Taxonomy & naming
Charax macrolepis was described in 1858 by Rudolf Kner, with the original name Epicyrtus macrolepis Kner, 1858. The species was subsequently transferred to the genus Charax Scopoli, 1777, which is the current valid placement. Kner's type material was collected from the Guaporé, a major tributary of the upper rio Madeira in what is now Brazil. Eschmeyer's Catalog of Fishes treats the valid name as Charax macrolepis (Kner, 1858), parenthetical because the original genus has changed.
The genus Charax belongs to the subfamily Characinae (family Characidae) and encompasses roughly two dozen species of elongate, dorsally humped characins distributed across tropical South America. Menezes and Lucena's 2014 taxonomic review of Charax, published in Neotropical Ichthyology, redescribed C. macrolepis and confirmed its validity, designating a female specimen of 3.5 in SL from the rio Guaporé tributary in the upper rio Madeira basin as reference material. Key characters of the genus include a predorsal gibbosity — the characteristic hump behind the head — a notch on the posteroventral cleithrum, and conical, unicuspid teeth suited to seizing slippery prey.
Morphology
Charax macrolepis reaches approximately 3.5 in standard length (SL), making it a mid-sized representative of the genus. Like all Charax it is elongate and laterally compressed, with the body deepest just behind the head, tapering rearward to a relatively narrow caudal peduncle. The dorsum shows the characteristic predorsal gibbosity of the genus — a raised profile that gives the fish a slightly hunchbacked silhouette when viewed from the side. The lower jaw protrudes beyond the upper, angling the mouth sharply upward and giving the fish an alert, predatory aspect.
The species name macrolepis ("large-scaled") refers to the noticeably large cycloid scales covering the flanks, which can number fewer rows than in finer-scaled congeners. The flanks are largely translucent in life, with musculature and internal organs visible through the body wall — a characteristic glass-fish transparency shared across Characinae that disappears shortly after death or preservation. Fin pigmentation is subdued: the fins are generally hyaline or faintly dusky, and there is no bold lateral stripe or caudal spot of the kind seen in some allied genera.
Habitat
The species is endemic to Brazil, where it inhabits the upper and middle rio Madeira basin, with type material from the rio Guaporé tributary. FishBase characterises it as a freshwater benthopelagic species occurring in tropical inland waters. The IUCN assessment notes its habitat as permanent rivers, streams, and creeks within inland wetland systems, with a stable and apparently widespread population across its Madeira-basin range.
Congeners in the genus Charax tend to favour slow-moving to moderately flowing water — blackwater or clear-water tributaries, creek margins, and seasonally flooded forest — where translucent flanks and ambush posture let them approach small prey without detection. Based on water-chemistry data for comparable Charax species and the broader Madeira system, C. macrolepis likely encounters soft, warm, moderately acidic to near-neutral water typical of Amazonian tributaries, though direct field measurements for this species are sparse in the available literature.
Feeding
Charax macrolepis is carnivorous, with FishBase recording a trophic level of 3.7 (±0.6), consistent with a diet centred on small fish and larger invertebrates. The upturned mouth and conical teeth are adaptations for seizing mobile prey, and the IUCN assessment notes the species as carnivorous and possibly piscivorous. In life, the combination of near-transparent flanks and a dorsally compressed, upward-angled jaw suggests an ambush-and-dart foraging mode rather than active open-water pursuit.
In captivity, similarly sized Charax species take live or frozen foods readily — small fish, large bloodworm, small earthworms, and prawns — and can generally be trained onto meaty prepared foods with patience. Because the genus is genuinely piscivorous, tankmates small enough to be swallowed are at risk and should be avoided.
Mating
No published field or captive observations of mating behaviour specific to Charax macrolepis are currently available in the accessible literature. Based on the biology of related Characinae, the species almost certainly reproduces by open egg-scattering, with fertilisation occurring as males and females drive alongside one another and release gametes simultaneously among fine-leaved vegetation or submerged debris. No courtship territory or lasting pair bond is expected, and males likely compete loosely for access to ripe females during spawning events.
Seasonal cues — rising water temperature, photoperiod changes, and the onset of the rainy season in the Madeira basin — probably trigger reproductive activity in wild populations, as is typical for Amazonian characins. Captive spawning of congeners has been achieved under similar stimulus conditions, with soft, warm water and a slight drop in barometric pressure mimicking the onset of rains.
Breeding
Charax macrolepis has not been bred in captivity to a degree documented in the hobby literature. By analogy with other Characinae it would be expected to scatter small, non-adhesive to slightly adhesive eggs among fine-leaved plants or open substrate, with no parental care whatsoever — adults would be expected to consume their own eggs and larvae if left in the breeding vessel.
A typical Characinae breeding setup involves a dimly lit tank with soft, slightly acidic warm water, fine-leaved plant clumps or spawning mops as egg-deposit substrate, and well-conditioned adults fed a varied diet of live and frozen meaty foods prior to pairing. Eggs would be removed promptly after spawning, and the small fry would require appropriately sized live foods — infusoria, micro-worms, or rotifers — before advancing to newly hatched brine shrimp. Given the genus's rarity in the trade, documented captive breeding would be a notable hobbyist achievement.
In the aquarium
Charax macrolepis is rarely encountered in the aquarium trade, and dedicated hobbyists who obtain it typically source it as a bycatch in shipments of wild-caught Amazonian fish. It is a predatory species that should not be trusted with tankmates it can swallow; appropriately sized companions would be robust, similarly sized characins or callichthyid catfish that occupy the lower water column and cannot easily be seized. The upturned jaw and large scales make it a visually striking specimen fish rather than a community-tank candidate.
Water conditions should replicate the soft, warm, slightly acidic to neutral chemistry of Madeira-basin tributaries: temperature approximately 77–82 °F, pH in the range of 6.0–7.0, and low to moderate hardness. Good filtration with gentle flow and ample cover — driftwood, leaf litter, and fine-leaved vegetation — helps the fish settle. Feeding presents the main husbandry challenge: live or frozen small fish and invertebrates are the most reliable first foods, though a transition to frozen prawns, large bloodworm, or carnivore pellets is achievable with patience. A spacious tank of at least 39.5 in is appropriate for an adult given its active hunting behaviour.
Conservation
Charax macrolepis is assessed as Least Concern (LC) on the IUCN Red List, with the assessment carried out on 4 March 2021 and published in 2023. The assessment was conducted by Lima and Reis, who noted a stable population trend, no obvious threats to the species, and a distribution encompassing the upper and middle rio Madeira basin — one of South America's major river systems. FishBase records a low fishing vulnerability index of 10/100, indicating the species is not a significant target of commercial or artisanal fisheries.
The broader rio Madeira basin faces pressures common to Amazonian systems — deforestation in the catchment, hydroelectric development, and agricultural runoff — but none of these are identified as driving declines in C. macrolepis specifically. The stable assessment reflects a species that is apparently secure within a large basin, though its rarity in collections and limited study mean that ecological requirements and population structure remain incompletely understood.