Taxonomy & naming
Ctenobrycon spilurus was described by Achille Valenciennes in 1850. The species is placed in the genus Ctenobrycon within the family Characidae, the large and diverse family of characins that spans South and Central America and Africa. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & van der Laan) is the governing authority for the valid name; the parenthetical notation around Valenciennes reflects the fact that the species was subsequently moved from the genus in which it was originally placed.
Ctenobrycon is a small genus of South American characins. Its position within the wider Characidae has been reviewed as the family's phylogeny has been progressively revised, but at the time of writing it remains allocated to Characidae in the broad sense. The silver tetra occupies the north-eastern corner of South America, from the Orinoco drainage south and east through the Guianan coastal systems.
Morphology
Ctenobrycon spilurus is a moderately deep-bodied, laterally compressed characin reaching approximately 3 in standard length. The body is silvery overall, giving rise to the common name, with the typical compact, fusiform shape of a schooling mid-water characin. Like other characids it carries an adipose fin between the dorsal and caudal fins.
A dark spot or blotch at the base of the caudal fin is a characteristic mark, as the species epithet spilurus (from Greek spilos, spot, and oura, tail) reflects. The fins are generally clear to slightly yellowish. There is moderate sexual dimorphism: females tend to be deeper-bodied and fuller when in spawning condition, while males are slimmer and sometimes show slightly more intense colouration.
Habitat
In the wild, Ctenobrycon spilurus inhabits slow-moving to still freshwater environments: rivers, streams, and floodplain pools across the north-eastern portion of South America. Its range encompasses the Orinoco River basin in Venezuela, coastal drainages of Guyana, Suriname, and French Guiana, and associated lowland waters including those of Trinidad.
It is a pelagic schooling species of tropical freshwater, found in the water column rather than tight to bottom structure. Recorded water temperatures span approximately 73–81 °F. The species is described as adaptable across a range of water chemistries, occurring in the soft, slightly acidic to neutral waters typical of Guianan lowland rivers. Floodplain pools and marginal areas with submerged or emergent vegetation figure prominently in its habitat, providing cover and feeding opportunities.
Feeding
Ctenobrycon spilurus is an omnivore in the wild, opportunistically taking invertebrates, plant matter, and organic debris available in its floodplain and river habitats. Insects, small crustaceans, algae, and vegetable material all likely contribute to the natural diet of a mid-water schooling characin in this region.
In the aquarium it accepts a wide variety of foods without difficulty: good-quality flake and small pellets, frozen and live foods such as Daphnia, bloodworm, and brine shrimp, as well as blanched vegetable matter. Its omnivorous flexibility makes it one of the easier tetras to feed well. Regular variety in the diet supports good condition and colour.
Mating
Like the great majority of characins, Ctenobrycon spilurus is an egg-scattering species with no pair bond and no parental investment. Courtship is brief and opportunistic: a male pursues a ripe female through fine-leaved vegetation or open water, and the pair release eggs and milt simultaneously in a characteristic side-by-side driving motion.
Spawning activity in this species is reported to occur in the morning hours. Males may pursue females actively, and group spawning with multiple males attending a single female is typical of the family. Conditioning groups of fish together on varied live and frozen foods brings both sexes into breeding condition.
Breeding
Ctenobrycon spilurus spawns freely once in good condition, scattering adhesive eggs among fine-leaved plants or over a spawning substrate. Reported clutch sizes are substantial — on the order of several hundred eggs per spawn — consistent with the egg-scattering strategy where high fecundity compensates for the absence of parental protection.
Adults will readily consume their own eggs, so breeders typically move eggs or adults after spawning to prevent predation. The eggs hatch within a few days at tropical temperatures. Fry are small at first and require appropriately sized first foods — infusoria or commercial liquid fry food — before transitioning to newly hatched brine shrimp and finely crushed dry foods. Water quality during early rearing is critical, as with all small characin fry.
In the aquarium
Ctenobrycon spilurus is described by hobbyists as hardy, active, and adaptable — qualities that make it suitable for community aquaria housing other similarly sized, peaceful species. It is a schooling fish and should be kept in groups of six or more to show natural behaviour and to reduce skittishness; isolated individuals or pairs tend to be nervous and colour up poorly.
A tank of moderate size — at least 23.5–31.5 in in length — with open mid-water swimming space suits the species well. Planted sides and background with open central lanes reflect its natural floodplain-pool environment. Water conditions of around 73–81 °F and a pH near neutral (around 6–7 is cited as suitable) cover the range it encounters in the wild. The species is not demanding about water chemistry and tolerates a broader range than many characins. Good filtration and regular water changes are standard. Suitable tankmates include other robust tetras, peaceful barbs, rasboras, and small bottom dwellers. It is not a fin-nipping species by reputation, though active individuals in an underfed school may pick at long-finned tankmates.
Conservation
The IUCN Red List assessed Ctenobrycon spilurus as Least Concern (LC) in March 2021, reflecting its broad distribution across multiple river systems in north-eastern South America and its apparent stability within that range. The species occupies the Orinoco basin, the coastal drainages of the Guianas, and associated lowland systems — a range encompassing several countries and a diversity of habitats that provides resilience against localised threats.
No major range-wide pressures are identified that would qualify the species for a threatened category under current assessment criteria. As with all freshwater fishes of lowland South America, ongoing deforestation, agricultural run-off, and changes to river hydrology remain background concerns for the broader fish fauna of the region, but Ctenobrycon spilurus is not currently considered at elevated risk.