Taxonomy & naming
Serrasalmus rhombeus was one of the first piranhas formally described, named by Linnaeus in 1766 and carrying the type locality "South America." It belongs to the family Serrasalmidae — piranhas, pacus and silver dollars — within the order Characiformes, and to the genus Serrasalmus, which the Catalog of Fishes (Eschmeyer, California Academy of Sciences) recognises as the largest genus of true piranhas, distinguished by a single row of interlocking, blade-like teeth and, in most species, a solitary rather than shoaling habit.
S. rhombeus is now understood to be a species complex: because its colouring and body proportions vary so much across its enormous range, older literature likely lumped several genetically distinct populations or cryptic species under this one name. The common names in circulation — redeye, black, white, spotted and yellow piranha — largely track this colour variation by locality and age class rather than describing separate taxa, and identification within Serrasalmus more broadly is difficult without close examination of dentition and body proportions.
Morphology
The black piranha has the deep-bodied, laterally compressed piranha silhouette, with a blunt head, a strongly underslung lower jaw and a continuous row of sharp, interlocking triangular teeth used like a pair of scissors — teeth that are replaced in sets rather than singly, so the fish is rarely without a fully functional bite. FishBase records a maximum length of 16.5 in standard length, making it one of the largest piranhas kept in the hobby, though most aquarium specimens are considerably smaller.
Colouration changes dramatically with age and appears to vary by population: juveniles are silver with dark speckling, and as the fish matures it typically darkens through grey and purple tones toward the deep blackish or slate colouring of large adults, sometimes with a notably red or orange eye — the source of the "redeye piranha" name. Growth is fast in the first year (SeriouslyFish notes roughly 6 in in year one) and slows markedly afterward to around 0.5–1 in per year, with a lifespan reported at over ten years.
Habitat
Serrasalmus rhombeus has one of the broadest ranges of any piranha, occurring across the Amazon and Orinoco river basins, the rivers of the Guiana Shield, and northeastern Brazilian coastal drainages. Within that range it is a benthopelagic, potamodromous fish of river channels rather than the still floodplain lakes favoured by some other serrasalmids, and hobby sources describe a preference for deep, well-oxygenated water with strong current.
FishBase gives a temperature range of roughly 73–81 °F and pH 5.8–7.0 for the species, while SeriouslyFish cites a wider aquarium tolerance of pH 4.5–7.5 and hardness 1–20°H at 72–82 °F, consistent with a fish that ranges across blackwater, whitewater and clearwater systems. In the aquarium, dim lighting and structure such as driftwood and roots are recommended to mimic the deeper, shaded water it favours in the wild.
Feeding
In the wild, S. rhombeus is an opportunistic carnivore, and FishBase's diet data lists small fish, crabs, insects, lizards and even mammals among items taken — a broader and more mammal-inclusive diet than the fin-and-scale-nipping omnivory typical of shoaling Pygocentrus species, reflecting its role as a larger, more solitary predator. Fishipedia notes strong fin-nipping and predatory behaviour as part of its feeding repertoire as well.
In the aquarium, SeriouslyFish recommends a varied diet of prawn, squid, whitebait, earthworm and mussel fed two to three times a week rather than daily, specifically cautioning against fatty mammalian or avian meat and against live "feeder" fish, which carry disease risk and add little nutritional value. As with other large piranhas, overfeeding and a monotonous diet are more of a practical concern for keepers than underfeeding.
Mating
Courtship and pairing behaviour in Serrasalmus rhombeus are not well documented, and unlike the commercially bred Pygocentrus nattereri, this species is rarely bred in captivity at all — SeriouslyFish records only a single well-known instance, at Duisburg Zoo in the 1970s, in a large 1,200-US-gallon planted aquarium. That scarcity of records reflects both the fish's solitary, aggressive temperament, which makes safely conditioning and pairing adults difficult in home aquaria, and the practical challenge of housing two large, territorial Serrasalmus together long enough for pairing to occur.
No reliable external sexual dimorphism is documented for the species; sources do not describe a consistent colour or fin difference between sexes, and the fish is not routinely sexed by hobbyists.
Breeding
Where breeding has been documented, S. rhombeus is a substrate egg-layer: in the Duisburg Zoo account, a pair spawned adhesive eggs among moss in a large, densely planted tank, and the parents defended the resulting brood as a nest site rather than scattering eggs freely into open water. This matches the general pattern for true piranhas (Pygocentrus and Serrasalmus), which clear and guard a nest rather than mouthbrood or build a bubble nest.
Notably, in that single documented case the parents' tolerance had limits: once the fry grew to roughly 6 in, the adults turned on and attacked their own offspring, illustrating just how territorial and aggressive this species remains even toward its own young. Breeding this species in the home aquarium should be considered exceptionally rare and difficult rather than a realistic goal for most keepers; almost all specimens in the trade are wild-collected rather than captive-bred.
In the aquarium
Serrasalmus rhombeus is kept strictly as a solitary specimen fish — every source consulted agrees there are no reliable reports of safely mixing it with tankmates, whether conspecifics or other species, and it should be assumed to view anything sharing its tank as a target or a threat. Given a maximum recorded length of 16.5 in SL, this is also one of the larger piranhas in the hobby, and a deep, roomy tank sized for that eventual size is essential; Fishipedia specifically recommends a deep, square-profile tank over a long, shallow one, reflecting the species' preference for depth over open swimming space.
Dim lighting, driftwood, roots and other cover help settle a naturally shy but powerful predator that in the wild favours shaded, deeper water. The jaws are genuinely formidable and this is not a fish to net casually — move it by container, and handle water changes and maintenance with the same caution given to any large, solitary predatory fish. As with all piranhas, ownership is restricted or banned outright in many U.S. states and countries; check local and state law before acquiring one, and never release a piranha into open water under any circumstances.
Conservation
The IUCN Red List assesses Serrasalmus rhombeus as Least Concern (2024 assessment), reflecting its very wide distribution across the Amazon, Orinoco, Guiana Shield and northeastern Brazilian coastal river systems and its recorded status as non-threatened across northern South America. FishBase's high resilience rating (population doubling time under 15 months) is consistent with a species facing no significant conservation pressure at range-wide scale.
The more relevant caution for this species runs through the aquarium trade and biosecurity rather than extinction risk: because almost all specimens are wild-collected rather than captive-bred, and because the species is large, predatory and restricted or banned in many jurisdictions, the conservation-adjacent concern is largely about preventing escapes or releases into non-native waters rather than about protecting wild populations, which remain broadly secure.