Taxonomy & naming
Hoplerythrinus unitaeniatus was described by Johann Baptist von Spix and Louis Agassiz in 1829 from material collected in the Rio São Francisco basin of Brazil, the species' type locality. It sits in the small family Erythrinidae — the wolf fishes and trahiras — within the order Characiformes, alongside the genera Hoplias and Erythrinus, from which it differs primarily in tooth and fin detail; the Catalog of Fishes (Eschmeyer, California Academy of Sciences) recognises Hoplerythrinus as a monotypic-leaning genus historically treated as containing this single widespread nominal species, though FishBase notes it likely represents an unresolved species complex given how broadly it ranges across South America.
The genus name Hoplerythrinus combines the Greek hoplon ("weapon" or "armour") with a reference to Erythrinus, the type genus of the family; the specific epithet unitaeniatus is Latin for "single-banded," describing the one dark lateral stripe that runs along the flank and is one of the fish's most consistent field marks. "Trahira" and "jeju" are vernacular names shared loosely across the family in Brazil and the Guianas and are not exclusive to this species.
Morphology
The striped trahira has the elongated, near-cylindrical body typical of the Erythrinidae: a robust, faintly compressed torpedo shape, a broad flattened head and a large mouth armed with strong conical teeth built for gripping and swallowing prey whole rather than shearing flesh. FishBase records a maximum length of 10 in, but Seriously Fish gives a more commonly cited maximum standard length of around 15.5 in for aquarium and wild specimens, and keepers should plan around the larger figure rather than assume the fish will stay small.
Colouration is variable but the species is most reliably identified by the single dark horizontal stripe running from behind the gill cover toward the tail — the feature enshrined in its Latin name — set against an olive-to-brown body that can shift tone with mood and habitat. Like its relatives, it carries a highly vascularised, air-breathing accessory respiratory structure derived from the swim bladder, letting it gulp atmospheric air at the surface, a critical adaptation for the low-oxygen waters it favours.
Habitat
Hoplerythrinus unitaeniatus has one of the broadest ranges of any Neotropical characin-relative, recorded across the Amazon, Orinoco, Paraná and Rio São Francisco basins, the Magdalena River in Colombia, and coastal river systems in Guyana, Suriname and French Guiana. Within that range it favours still and slow-moving habitats — floodplain pools, swamps, oxbow lakes and vegetated backwaters — rather than fast-flowing river channels, often in water that is warm, soft and mildly acidic to neutral.
Seriously Fish gives a temperature range of 73–81 °F and a wide pH tolerance of 5.5–7.5, describing the species as highly adaptable to local water chemistry. Its air-breathing capacity is the key to this ecological niche: it can persist in shallow, weed-choked, oxygen-depleted water that would be uninhabitable for most fish, lurking near the surface among floating and marginal vegetation to ambush prey and to gulp air.
Feeding
The striped trahira is an unambiguous piscivore and ambush predator. In the wild it takes fish, aquatic invertebrates and any other animal prey it can engulf, striking from cover near the surface or amid vegetation rather than pursuing prey over distance. Its wide gape and strong conical teeth are built for seizing and swallowing whole prey headfirst, not for grazing or picking at food items.
In the aquarium, Seriously Fish reports it takes meaty foods readily — earthworms, prawns, mussels and strips of white fish — but tends to reject dried and manufactured foods outright. Any tankmate must be too large to be swallowed whole; feeding should never include fish it can fit in its mouth, since it will treat them as prey rather than company.
Mating
Little is documented in the available sources about courtship behaviour in Hoplerythrinus unitaeniatus specifically. Within the wolf-fish family Erythrinidae more broadly, several species are known to be substrate nest-spawners in which a male establishes and defends a spawning site — typically a shallow pit or a patch cleared among plants — that he continues to guard after the female has deposited eggs there. No species-specific pairing, courtship display or sexual dimorphism is documented for H. unitaeniatus in the sources available here, and the species should not be assumed to pair or court the way open-water shoaling characins do; data sparse.
Breeding
Consistent with the pattern reported for its relatives in Erythrinidae, Hoplerythrinus unitaeniatus is believed to be a substrate nest-spawner in which the male guards a pit or plant-associated nest and the eggs and fry within it, but Seriously Fish states plainly that the species is not currently bred in the hobby. No aquarium spawning accounts, egg counts, incubation times or fry-rearing details are available in the sources consulted for this article.
Prospective keepers should treat this as a species without an established captive-breeding pathway: any specimens in the trade are wild-caught, and there is no reliable hobbyist protocol to draw on. This should be stated honestly rather than extrapolated from unrelated breeding modes.
In the aquarium
This is a predator-only fish, not a community species. Seriously Fish describes it as solitary and intolerant of its own kind — two individuals housed together may fight to the death — though it can sometimes cohabit with other similarly sized, robust species in a sufficiently large tank. A single specimen approaching its reported maximum of 15.5 in needs a large aquarium with open swimming space, sturdy cover such as driftwood and floating or emergent plants to mimic its ambush habitat near the surface, and — critically — a tight-fitting, weighted lid, since the species is noted as an excellent jumper.
Water should be kept warm, in the 73–81 °F range, with a wide tolerable pH of roughly 5.5–7.5 reflecting its adaptability in the wild. Because of its air-breathing habit it benefits from ready surface access and should never be kept in a fully sealed or heavily restricted-airspace setup. Feeding must be exclusively meaty foods — it is likely to refuse pellets and flake outright — and any tankmate must be too large to be viewed as prey. This is a specialist's fish for a dedicated predator setup, not a beginner or community purchase.
Conservation
Hoplerythrinus unitaeniatus is assessed by the IUCN Red List as Least Concern, reflecting its exceptionally wide distribution across the Amazon, Orinoco, Paraná, São Francisco and Magdalena basins plus coastal drainages in the Guianas, and its evident tolerance of a broad range of habitat conditions, including degraded or low-oxygen water that few other fish can use. No major range-wide threats are documented in the sources available here.
As a widespread, adaptable species occupying marginal and floodplain habitats that are often lower priorities for conservation attention than main river channels, it is not a species of particular concern, though — as with most Neotropical freshwater fish — ongoing wetland drainage, damming and floodplain habitat loss across its range remain long-term watch items rather than documented current threats to this species specifically.