Taxonomy & naming
Hydrolycus scomberoides was described by Georges Cuvier in 1819. It belongs to the family Cynodontidae, a small group of predatory characins restricted to South America and closely related to the Characidae; the genus Hydrolycus currently holds a handful of valid species, all commonly called payaras. The genus name comes from the Greek hydro- (water) and lykos (wolf) — "water wolf" — a fitting description for a streamlined ambush-and-pursuit fish built almost entirely around its jaws and fangs; the species epithet scomberoides ("mackerel-like") refers to its fast, streamlined, mackerel-shaped body.
Several Hydrolycus species are traded under the same "payara" or "vampire fish" common names and are frequently confused with one another and with the larger Hydrolycus armatus; H. scomberoides is distinguished mainly by being the smallest member of the genus reaching maturity at 10–12 in SL, well under the roughly one-metre size some of its relatives attain. Field reports of individuals up to around 46 in total length exist but are exceptional outliers rather than typical adult size for this species. The name Cynodon scomberoides is treated as a synonym of this species in Eschmeyer's Catalog of Fishes. The name Cynodon scombroides is treated as a synonym of this species in Eschmeyer's Catalog of Fishes.
Morphology
The payara's body is elongated, laterally compressed and built for speed, with a forked tail and a silvery, almost featureless flank that gives it a strong resemblance to a marine jack or barracuda — hence the common name "freshwater barracuda." Its most striking feature is the pair of long, curved fangs projecting from the tip of the lower jaw; when the mouth closes, these fangs pass through two bony sockets in the upper jaw and snout, allowing the fish to close its mouth completely despite teeth that can be several centimetres long.
Adults of H. scomberoides typically measure 10–12 in standard length, making this the smallest of the payara species; other Hydrolycus can approach a metre. The skin and fins are notably delicate for a fish of its size and speed — a trait that matters enormously in captivity, since collisions with tank walls or decor readily tear the skin or snap the fangs. Colouration is a plain, unmarked silver over the body, with little sexual dimorphism reported in the literature or hobby accounts.
Habitat
Hydrolycus scomberoides is native to the Amazon River basin of South America, where it occupies open channels of large rivers as a benthopelagic fish — cruising the water column rather than sitting on the bottom or in dense cover. It favours clear-to-turbid, well-oxygenated, fast-flowing water typical of the main river channels rather than still floodplain lakes or heavily vegetated backwaters.
Water conditions across its range run warm, in the region of 75–82 °F, with a broad tolerated pH of roughly 6.0 to 8.0 and soft-to-moderately-hard water (cited hardness roughly 36-268 ppm). In the aquarium this translates into a demand for spotless, well-oxygenated water and strong current — sources recommend flow rates on the order of four to five times the tank volume per hour, reflecting a fish evolved for open, moving river water rather than a static tank.
Feeding
The payara is an obligate ichthyophage — a fish that eats fish and essentially nothing else in the wild — hunting by fast pursuit in open water rather than ambush from cover. In the aquarium it should be fed a varied diet of whole fish, fish fillet and other meaty aquatic foods; several sources are explicit that mammalian or avian meat such as beef heart or chicken should not be fed, as it is nutritionally inappropriate for a fish adapted to an all-fish diet. Most individuals can be trained over time to accept dead or frozen fish rather than requiring a constant supply of live feeders, which matters both for cost and for reducing disease risk from feeder fish.
Because of its predatory nature, any tankmate small enough to be engulfed will eventually be eaten; suitable companions are limited to fish too large or too fast to be viewed as prey. Feeding behaviour in a large, well-conditioned specimen can be trained to a degree, but the species remains, at its core, a specialised piscivore rather than a flexible omnivore.
Mating
Little is documented about courtship behaviour in Hydrolycus scomberoides specifically. What is known relates to its reproductive migration: the species undertakes a seasonal spawning migration in its native rivers, reported to run from roughly November through April, moving to specific reaches of large rivers to spawn in a pattern typical of many big-river characins. There is no aquarium-derived account of pairing, courtship display or nest preparation, because the species is not known to be bred in captivity.
Breeding
Hydrolycus scomberoides is a big-river migratory spawner, and captive breeding is not established. The species relies on a long-distance spawning migration through open river channels — conditions that cannot be replicated in a home or even most public aquarium setups — and no hobby or commercial breeding programme for this species is documented in the sources available. Every specimen in the trade is understood to be wild-collected rather than aquacultured.
Given this, prospective keepers should treat every individual as a wild animal taken from a river population, with all the added responsibility that implies for water quality, handling and long-term care, since there is no captive-bred supply chain to fall back on.
In the aquarium
The payara is one of the more genuinely difficult predatory characins in the trade, and most sources are blunt that it is unsuitable for typical home aquaria. Its combination of large adult size, high-speed swimming habit and extremely delicate skin means it needs a very large, long, obstacle-free tank — public-aquarium-scale housing is often cited as the realistic minimum for keeping this fish long-term — because a startled payara in a small or cluttered tank will collide with the glass or decor hard enough to split its snout or tear its skin, injuries from which it frequently does not recover. Water must be kept spotless, well-oxygenated and moving strongly, with flow on the order of four to five times tank volume per hour recommended, and any change in water quality is poorly tolerated.
Juveniles are reported as relatively cryptic and secretive, while adults become active, fast-cruising fish; a suitably enormous, current-rich tank can house a small group (three or more individuals cited), but the space and filtration demands scale up accordingly. Tankmates must be fish too large to be swallowed and fast enough to avoid being seen as target practice — in effect the payara is usually kept alone or with a very limited set of large, robust dither fish. This is a specialist fish for keepers with the tank size, plumbing and experience to match; it is not a fish to buy on impulse.
Conservation
Hydrolycus scomberoides is assessed by the IUCN Red List as Least Concern, reflecting its wide distribution across the Amazon basin and the absence of known major population threats specific to the species. As with many Amazonian river fish, broader pressures on the basin — damming of major tributaries, water-quality change and habitat disturbance — are long-term watch items for the ecosystems it depends on, even though the species itself is not currently considered threatened.
Because the aquarium trade in this species relies entirely on wild-caught fish rather than captive breeding, collection for the trade is a direct, if apparently currently sustainable, draw on wild populations; prospective keepers should weigh this alongside the species' demanding and often unforgiving care requirements before acquiring one.