Taxonomy & naming
Ctenobrycon hauxwellianus was described by Edward Drinker Cope in 1870, based on specimens collected by the English naturalist and collector John Hauxwell during his surveys of the Peruvian Amazon. The species is placed in the genus Ctenobrycon, a small characid genus within the family Characidae; Eschmeyer's Catalog of Fishes (the authority for valid names) lists the combination as Ctenobrycon hauxwellianus (Cope, 1870), with the parenthetical authority reflecting transfer from Cope's original generic placement.
Ctenobrycon is a compact, historically somewhat unstable genus of mid-water tetras. Its species share the typical characid body plan — a compressed, laterally flattened form with the adipose fin that is the calling card of the family — and are grouped by a combination of meristic and morphological characters. The genus is native to South America, with species distributed across the Amazon and Orinoco drainage systems. C. hauxwellianus is the species most frequently encountered in the aquarium hobby from this genus, though it has never become a mainstream trade item.
Morphology
Ctenobrycon hauxwellianus is a small, slender-to-moderately deep-bodied tetra reaching approximately 2 in standard length. Like all characids it bears the family's hallmark adipose fin — a small, rayless fin between the dorsal and caudal — and has a complete lateral line. The overall body form is typical of mid-water, open-water characins: streamlined, laterally compressed, and well-suited for free swimming in still or slow-moving water.
Colouration is relatively subdued compared to the most commercially prominent tetras. The body is silvery with a slightly greenish or yellowish iridescence along the flanks, and a dark or dusky lateral stripe or blotch may be present toward the caudal peduncle. The fins are generally transparent to lightly pigmented. As with many small characins, males and females are similar in appearance, with females tending to be noticeably fuller-bodied when in breeding condition. Data on precise meristic counts are sparse in readily accessible literature.
Habitat
Ctenobrycon hauxwellianus is distributed in the upper Amazon basin, with Peru as the primary known range and records from the broader upper Amazon drainage. The type locality is in Peru, and specimens have been associated with the Ucayali river system and related Amazonian tributaries. This region is characterised by lowland rainforest draining into slow-moving blackwater or whitewater streams and flooded forest margins.
The species inhabits warm, typically soft and acidic water consistent with western Amazonian conditions. Water temperatures in the natural range fall within approximately 75–82 °F, and the soft, low-conductivity, mildly acidic conditions (pH around 5.5–6.5) typical of the upper Amazon are the presumed natural baseline. Like many tetras of this region, it is likely to frequent areas of moderate cover — submerged vegetation, roots, leaf litter — rather than fully open water, though detailed field habitat data for this species are sparse.
Feeding
Ctenobrycon hauxwellianus is an omnivore, as is typical for small mid-water Amazonian tetras. The natural diet is presumed to consist of small invertebrates — insect larvae, microcrustaceans, and similar small prey — along with algal or plant material opportunistically encountered in the water column and along submerged surfaces. The genus Ctenobrycon is characterised by small live and prepared foods in captivity.
In the aquarium the species accepts a range of small prepared foods including micro-pellets and quality flake, alongside frozen and live offerings such as Daphnia, brine shrimp, and bloodworm. A varied diet supports good colouration and breeding condition. Like most tetras, it does best fed small portions regularly rather than a single large daily meal, and benefits from live or frozen food at least a few times per week.
Mating
Ctenobrycon hauxwellianus reproduces by open egg-scattering, the ancestral reproductive mode of most characids. Males pursue and court receptive females, displaying alongside them and driving spawning runs into fine-leaved vegetation. There is no pair bond, no territory defence, and no parental care of any kind — the adults scatter and fertilise small adhesive eggs among plants or substrate and take no further interest in them.
Males in breeding condition may show slightly more intense colouration or leaner body condition relative to the obviously rounded abdomen of a ripe female. Spawning activity is stimulated by the standard triggers common to Amazonian tetras: a gradual temperature rise, a water change with slightly cooler, very soft water, and the presence of fine-leaved plants or spawning mops. Detailed hobbyist accounts of courtship behaviour in this specific species are rare in the accessible literature.
Breeding
Breeding Ctenobrycon hauxwellianus follows the standard protocol for small, egg-scattering Amazonian tetras. A conditioned pair or small group is placed in a separate spawning tank furnished with fine-leaved plants (Java moss, Cabomba, or synthetic spawning mops) and filled with very soft, slightly acidic water at approximately 79–82 °F. A mesh bottom or dense plant cover helps protect the eggs from the adults, which will consume them readily.
The eggs are small and adhesive, and hatch within approximately 24–48 hours at spawning temperatures. The fry are tiny and require infusoria or commercial liquid fry food as a first food before graduating to freshly hatched brine shrimp nauplii and finely crushed flake. The adults provide no parental care and should be removed promptly after spawning. Documented breeding in captivity for this species is not widely reported in hobbyist literature, reflecting its niche status in the trade, but the biology follows the predictable egg-scatterer pattern of the family.
In the aquarium
Ctenobrycon hauxwellianus is a peaceful, active tetra best kept in a group of at least six individuals in a well-planted aquarium. A tank of 23.5–31.5 in or longer suits a small school comfortably, with open swimming space in the midground and fine-leaved or feathery plants toward the back and sides. Soft, slightly acidic water (pH 5.5–6.5, very soft) and temperatures of 75–82 °F replicate the natural conditions of the upper Amazon and support best colour and health.
As a small, inoffensive characin it mixes well with other soft-water, peaceful community fish of similar size — other small tetras, dwarf cichlids, corydoras catfish, and small hatchetfishes are natural companions. It is not aggressive and will not harass tankmates or nip fins on peaceful fish. The species is rarely available from major retailers and typically appears only through specialist importers or hobbyist networks. Water quality and the provision of live or frozen foods alongside prepared foods are the main keys to keeping it in good condition.
Conservation
Ctenobrycon hauxwellianus has not been formally assessed by the IUCN Red List and carries a status of Not Evaluated (NE). No published conservation assessment has been located for the species. It is a minor species in the ornamental fish trade, not subject to the large-scale collection pressure that has affected more commercially prominent Amazonian tetras.
The upper Amazon basin of Peru, where this species is distributed, faces ongoing pressure from habitat conversion — deforestation, agricultural expansion, and associated sedimentation and water-quality change — that affects a broad range of Amazonian fish fauna. Without a formal range assessment or population data, the specific vulnerability of C. hauxwellianus to these pressures is unknown. For aquarists, sourcing captive-bred specimens when available is a sound default for any Amazonian species, reducing reliance on wild collection.