Taxonomy & naming
Gymnotus stenoleucus was described by Francisco Mago-Leccia in 1994, part of his broader revision of the genus that split what had been treated as a single wide-ranging "Gymnotus carapo" into more than twenty distinct species. It belongs to the family Gymnotidae within the order Gymnotiformes, the New-World electric knifefishes — bony fish, but unusual ones, defined by a weak electric organ used for electrolocation and communication and by a body plan built around a single long anal fin rather than a dorsal fin or a conventional tail.
Gymnotidae is one of several families in the order; it is distinguished from the wave-type Apteronotidae and Sternopygidae by its pulse-type discharge and by the presence of the electric eel, Electrophorus, among its close relatives. Despite the shared common name, Gymnotiformes have no relationship to the Asian featherback and clown knifefishes (Chitala, Notopterus) — the resemblance is convergent body shape, not kinship. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the authority for the valid name; no synonyms or other treatment history are noted in the sources consulted here.
Morphology
Gymnotus stenoleucus has the stout, laterally compressed, boldly barred body typical of the banded knifefishes, tapering from a fairly deep trunk — body depth roughly 7–9% of total length in the collections examined — to a whip-thin tail. Anal-fin ray counts run high, around 199–226, reflecting the very long ribbon-like fin that runs beneath most of the body and provides essentially all of the fish's propulsion, driving it forward or backward with equal ease. Precaudal vertebrae number 42–43 and caudal vertebrae 53–57. The eye is proportionately small, its diameter only 20–30% of the interocular distance, consistent with a fish that relies on electrolocation rather than vision to find its way after dark.
Museum specimens range roughly 2.5–5.5 in standard length, with head length making up about 60–66% of the preanal fin distance. The species is scaleless, like all gymnotiforms, and carries the electric organ running the length of its body beneath the skin, together with the sensory pores that read the field it generates. Recorded maximum size is 5.5 in total length — a small member of the genus. Sexes are not reported to differ externally in the sources available; no dimorphism has been documented for this species.
Habitat
The species is known from the upper Orinoco drainage of southern Venezuela's Amazonas state — including the Río Cataniapo, Cunucunuma and Casiquiare tributaries, with the type locality among them — and from adjacent Vaupés department in Colombia. It appears to be a narrow-range endemic of these upper Orinoco tributaries rather than a wide-ranging species.
It lives in slow-moving Venezuelan waters with dense marginal and submerged vegetation, the kind of dim, structurally complex habitat where an electric sense is a real advantage over eyesight. Like other Gymnotus it is a facultative air-breather, able to gulp atmospheric oxygen at the surface, which lets it tolerate the low-oxygen conditions common in still, vegetation-choked backwaters and flooded margins.
Feeding
Gymnotus stenoleucus is carnivorous, with a reported trophic level of 3.2 ± 0.4 placing it among the predatory fishes of its community. It is a nocturnal, bottom-oriented hunter that spends the day hidden and becomes active after dark, locating prey substantially through electrolocation in water where vision offers little help.
Where its diet has been characterised in the hobby literature, it takes bloodworms, midge larvae, tubifex, mysids, shrimp and earthworms — small live and frozen invertebrate prey consistent with what is known of banded knifefish feeding generally. Multiple small feedings are noted as preferable to one large one, in keeping with its habit of foraging steadily through the night rather than taking one large meal.
Mating
As in other Gymnotus, the electric organ discharge is understood to carry a social as well as a sensory function, and courtship in the genus is thought to involve electric signalling between potential mates alongside physical approach, with pulse type and rate distinguishing individuals and sexes. No species-specific account of courtship behaviour in Gymnotus stenoleucus was found in the sources consulted; the hobby literature notes only that the species is highly territorial, which likely complicates courtship in confined spaces and argues against keeping more than one individual together outside a deliberate breeding attempt.
Data on this species' reproductive behaviour is sparse, and nothing beyond general genus-level inference should be asserted here.
Breeding
Gymnotidae are unusual among fishes for showing real parental care, and in Gymnotus generally the male is understood to build and guard a nest — often a sheltered hollow or a mass of vegetation and floating material — into which the female spawns, with the male then guarding the eggs and larvae, typically on a seasonal, flood-linked cycle. Whether Gymnotus stenoleucus follows this pattern specifically has not been documented; no aquarium spawning of the species has been reported in the sources available.
The honest summary for this species is that breeding is undocumented rather than that it follows a known protocol: assume the genus-typical guarded-nest mode as the most likely pattern, but treat any specific claim about this species' spawning as unconfirmed.
In the aquarium
Gymnotus stenoleucus is essentially unknown in the aquarium trade and is not a species aquarists are likely to encounter for sale; what little is written about it in the hobby comes from single-species profile pages rather than widespread keeping experience. Where kept, it wants warm water, 72–82 °F, soft to moderately hard (roughly 2–12 dGH) and mildly acidic to neutral, pH 6.0–7.0, mirroring the vegetated, slow-flowing Orinoco tributaries it comes from.
Being scaleless, it needs the same caution around medications and water quality as other gymnotiforms — copper-based treatments should be avoided and other medications dosed conservatively. It is described as highly territorial and solitary, with group housing to be avoided, and as a nocturnal predator it is not a safe companion for anything small enough to be eaten after dark. A dim, well-covered tank with hiding places for daytime rest suits it best, and as with all banded knifefishes it should not be assumed community-safe.
Conservation
Gymnotus stenoleucus is assessed as Least Concern on the IUCN Red List, an assessment dated 13 August 2020. As a narrow-range endemic of the upper Orinoco tributaries in southern Venezuela and adjacent Colombia, its security depends on the condition of that relatively limited range rather than on a broad continental distribution.
No specific population threats are documented in the sources consulted here; narrow-range Orinoco tributary endemics generally face pressure from habitat alteration, deforestation and water-quality change in their basin, but no claim beyond the Least Concern assessment itself should be made for this species without further documentation.