Taxonomy & naming
Serrasalmus marginatus was described by the French zoologist Achille Valenciennes in 1837, as part of the same wave of Cuvier-and-Valenciennes descriptions that named many of the classic Serrasalmidae. It belongs to the family Serrasalmidae — the piranhas, pacus and silver dollars — within the order Characiformes, and to the genus Serrasalmus, whose roughly thirty valid species (per the Catalog of Fishes, Eschmeyer, California Academy of Sciences) are the solitary, territorial true piranhas, as distinct from the shoaling genus Pygocentrus.
Serrasalmidae taxonomy has been reworked substantially in recent decades — silver dollars have been split out of the old catch-all genus Myleus into Myloplus, Tometes and Ossubtus, and pacus separated from Colossoma — but Serrasalmus itself remains stable, and marginatus is a well-established member of it. The species is most reliably told apart from similar Serrasalmus by a distinctive thick, black, V-shaped marking on the caudal fin, a field mark noted consistently by hobby and identification sources.
Morphology
Serrasalmus marginatus shows the classic piranha silhouette in miniature: a deep, laterally compressed body, a blunt head with a strongly underslung lower jaw, and a single row of sharp-edged, interlocking triangular teeth in both jaws. FishBase records a maximum published length of 13 in total length for females, with males and unsexed individuals topping out smaller at around 10.5 in TL; Seriously Fish gives a more conservative typical adult size of about 8.5 in standard length, and fish mature at roughly 5 in.
The most useful identification feature is a bold, thick black marking in a V shape on the caudal fin, which helps separate this species from the many superficially similar small Serrasalmus found in the same rivers. Body colouration otherwise follows the familiar piranha pattern of a silvery flank, though as with its relatives, precise shade and intensity vary with mood, water chemistry and individual condition.
Habitat
Serrasalmus marginatus is native to the Paraguay-Paraná river basin of South America, recorded from Brazil, Paraguay, Uruguay and Argentina, and occurring specifically in the Rio Paraguay and Rio Paraná drainages. This puts it in a more subtropical setting than the classic Amazonian piranhas, with a correspondingly cooler thermal range.
Seriously Fish gives water parameters of roughly 72–77 °F, pH 6.5–7.0 and 5–15° dH — mild, moderately soft-to-hard water rather than the extreme soft blackwater sometimes assumed for piranhas generally. As a freshwater, benthopelagic species it is found through the water column in river channels and associated habitats of the Paraguay-Paraná system, and in the aquarium it does best with dense vegetation and strong water quality reflecting that river environment.
Feeding
FishBase describes Serrasalmus marginatus as feeding primarily on the fins of other fish, using a stealthy stalking approach and striking at the tail or anal fin of its prey rather than attempting to kill and consume it outright — a hunting style shared with other solitary Serrasalmus and quite different from the popular image of a piranha shredding whole prey. This is a specialised, opportunistic strategy that lets a comparatively small fish exploit much larger neighbours without direct confrontation.
In the aquarium, Seriously Fish recommends a varied diet that shifts with age: juveniles take bloodworm, tubifex and artemia readily, while adults need larger, tougher fare such as mussel, prawn, squid and whitebait. Mammalian or avian meat (such as beef heart or chicken) should be avoided, as it is poorly suited to the fish's nutritional needs and can cause long-term health problems — a caution that applies across piranha-keeping generally.
Mating
Detailed courtship and pairing behaviour for Serrasalmus marginatus is not well documented in the available literature. As a solitary, territorial species, wild spawning is presumed to follow the general pattern described for other true piranhas — a receptive pair coming together briefly around a cleared substrate site — but specific field or aquarium observations of courtship in this species are sparse.
External sexual dimorphism has not been documented for this species; there is no reliably reported colour or morphological difference between males and females.
Breeding
Like other true piranhas, Serrasalmus marginatus is presumed to be a substrate-spawning egg-layer rather than a mouthbrooder or bubble-nest builder, consistent with the breeding mode documented across the genus. However, Seriously Fish states plainly that the species is not bred in aquaria, and no aquarium or commercial spawning accounts for S. marginatus were found in the sources reviewed here.
Given the lack of direct breeding records, this account does not speculate further on egg number, incubation time or the specifics of parental care for this species — data is sparse. Fish available in the hobby should be assumed wild-collected rather than captive-bred.
In the aquarium
Serrasalmus marginatus is solitary and incompatible with tankmates, and Seriously Fish is explicit that it should be kept alone rather than in a group or a mixed community — a direct contrast to shoaling Pygocentrus species. It is rarely available in the hobby, so keepers who do source one should expect a wild-caught, somewhat sensitive fish that demands stable, high water quality.
A planted tank with dense vegetation gives the fish cover and reduces stress, and water should be kept in the mild range noted above (roughly 72–77 °F, pH 6.5–7.0, 5–15° dH) rather than assuming the warmer, softer conditions typical of Amazonian piranhas. As with all Serrasalmus, the jaws and teeth are genuinely capable of a serious bite, so fish should be moved by container rather than net, and hands kept clear at feeding time. Finally, piranhas of this genus are restricted, permit-controlled or banned outright in many U.S. states and countries because of invasive-species concern, so a keeper must check local and state law before acquiring one, and must never release a piranha into open water.
Conservation
Serrasalmus marginatus is assessed by the IUCN Red List as Least Concern, an assessment dated 22 December 2020, reflecting a wide distribution across the Paraguay-Paraná basin in Brazil, Paraguay, Uruguay and Argentina and no evidence of significant population decline.
Because the species is not bred commercially or in the hobby, any specimens reaching the aquarium trade are wild-collected, though it is described as rarely available, suggesting collection pressure specifically for the aquarium trade is low. Broader pressures on the Paraguay-Paraná system — damming, water-quality change and habitat modification — are the general long-term watch items relevant to fishes of this basin, even though S. marginatus itself is not currently considered threatened.