Taxonomy & naming
Acestrorhynchus nasutus was described by Carl H. Eigenmann in 1912. It belongs to Acestrorhynchidae, the pike-characin family, a group of slender predatory characins that was for decades folded into Characidae before molecular and morphological work split it out as its own family — a lineage that also includes the far larger genus Hydrolycus, the true payaras. Acestrorhynchus itself holds more than a dozen valid species, most sharing the same silhouette and differing mainly in snout proportions, fin colour and the extent of a dark lateral stripe.
The species epithet nasutus is Latin for "large-nosed" or "big-snouted," and it is apt: FishBase gives a snout that makes up roughly 43–48% of head length, unusually elongated even by the standards of a genus defined by long jaws. The type locality is Rockstone, Guyana. Within the trade the species is sold under names like "dwarf redtail barracuda," which — like most of the common names attached to this genus — borrows the marine barracuda's reputation for a fish that is convergent in shape only.
Morphology
This is a small, elongated, laterally compressed characin built for a burst-and-glide hunting style rather than sustained cruising. The head is drawn out into that long, pointed snout, and the lower jaw projects slightly, lined with a row of narrow, needle-like teeth suited to gripping small fish rather than shearing flesh. FishBase records a maximum size of 2.5 in standard length, making A. nasutus one of the smallest members of its genus and a fraction of the size its famous relative Hydrolycus reaches.
Colouration follows the family pattern: a bright silver body with a dark longitudinal band running from the snout through the eye and along the flank to the base of the caudal fin, which in several Acestrorhynchus species (including trade fish sold under this and related names) carries a reddish or orange tinge — the basis for the "redtail" name. The skin and fins are notably thin and easily damaged, a trait shared across the pike-characins that makes careful handling and netting a real concern rather than an afterthought.
Habitat
Acestrorhynchus nasutus is native to the Amazon and Orinoco River basins and the rivers of Guyana, with confirmed records from Brazil, Colombia, Guyana, Peru and Venezuela. FishBase classifies it as a benthopelagic freshwater fish of tropical South America, occupying the water column over open channels and along margins of larger rivers rather than dense vegetation or headwater streams.
Detailed water-chemistry data specific to this species is sparse in the available record. Aquarium-trade sources report it doing well at pH 6.0–7.5, 8–16 dH and 75–82 °F, consistent with the broader Amazon/Orinoco lowland pattern of warm, soft-to-moderately-hard, mildly acidic-to-neutral water; treat the wild figures as approximate rather than tightly documented, and favour stable, well-oxygenated conditions with some current, mimicking open river reaches.
Feeding
Acestrorhynchus nasutus is an obligate piscivore — a fish-eating specialist, not an opportunistic omnivore. In the wild it hunts small fish in open water using its elongated jaws and needle teeth to seize prey rather than crush it. Aquarium-trade accounts describe it accepting live fish readily, along with earthworms and aquatic insects, and note that some individuals can be weaned onto frozen or prepared meaty foods over time, though this is not guaranteed and dry foods are typically refused outright.
Because it is a genuine predator, any tankmate small enough to fit in its mouth is at risk regardless of how it is fed; the practical implication for feeding is the same one that shapes stocking decisions — this is not a fish that can be trusted around small companions, live-food dependence aside.
Mating
Courtship and pairing behaviour for Acestrorhynchus nasutus is not documented in the sources available for this account. What is recorded is that, in the wild, spawning is tied to the rainy season and rising water levels — the same flood-pulse cue that triggers reproduction across many Neotropical river fish, dispersing eggs and fry into newly inundated floodplain habitat rather than a fixed nest site. No description of pair formation, courtship display or external sexual dimorphism could be confirmed from the cache; this account does not speculate beyond that.
Breeding
Acestrorhynchus nasutus is, like its larger relatives the payaras, a big-river, flood-season spawner, and captive breeding is not established in the aquarium hobby. Available sources describe wild spawning coinciding with rainy-season water-level rises but do not document a nest, a defended site, or any form of parental care — consistent with an open-water broadcast strategy typical of large-river characins rather than the guarded, substrate-spawning behaviour seen in true piranhas.
There are no verified reports of this species being bred in home aquaria or commercial facilities. Prospective keepers should treat it, honestly, as a species that is wild-collected for the trade and not reproduced in captivity.
In the aquarium
At under 3 in, Acestrorhynchus nasutus is far more approachable in scale than the metre-long payaras of genus Hydrolycus, but it inherits the same fragile, high-strung temperament that makes the whole pike-characin group a specialist's fish rather than a casual purchase. It is skittish, prone to panic in a bare or brightly lit tank, and its thin skin and fins are easily damaged by collisions with glass or decor during a startled dash — so a long, open tank with soft, dim lighting and cover such as floating plants or driftwood near the surface is essential, not decorative.
Hobby sources recommend keeping it in a group of three or more, which appears to settle its nervous disposition, and pairing that with tankmates chosen purely on the basis of being too large to be seen as food — this remains an obligate piscivore, and any fish it can fit in its mouth is at risk. Recommended water parameters from trade sources are pH 6.0–7.5, 8–16 dH and 75–82 °F. Because it is not bred in captivity, aquarium stock is wild-collected, which is worth factoring into sourcing decisions.
Conservation
The IUCN Red List assesses Acestrorhynchus nasutus as Least Concern, reflecting its wide distribution across the Amazon and Orinoco basins and the rivers of Guyana. General threats noted for the species and its habitat include deforestation-driven habitat degradation, pollution and overfishing pressure across the Neotropical river systems it occupies, but none of these currently elevate its extinction risk.
Because the aquarium trade relies on wild-caught individuals rather than captive-bred stock, sustained collection pressure is a relevant long-term consideration even though the species is not currently threatened; there is no documented population-level concern specific to the aquarium trade in the available sources.