Taxonomy & naming
Serrasalmus spilopleura was described by the Austrian ichthyologist Rudolf Kner in 1858. It belongs to the family Serrasalmidae — the piranhas, pacus and silver dollars — within the order Characiformes, and to the genus Serrasalmus, the most species-rich genus of true piranhas, distinguished from the single-row-toothed, shoaling Pygocentrus by generally smaller adult size, a more solitary and territorial temperament, and subtler differences in tooth and body shape used to separate the genus's many similar-looking species.
Serrasalmus as a genus has a long history of taxonomic confusion, and S. spilopleura is one of the names that has at times been applied broadly to a complex of similar-looking, speckled Serrasalmus across South America; the Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the reference for which populations currently fall under this name versus close relatives such as S. maculatus. The common name "speckled piranha" refers to the dark spots or flecks scattered across the silvery flanks, most pronounced in juveniles and subadults.
Morphology
Serrasalmus spilopleura has the classic piranha profile: 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 that work like a pair of scissors. FishBase records a maximum published size of 10 in total length, with a common size closer to 10 in standard length — noticeably smaller than the large Pygocentrus piranhas, which is consistent with its more modest tank requirements relative to its larger cousins.
Colouration is silvery overall, deepening with maturity, with the dark speckling across the flanks that gives the species its common name. As in other Serrasalmus, colour and pattern vary with age, mood and water conditions, and are not always a reliable field mark for separating this species from its close relatives within the genus.
Habitat
Serrasalmus spilopleura is native to South America, recorded from the Guaporé River basin and the Paraná River basin, including Argentina, placing it partly in a more subtropical, seasonally cooler system than the classic Amazonian piranhas. FishBase describes it as a tropical, benthopelagic freshwater fish found across a fairly wide temperature and pH range within that system.
Water across its recorded range spans roughly 73–82 °F and pH 5.0–7.0, a broader and somewhat softer-to-neutral window than many aquarium accounts assume; because part of its range lies in the more temperate Paraná system, keepers should not treat it as a strict warm blackwater specialist without checking the population in question. As with other piranhas, submerged wood, roots and dense cover are likely important in the wild for a fish that behaves as an ambush forager rather than an open-water pursuit predator.
Feeding
Serrasalmus spilopleura is an opportunistic carnivore and scavenger, and — as with its relatives — a fin-nipper: hobby sources describe it taking fin and scale fragments from other fish as well as whole prey, insects, crustaceans and carrion, rather than behaving as a constant, indiscriminate predator. The "feeding frenzy" behaviour associated with piranhas in popular culture is a response to scarcity or a concentrated food source, not an everyday hunting style.
In the aquarium it takes frozen foods, earthworms and smelts readily, along with quality carnivore pellets. Keepers are cautioned against relying on live "feeder" fish, which carry disease and parasite risk and are nutritionally inconsistent; meaty prepared foods are the safer and more reliable diet.
Mating
Detailed courtship description specific to Serrasalmus spilopleura is sparse in the available literature, but breeding accounts for the genus Serrasalmus — documented in the closely related S. maculatus — describe a male selecting and preparing a spawning site among vegetation, with a period of pairing and site-clearing preceding the actual spawn. Hobby sources note that captive-bred S. spilopleura respond to a rise in temperature toward around 77 °F and softer, more acidic water near pH 6.5 as a spawning trigger, consistent with a seasonal or flood-linked cue.
External sexual dimorphism is not documented for this species; as with other Serrasalmus, breeders typically identify a compatible pair by behaviour — pairing off and territorial site-guarding — rather than by any consistent difference in colour or fin shape between the sexes.
Breeding
Serrasalmus spilopleura is a substrate egg-layer, not a mouthbrooder or bubble-nest builder. Hobby sources identify it as one of the few Serrasalmus species that has actually been bred in captivity, with a pair clearing and defending a spawning site among plants or fine substrate; the closely related S. maculatus — used here as a same-genus reference point for breeding behaviour — shows the male preparing and guarding the site, with adhesive eggs (broods reported in the hundreds to low thousands for the genus) hatching in roughly two to three days at warm temperatures, and the male becoming markedly aggressive toward intruders near the nest.
Because this is a solitary, territorial species outside of a compatible pair, breeding attempts in the aquarium require careful, cautious pairing in a large, well-covered tank rather than the group-conditioning approach used with shoaling Pygocentrus; unsuccessful pairings can result in one fish injuring or killing the other. Data specific to fry-rearing in S. spilopleura is sparse in the cache; general Serrasalmus practice is early feeding on infusoria followed by brine shrimp nauplii.
In the aquarium
Unlike the shoaling Pygocentrus piranhas, Serrasalmus spilopleura is usually kept as a solitary specimen — it is territorial and unpredictable toward conspecifics and tankmates, and group-keeping is only realistic in very large tanks (hobby sources cite roughly 1,000+ litres) with heavy cover and multiple sightline breaks to reduce aggression. A single adult is comfortable in a considerably smaller footprint, with one hobby source citing a minimum of around 75 US gal for a solitary fish, though bigger is always safer for a fish of this temperament.
Water should be kept warm (roughly 73–82 °F) and on the soft, mildly acidic to neutral side (pH around 5.0–7.5 depending on source), with strong filtration for a heavy carnivore bioload. This species is noted as prone to stress-related illness if kept lonely and under-stimulated despite its solitary preference — adequate cover, a settled routine and minimal disturbance matter as much as tank size. The jaws are genuinely capable of a serious bite, so the fish should be moved by container rather than net. Finally, as with all true piranhas, keepers must check local and state law before acquiring one, since piranhas are restricted or outright banned in many U.S. states and countries, and a piranha should never be released into open water.
Conservation
Serrasalmus spilopleura is assessed by the IUCN Red List as Least Concern, consistent with a wide-ranging South American characin that supports a commercial fishery and is also present in the aquarium trade. FishBase notes both commercial fishing and aquarium collection across its range in the Guaporé and Paraná basins.
No major species-specific conservation concerns are documented for S. spilopleura in the available sources. As with other Neotropical river fish, broader pressures on the Paraná and Guaporé systems — damming, water-quality change and habitat modification — are the relevant long-term watch items even though the species itself is not currently considered threatened.