Taxonomy & naming
Semaprochilodus taeniurus was described by the French zoologist Achille Valenciennes in 1821, originally placed in the genus Prochilodus before later authors split off Semaprochilodus to hold the smaller-scaled, more strongly migratory "flagtail" species. It belongs to the family Prochilodontidae, a small Neotropical family of detritivorous characiforms closely allied to the larger Serrasalmidae (piranhas, pacus and silver dollars) and the wider characin radiation covered elsewhere in this atlas.
The genus Semaprochilodus is frequently confused in the trade with its close relative S. laticeps, the other widely traded "flagtail," and older aquarium literature sometimes lists both under the older combination Prochilodus taeniurus; the Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the authority for the current valid combination and this article follows it. The common names "flagtail prochilodus" and "red flagtail" describe the banded tail and dorsal fin that make the species instantly recognisable in dealer tanks.
Morphology
Semaprochilodus taeniurus has a deep, laterally compressed, almost oval body typical of the family, with a small, thick-lipped, protrusible mouth adapted for grazing rather than grasping prey. The overall body colour is bright silver, sometimes with a faint golden sheen, and the species' signature feature is its finnage: the caudal fin and the tall dorsal fin both carry bold, alternating black and red (or orange) bands, giving the fish its "flagtail" name and making it one of the more visually striking mid-sized characins in the hobby.
Adults are a genuinely substantial aquarium fish. Available size data (a maximum standard length of roughly 10.5 in reported from wild specimens, with total length commonly cited around 12 in) confirms this is not a small community-tank characin — it is a large-bodied, fast-swimming shoaler that needs to be sized for accordingly. Sexes are outwardly similar; no reliable external dimorphism is documented for the species.
Habitat
The species is native to the central Amazon basin in Brazil, where it inhabits the main river channel and its larger tributaries, moving between whitewater and clearwater systems as part of its seasonal migration. Like other Prochilodontidae, it uses the Amazon's flood pulse as the organising rhythm of its life: it feeds across the nutrient-rich, algae- and detritus-covered surfaces of flooded forest and floodplain lakes during high water, then forms immense shoals that migrate, salmon-like, into river channels and upstream tributaries as water levels shift.
Water across its range is warm and mildly acidic to neutral, with reported habitat temperatures of roughly 73–84 °F and pH in the range of 5.5–7.5. As a wide-ranging, actively swimming river and floodplain fish rather than a still-blackwater specialist, it tolerates a broader chemistry window than many Amazon characins, though aquarists should still avoid harsh or alkaline extremes.
Feeding
Semaprochilodus taeniurus is fundamentally a grazer, not a predator. In the wild it feeds on algae, biofilm and organic detritus scraped from submerged surfaces and flooded vegetation using its small, downturned, sucker-like mouth, supplementing this with fine particulate matter filtered from the substrate — a diet and feeding mode shared with its Prochilodontidae relatives and the detritivorous curimatas covered elsewhere in this atlas.
In the aquarium this translates into a herbivore-leaning omnivore that does best on a diet built around blanched vegetables (spinach, courgette, cucumber, lettuce), algae wafers and spirulina-based foods, with occasional frozen bloodworm or other meaty foods taken opportunistically. It is not a fin- or scale-eater and poses no predatory risk to tankmates its own size or larger; its feeding behaviour is entirely consistent with the grazing, algae-scraping habit typical of its genus.
Mating
Courtship and pair formation are not documented for Semaprochilodus taeniurus in the way they are for nest-guarding characins. What is documented is the species' broader reproductive strategy: like other Prochilodontidae, it is a mass migratory spawner, with huge shoals undertaking coordinated upstream movements timed to the rise of the flood-season water level, spawning communally in open water at river and tributary mouths rather than pairing off at a fixed site.
No external sexual dimorphism has been reliably documented for this species, and because spawning is a large-scale, open-water migratory event rather than a paired courtship display observable in captivity, aquarium and hobby literature has essentially nothing to add on individual mate choice or dimorphism beyond what applies to the genus as a whole.
Breeding
Semaprochilodus taeniurus is a migratory food-fish, and its reproduction reflects that: in the wild it undertakes long-distance spawning migrations covering many kilometres of river, releasing eggs and milt in open water during the rise of the seasonal flood rather than at a defended nest. This life history is essentially impossible to replicate in a home aquarium, and captive reproduction of this species is unrecorded in the hobby.
Where Semaprochilodus taeniurus is propagated at all, it is on a commercial or research scale, using hormone-induced spawning (analogous to the techniques used for related food-fish Prochilodus species) to trigger release of eggs from broodstock held in ponds or tanks — not a technique available to, or intended for, home aquarists. Prospective keepers should treat every specimen in the trade as wild-collected or farm-raised from hormone-induced commercial spawning, not home-bred, and should not expect this species to breed in a display aquarium.
In the aquarium
Semaprochilodus taeniurus is an active, fast-swimming shoaling fish that reaches a real adult size of around 12 in, and it should be sized and grouped accordingly: a large, long aquarium with open swimming space, strong filtration and a group of six or more individuals to satisfy its shoaling instinct. Kept singly or in too small a group it becomes territorial and can squabble with its own kind and with similarly shaped tankmates; in an adequately sized group it is reported as a peaceful, active midwater fish that does well with other large, robust Amazonian community species.
Because it is primarily a grazer, the tank benefits from some algae growth or regular offerings of blanched vegetables and algae wafers, and, as with pacus and other large characins in this atlas, keepers should commit to its full adult size before purchase — this is not a fish that stays small, and it should never be acquired on the assumption it can be rehomed easily once it outgrows a starter tank. As with all Neotropical fish covered here, it should never be released into open waters outside its native range.
Conservation
The IUCN Red List assesses Semaprochilodus taeniurus as Least Concern, consistent with its wide distribution across the central Amazon basin and its abundance as a shoaling, migratory species that supports both subsistence and commercial fisheries as well as the aquarium trade. As a widely distributed, still-abundant Amazonian species it is not currently a conservation priority.
The more relevant long-term concern, shared with other Amazon basin migratory fish, is the vulnerability of flood-pulse migrators to river fragmentation: dams and other barriers on Amazon tributaries can disrupt the long-distance spawning migrations this species depends on, even where standing population numbers remain healthy today. No population-specific threat data beyond the general LC assessment was found in the sources consulted for this article.