Taxonomy & naming
Charax delimai was described by Naércio A. Menezes and Carlos A. S. de Lucena in 2014. The type locality is in the Rio Negro system, Brazil. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) is the authority for the valid name Charax delimai Menezes & Lucena, 2014.
The species belongs to the genus Charax Scopoli, 1777, within the subfamily Characinae of the family Characidae. Characinae is characterised by an inferior or subterminal mouth oriented for surface or mid-water predation, a strongly compressed, deep body, and in many species a high degree of body transparency — features that collectively suit an ambush-and-lunge feeding strategy. Charax is one of the larger genera in the subfamily, distributed widely across South American river basins, and Charax delimai represents one of the more recently formalised additions to the group.
Morphology
FishBase records a maximum standard length of 4 in for Charax delimai — a typical size for the genus, which ranges from small to medium-sized among the Characinae. Like other members of Charax, it is expected to show the family hallmarks: a deep, laterally compressed body, a subterminal mouth with visible teeth suited to seizing prey, a relatively large eye, and a scaled, somewhat translucent flank that gives many congeners a glassy appearance.
Specific meristic or colour data for this species beyond size are sparse in publicly available records, as the species is known mainly from type and referred museum material rather than from extensive aquarium observation. The body form in Charax typically features a concave dorsal profile ahead of the dorsal fin, a well-developed adipose fin, and a forked caudal fin — traits the species almost certainly shares.
Habitat
Charax delimai is recorded from the Rio Negro drainage of Brazil. The Rio Negro is the largest left-bank tributary of the Amazon and is renowned for its blackwater chemistry: dark, tea-coloured water stained by dissolved humic and fulvic acids leached from surrounding rainforest soils, with very low pH (typically 3.5–5.5 in blackwater reaches), negligible hardness, and low conductivity.
Specific microhabitat data for this species are not available in the published literature reviewed, but Charax species in general favour vegetated margins, submerged structure, and mid-water positions in slow or standing water, where they can ambush small fish and invertebrates. The Rio Negro system is species-rich and extraordinarily diverse in habitat type, ranging from main channel runs to flooded forests (igapó) and isolated lakes (restingas).
Feeding
Charax species are generalised predators, and Charax delimai is expected to share this strategy. The subterminal, toothed mouth of Charax is adapted for seizing small fish and larger invertebrates rather than grazing on plankton or detritus. Congeners have been recorded taking small characins, other small fish, and aquatic invertebrates in the wild.
In an aquarium setting, members of Charax typically accept small live or frozen prey such as bloodworm, brine shrimp, and small feeder organisms, and can sometimes be transitioned to meaty frozen foods. Pellets and flake are less readily accepted by wild-caught individuals, though captive-raised fish may be more flexible. Specific dietary data for Charax delimai from field studies are not documented in the sources available.
Mating
No published observations of courtship or spawning behaviour in Charax delimai are available. Within the Characinae broadly, reproduction follows the open egg-scattering pattern typical of characins: males pursue females and spawning occurs with eggs released into the water column or among fine-leaved vegetation, with no parental care extended to eggs or larvae after broadcast.
Seasonal reproductive activity in Amazonian characins is often linked to the flood cycle, with rising waters triggering spawning in many species. Whether Charax delimai shows a strongly seasonal pattern is not documented.
Breeding
Charax delimai has not been reported as bred in captivity in available hobby or scientific literature. General expectations for a member of Charax in the aquarium follow the typical characid pattern: egg-scattering spawners releasing small, adhesive or semi-adhesive eggs among fine-leaved plants or over open substrate, with incubation times of a few days at tropical temperatures and no parental investment.
For hobbyists attempting breeding, the soft, acidic, blackwater conditions of the Rio Negro — low pH (4.5–6.0), very soft water, and temperatures in the 79–86 °F range — would be the logical starting point, as these mirror the natural habitat and trigger spawning in many Rio Negro endemics. Data on fry size, first foods, and development rate for this species remain undocumented.
In the aquarium
Charax delimai is not an established aquarium species, and it rarely if ever appears in the hobby trade. It is known primarily from scientific collections rather than from aquarists' tanks. As a member of Charax it would be classed as a predator unsuitable for community tanks containing small fish.
For specialist keepers interested in Rio Negro Characinae, a biotope aquarium replicating blackwater conditions — very soft, highly acidic, darkly tinted water with minimal hardness, subdued lighting, leaf litter, and driftwood — would suit the species' natural environment. Small live or frozen prey would be the expected diet. Given the scarcity of hobby records, detailed care information is not available.
Conservation
Charax delimai is assessed as Least Concern (LC) on the IUCN Red List, with an assessment conducted in 2021 (revalidated 2025) by Flávio Lima (UNICAMP). The population trend is recorded as stable. The species' range within the Rio Negro system of Brazil means it falls within one of the largest, most intact tropical river systems on Earth, which provides some baseline protection simply by virtue of the drainage's remoteness and scale.
The Rio Negro headwaters and much of its basin are within or adjacent to protected areas, and the blackwater conditions that characterise the system make large-scale agriculture less viable than in the fertile várzea floodplains of the white-water Amazon. No specific threats are documented for this species in the IUCN assessment. Continued monitoring of the broader Rio Negro ichthyofauna is important given ongoing pressures in the Amazon basin from deforestation, mercury contamination from artisanal gold mining, and climate-driven hydrological change.