Taxonomy & naming
Charax pauciradiatus was described by Albert Günther in 1864. The valid name is recorded in Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) as Charax pauciradiatus (Günther, 1864); the parenthetical authority indicates the species was originally described in a different genus before being transferred to Charax.
The genus Charax belongs to the family Characidae, the largest family of freshwater fishes, which spans both South America and Africa. Within Characidae, Charax is placed in the tribe Characini and is one of several genera whose members are loosely grouped as glass tetras or glass characins, though that common name is applied inconsistently across related genera. No stable English common name is widely used for this species specifically.
Morphology
Charax pauciradiatus reaches a maximum recorded standard length of about 4 in. The body is laterally compressed and somewhat deep through the anterior trunk, tapering to a slender caudal peduncle — a shape shared with other Charax species and suited to rapid, short bursts of pursuit predation. The jaw is underslung and armed with teeth capable of gripping slippery prey.
Sexual dimorphism in this species is notable in size: males are reported to be considerably smaller than females, an unusual asymmetry in which the female is the larger sex. The flanks have the silvery, semi-translucent appearance typical of small characins, though no detailed colour description is available in the published literature cached for this article.
Habitat
The species is known from four river basins in Brazil: the Capim (Pará), Madeira (Rondônia), Aripuanã (Mato Grosso), and Negro (Amazonas). There are also reports from Suriname. The IUCN assessment notes the species is known only from Brazil in these four basins; FishBase records the Suriname occurrence but the IUCN range is more conservative.
Within those drainages, Charax pauciradiatus inhabits small black-water and clear-water rivers. FishBase records it as a benthic-pelagic freshwater species, occurring in water with pH 6.2–7.5, hardness around 25 dH, and temperatures of 72–81 °F (72–81 °F). The IUCN assessment describes the habitat as small rivers with no significant threats identified and notes that no protected areas within the range have been recorded.
Feeding
Charax pauciradiatus is carnivorous, and its diet shifts markedly with age. Juveniles under 2.5 in standard length feed primarily on Chironomidae (midge) larvae and Cladocera (water fleas). Adults switch to larger, more mobile prey: shrimps, Odonata nymphs (dragonfly and damselfly larvae), and juvenile fishes.
This ontogenetic diet shift is consistent with the jaw morphology and predatory habits seen across the genus. The species is likely an opportunistic ambush or active pursuit predator of small animals in the water column and near the substrate of small rivers. In the aquarium, suitably sized live and frozen invertebrates — bloodworm, brine shrimp, small river shrimp — would match the natural feeding profile.
Mating
FishBase records reproduction as occurring in the rainy season, placing Charax pauciradiatus among the many Amazonian characins whose spawning is tied to seasonal flooding. Eggs are deposited on vegetation, and incubation is reported at approximately 30 hours — a short incubation period consistent with small, warm-water species.
Beyond these basic parameters, detailed observations of courtship in Charax pauciradiatus are not available in the cached literature. Based on the biology of related Charax species, egg-scattering over submerged plants with no parental care is the expected pattern. The pronounced size dimorphism — females significantly larger than males — likely influences the mating dynamic, with larger females producing proportionally greater egg loads.
Breeding
The available data describe egg deposition on vegetation with approximately 30 hours of incubation at tropical temperatures. Like other open-water egg-scattering characins, Charax pauciradiatus provides no parental care, and the eggs and larvae are left to develop unguarded.
No aquarium breeding records for this species are available in the sources consulted. Given its predatory nature and modest maximum size, it would likely require live or meaty frozen foods to reach breeding condition in captivity, and small mesh or dense plant cover would be needed to protect eggs from consumption by the parents. Data on clutch size, larval development, and fry feeding in this species are sparse.
In the aquarium
Charax pauciradiatus is an uncommon aquarium fish rarely encountered in the trade, and little husbandry experience is documented for the species specifically. Its size of up to 4 in and carnivorous diet mean it should be kept away from very small tankmates that fall within prey size. A species appropriate to black-water or clear-water setups — soft to moderately hard, slightly acidic to neutral water in the range of pH 6.2–7.5 and 72–81 °F — would match the natural habitat parameters recorded by FishBase.
Based on the natural history, the species would likely do best in a spacious tank with subdued lighting, dark substrate, and plenty of cover in the form of driftwood and fine-leaved plants, replicating the small, tannin-stained rivers it inhabits. Tankmates should be large enough not to be eaten and ideally originate from the same Amazonian black-water biotope. Data sparse — hobbyist records of this species in aquaria are essentially absent from the literature.
Conservation
Charax pauciradiatus is assessed as Least Concern (LC) on the IUCN Red List, with the assessment published in 2022 based on a November 2018 evaluation. The species is described as frequent but not abundant within its range of four Brazilian river basins. No significant threats have been identified, and the IUCN notes no research programs are in place and that population trend is unknown.
The absence of recorded protected areas within the range is noted in the assessment as a potential gap, though it has not yet been weighed as a threat driver. As a specialist of small black-water and clear-water rivers within the Amazon system, the species would be vulnerable to any future degradation of those habitats — deforestation, agricultural run-off, and river channelisation all affect the small tributary systems that Amazon specialists like Charax pauciradiatus depend on.