Taxonomy & naming
Serrasalmus hastatus was described in 2001 by William L. Fink and Antonio Machado-Allison as part of a revision that untangled several deep-bodied, superficially similar piranhas from the lower Rio Negro region of Brazil. It sits in the family Serrasalmidae (piranhas, pacus and silver dollars) within the order Characiformes, in the large and taxonomically difficult genus Serrasalmus, whose valid species boundaries are governed by the Catalog of Fishes (Eschmeyer, California Academy of Sciences).
Within Serrasalmus, S. hastatus belongs to what researchers informally call the "compressed group" — a cluster of species (S. altuvei, S. geryi, S. compressus, S. altispinis and S. hastatus itself) united by an unusually flattened, laterally compressed body and a pointed snout, distinguishing them from the deeper-jawed, more heavy-bodied Serrasalmus typified by S. rhombeus. The species epithet describes exactly that snout shape; see Morphology and the etymology note below. It is a poorly known, rarely collected fish, and no common name has taken hold in the trade or the literature.
Morphology
Serrasalmus hastatus is a small piranha, reaching a documented maximum of only 6 in standard length, making it one of the more diminutive members of its genus. Its defining feature is a strongly compressed, almost blade-like body with a pointed, spear-shaped snout — the character that separates the compressed group from bulkier Serrasalmus species and that gives this fish its name.
Like other piranhas it carries a single row of interlocking, triangular, razor-edged teeth in each jaw, replaced in sets through life. Detailed colouration and fin-pattern descriptions have not been widely published for this species; the type series and subsequent records come from a small number of museum specimens rather than field or aquarium photography, so identification in the hobby should be treated with caution and left to specialists familiar with the compressed-group species.
Habitat
Serrasalmus hastatus is known only from the lower Rio Negro drainage in Brazil, with records from the confluence area of the Rio Branco and Rio Negro, Marara beach, the Maguari flooded forest, the mouth of the Rio Branco, the Rio Uraricoera, and the Rio Negro near its confluence with the Rio Solimões close to Manaus. This is classic blackwater and mixed blackwater-whitewater Amazonian habitat — tannin-stained river channels, flooded forest margins and sandy beach shorelines.
No published water-chemistry data exist specifically for S. hastatus. Its close relative S. altuvei, from similar habitat, is documented at 75–82 °F, pH 4.5–7.0 and hardness 2–15° dH, and the Rio Negro system generally runs soft and acidic; those parameters are a reasonable proxy but not a confirmed reading for this species.
Feeding
No diet studies have been published for Serrasalmus hastatus specifically. Members of the compressed group, including the well-documented S. altuvei, are understood to be fin- and scale-biters in the wild — opportunistic predators that take bites from other fish rather than swallowing them whole, alongside smaller prey items typical of Serrasalmus generally.
On the strength of that pattern, S. hastatus is very likely a similar fin-biter, though this remains an inference rather than a confirmed observation for this exact species. No aquarium feeding records exist; a keeper attempting to hold one would be extrapolating from congeners rather than following any established protocol for this fish.
Mating
No courtship or pairing behaviour has been documented for Serrasalmus hastatus. Solitary, territorial Serrasalmus species in general do not shoal, and any pairing for spawning would be expected to be brief and closely tied to seasonal water changes, as in other true piranhas, but this has not been observed or recorded for this species.
External sexual dimorphism has not been described for S. hastatus; data is sparse to nonexistent for this poorly known species.
Breeding
There are no breeding records for Serrasalmus hastatus in the wild or in captivity. True piranhas in genera Serrasalmus and Pygocentrus are, as a rule, substrate egg-layers: a pair clears and defends a spawning site among plants or roots, the adhesive eggs are guarded and fanned — most often by the male — and hatch within a few days. That general pattern is the best available inference for this species, but it has not been confirmed for S. hastatus itself.
This species has never been reported bred in aquaria, and given the total absence of aquarium husbandry records, that is unsurprising rather than notable. Any breeding claim beyond the genus-level pattern above would be speculation.
In the aquarium
Serrasalmus hastatus has no established presence in the aquarium hobby; there are presently no aquarium care records for this species. What can be said comes from its close relative S. altuvei, which is kept solitarily and aggressively, requires a minimum tank on the order of 120 x 45 x 17.5 in (roughly 65 US gal), and will fight conspecifics to the death if housed together — behaviour typical of solitary Serrasalmus rather than the shoaling Pygocentrus piranhas.
A prospective keeper should assume S. hastatus needs the same one-fish-per-tank approach, dense cover, and a diet of robust meaty foods (mussel, prawn, squid, fish fillet) once past the juvenile stage, while recognising this is an extrapolation, not documented fact for the species. As with all piranhas, local and state law should be checked before acquiring one — piranhas are restricted or banned outright in many U.S. states and countries — and a piranha of any species should never be released into open water.
Conservation
The IUCN Red List assessed Serrasalmus hastatus as Least Concern in 2018, reflecting its occurrence across a reasonably broad stretch of the lower Rio Negro drainage rather than any evidence of abundance or active study — data on population trends and threats for this species are essentially absent from the literature.
Its native range within the Rio Negro basin faces the general long-term pressures common to Amazonian blackwater systems, including any future hydrological change, deforestation-linked sedimentation and water-quality shifts, though none of these has been specifically linked to this species. Because it is not present in the aquarium trade, collection pressure is not considered a relevant factor for its conservation status.