Taxonomy & naming
Hydrolycus tatauaia was described in 1999 by Mônica Toledo-Piza, Naércio Menezes and Osvaldo Santos, part of a revision that split the payaras into several distinct species after decades of most specimens being lumped under Hydrolycus scomberoides. It belongs to the family Cynodontidae — the payaras, or pike characins — within the order Characiformes, a small family of streamlined, fast-swimming predators closely related to the piranhas and pacus of Serrasalmidae but built for open-water pursuit rather than an ambush-and-bite strategy.
The genus Hydrolycus currently holds several valid species distinguished mainly by dentition, fin-ray counts and subtle proportional differences that make field identification difficult; H. tatauaia is one of the larger-bodied members of the genus. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the authority for the currently accepted name and synonymy, and GBIF's backbone taxonomy confirms the Toledo-Piza, Menezes & Santos, 1999 authorship as the valid combination — the species has not needed correcting to an older name.
Morphology
Hydrolycus tatauaia has the classic payara build: a long, laterally compressed, torpedo-shaped body, deeply forked tail and a silvery, almost chrome flank built for speed rather than manoeuvring in cover. The most striking feature is the dentition — the lower jaw carries two enormously elongated, needle-like canine teeth that project upward past the closed mouth and pass through sockets in the upper jaw and skull when the jaws shut, an adaptation for spearing prey sideways rather than crushing it.
FishBase records a maximum length of 23 in total length and a maximum weight of 2.7 kg for the species, making it one of the larger cynodontids kept (rarely) in captivity. The skin and fins are comparatively delicate for a fish of this size and speed, which matters directly for husbandry: a startled payara that strikes the glass or a tank wall can split its snout or damage its jaw, an injury that is difficult to treat and often fatal. Sexual dimorphism has not been documented in the cache material for this species.
Habitat
Hydrolycus tatauaia is native to tropical South America, recorded from the Amazon, Orinoco and Essequibo river basins. It is a migratory species, moving upstream in the wet season, broadly November to April, in step with breeding activity, and otherwise occupying open water in the main channels and larger tributaries of these river systems rather than flooded forest or vegetated margins.
Water across its range is warm, and SeriouslyFish gives a working range of roughly 75–82 °F, pH 6.0–8.0, and moderate hardness (36–268 ppm), broadly neutral to slightly alkaline rather than the strongly acidic blackwater sometimes assumed for Amazonian fish. High dissolved oxygen and a degree of water movement are recurring notes in husbandry accounts, consistent with a fast-swimming, open-water predator rather than a still-water ambush specialist.
Feeding
Hydrolycus tatauaia is an obligate piscivore — a pursuit-and-spear predator that uses its elongated canines to impale smaller fish before swallowing them, rather than a fish that bites off pieces the way a piranha does. In the wild its diet is essentially other fish, taken in open water. Newly imported specimens typically will only take live fish, though SeriouslyFish notes some individuals can be weaned onto dead whole fish or strips of fish and prawn over time — a genuinely difficult transition that many specimens never make.
Any animal sharing its water needs to be too large to be engulfed; this is not a fish that can be trusted with smaller tankmates under any circumstances, and its willingness to strike at anything moving that fits its mouth is a defining part of its biology, not an exaggeration.
Mating
What little is documented about reproduction in Hydrolycus tatauaia comes from its migratory behaviour: the species moves upstream in large river systems in a wet-season migration running roughly November to April, and spawning is associated with that migration rather than with any fixed nest site or territory. This pattern — a long-distance migratory spawning run timed to seasonal flooding — is typical of big-river characins and is fundamentally different from the paired, territorial courtship seen in substrate-spawning piranhas.
No courtship display, pairing behaviour or external sexual dimorphism is documented for this species in the available sources; the cache material does not support any specific claim beyond the migratory timing.
Breeding
Hydrolycus tatauaia is, honestly, not a species that is bred in the aquarium. Like other big-river migratory characins, its reproduction depends on long-distance upstream migration and the scale of a real river system, conditions that cannot be replicated in a home or even most public aquarium settings. There are no documented accounts of captive spawning for this species in the sources consulted.
Given its huge adult size, its need for enormous swimming space and its fragility around hard surfaces, H. tatauaia should be understood purely as a wild-collected, non-reproducing display fish in the trade — anyone acquiring one is not working toward breeding it, only toward keeping it alive and uninjured.
In the aquarium
Hydrolycus tatauaia is not a home-aquarium fish. SeriouslyFish specifies a minimum aquarium volume of 10,0.5 US gal or more, explicitly describing it as suitable only for large public aquarium installations, and lists the appropriate keeper experience level as public aquarists rather than home hobbyists. At an adult size approaching 23 in and 2.7 kg, and needing genuine swimming length rather than floor area, no domestic tank comes close to meeting its needs.
The species is also a prolific jumper, so any enclosure needs a completely secure lid, and its delicate skin and jaw structure mean it is prone to fatal snout and jaw injuries if it panics and strikes glass or hardscape — a real risk in any confined space, however large. Water needs strong movement and high dissolved oxygen, with SeriouslyFish recommending 30–50% weekly water changes to maintain quality for a large, high-metabolism predator. It is best kept alone or with similarly large fish it cannot swallow; small groups of three or more can work only where space is genuinely adequate. Given the specialist facility, feeding difficulty (many specimens refuse anything but live fish) and injury risk involved, this is a species that belongs in the hands of institutions, not home aquarists.
Conservation
Hydrolycus tatauaia is assessed as Least Concern by the IUCN Red List (assessment dated 2020), reflecting its wide distribution across the Amazon, Orinoco and Essequibo basins and the absence of evidence for significant population decline. It is not a species under particular conservation pressure at present.
The more relevant concerns for this species are indirect: as a large-river migratory fish, it depends on unobstructed upstream movement and intact seasonal flood dynamics, both of which are vulnerable to damming and hydrological alteration across its range, pressures that affect many big-river Amazonian and Orinocan fish even where the species itself is not individually threatened.