairing behaviour would need to be inferred from temporary tolerance between individuals rather than appearance.
Breeding
True piranhas of the genera Serrasalmus and Pygocentrus are substrate egg-layers: a pair clears and defends a nest site among plants or roots, the female deposits adhesive eggs that are fertilised and then guarded and fanned, most often by the male, and the eggs hatch within a few days. Serrasalmus brandtii is presumed to follow this same reproductive pattern based on its genus and its documented year-round, wet-season-peaked reproduction in the wild.
However, the available sources are explicit that breeding has not been documented in aquaria for this species. Unlike Pygocentrus nattereri, which is bred by hobbyists and commercially, S. brandtii is not established in the trade as a captive-bred fish, and a prospective keeper should not expect to reproduce it at home; this account states that plainly rather than assuming a spawning mode it has not actually inferred from data on this species.
In the aquarium
Serrasalmus brandtii is solitary and highly aggressive, and it should be kept alone rather than in a group — unlike shoaling Pygocentrus species, a lone Serrasalmus in its own territory is the normal, settled state for the fish, not a stress condition. Sources agree on a large species tank: Aquarium Glaser recommends a minimum of roughly 120 US gal, and The Aquarium Wiki likewise specifies a 120-US-gallon minimum, rising further for adults exceeding 12 in. Recommended temperature runs from about 73–82 °F with a pH of roughly 5.5–7.5, sitting within the wider 68–79 °F / pH 5.0–8.0 range the species tolerates in the wild.
Feeding-wise, expect to offer fish meat, shrimp and mussel, with some individuals accepting pellets once settled; newly acquired fish may need live food initially before converting to prepared diets. The jaws are genuinely capable of a serious bite and the fish should be moved by container rather than net. As with all piranhas, check local and state law before acquiring one — Serrasalmus species are restricted or banned outright in many U.S. states and countries — and never release a piranha into open water.
Conservation
Serrasalmus brandtii is assessed by the IUCN Red List as Least Concern, an assessment dated 7 November 2018. As an endemic of the rio São Francisco basin, its stable status reflects populations that persist both in protected river reaches and in reservoir habitat such as Pedra do Cavalo, where the species has become established following introduction.
Because it is restricted to a single, though large, Brazilian drainage rather than distributed across multiple river basins like many wide-ranging Amazonian piranhas, its long-term outlook is tied specifically to the health of the São Francisco system — a heavily dammed and increasingly water-stressed basin — even though the species itself is not currently considered threatened. Commercial availability in the aquarium trade is described as occasional rather than steady, consistent with a regionally endemic species that is not farmed for the hobby.