Taxonomy & naming
Gymnotus bahianus was described by Campos-da-Paz and Costa in 1996 and belongs to the family Gymnotidae, within the order Gymnotiformes — the New-World electric knifefishes. The genus Gymnotus is the banded knifefish genus: stout, boldly barred, pulse-type electric fish, and the only gymnotiform genus to range north out of South America into Central America. Gymnotus bahianus is a member of the wider Gymnotus carapo species-complex that has been progressively split into many geographically restricted species over the past three decades, of which this is one, named for its home state of Bahia.
As Gymnotiformes, these fish are bony but unusual: they are defined by an electric organ and by a ribbon-like anal fin used for locomotion, and despite the shared common name "knifefish" they are not related to the Asian featherback and clown knifefishes (Chitala, Notopterus), which belong to an entirely different order and resemble Gymnotus only by convergence. The Catalog of Fishes (Eschmeyer, CAS) is the authority for the valid name; FishBase lists no older combination, confirming Gymnotus bahianus as the currently accepted name.
Morphology
Gymnotus bahianus reaches a recorded maximum of about 11 in in total length. Like other banded knifefishes it has an elongate, laterally compressed body, no dorsal fin and no distinct tail fin, with propulsion supplied almost entirely by a long anal fin run as a travelling wave along the underside of the body — allowing it to move forwards and backwards with equal ease. The skin is scaleless, a trait shared across the family that leaves these fish more sensitive to water quality and to certain medications than scaled fish.
Running along the body is the electric organ that gives the order its name, paired with electroreceptive pores across the skin. In Gymnotus this organ produces a pulse-type electric organ discharge — a train of brief, stereotyped pulses, rather than the continuous wave-type signal of families like the ghost knifefishes (Apteronotidae). The specific coloration and banding pattern of G. bahianus is not detailed in the sources reviewed; data is sparse on external field marks beyond its inclusion in the barred, robust Gymnotus body plan typical of the genus.
Habitat
Gymnotus bahianus is known from the Rio Almada basin near Ilhéus in Bahia State, northeastern Brazil — its type locality and, per FishBase, also the known limit of its range. It shares the Atlantic Forest coastal-basin region with congeners such as Gymnotus interruptus and Gymnotus capitimaculatus, which occupy the neighbouring Rio de Contas and Rio Jucuruçu basins; a separate genetic study on the species' sex-chromosome system sourced its specimens from the Contas River basin, suggesting the species' range may extend across more than one of these short coastal drainages. FishBase describes it as a freshwater, benthopelagic fish.
Detailed water-chemistry data for G. bahianus itself is sparse in the available sources, but the Atlantic Forest coastal streams of this part of Bahia are typically warm, tannin-stained to clear, and near-neutral to acidic — conditions consistent with the broader Gymnotus preference for dim, structurally complex water where an electric sense outperforms eyesight.
Feeding
Specific dietary data for Gymnotus bahianus is not documented in the sources reviewed, but FishBase records a trophic level of 3.3 ± 0.4, consistent with a carnivorous diet. Across the genus Gymnotus, banded knifefish are nocturnal predators of small invertebrates and fish, hunting after dark using electrolocation to detect prey hidden in sediment, leaf litter or root tangles where vision is of little use.
As facultative air-breathers, Gymnotus species are also able to exploit oxygen-poor water and swamp-like margins that exclude many other predators, foraging methodically over the substrate rather than ambushing from cover. Nothing in the reviewed sources indicates G. bahianus departs from this general genus pattern, but the species-specific detail should be treated as data sparse rather than confirmed.
Mating
As in other Gymnotus, electric organ discharge in G. bahianus almost certainly plays a role in communication as well as electrolocation — EOD pulses in the genus are species- and sex-specific and are used for territorial and courtship signalling among the barred, often territorial banded knifefishes. Beyond that inference from the wider genus, courtship behaviour specific to Gymnotus bahianus is not documented in the sources reviewed.
The clearest species-specific finding is genetic rather than behavioural: G. bahianus carries a novel XX/XY1Y2 multiple sex chromosome system, with a modal diploid count of 2n=36 in females (30 metacentric/submetacentric + 6 subtelocentric chromosomes) and 2n=37 in males (32 metacentric/submetacentric + 5 subtelocentric chromosomes). This cytogenetic distinction is notable within the genus but does not, on its own, describe how the fish pairs or spawns.
Breeding
No breeding record specific to Gymnotus bahianus was found in the sources reviewed — this is a poorly documented, narrow-range species with no history in the aquarium trade to draw on. Gymnotidae as a family show real parental care, unusual among fishes: in Gymnotus generally, the male builds and guards a nest, often a sheltered hollow or a mass of vegetation and floating cover, into which the female spawns, with the male then guarding the eggs and larvae through to independence, typically tied to seasonal flooding.
There is no reason from the available sources to expect G. bahianus to differ from this genus-wide pattern, but in the absence of a direct observation for this species it should be presented as inferred from the genus rather than confirmed. Data is sparse; nothing here should be read as an established breeding protocol.
In the aquarium
Gymnotus bahianus has no established presence in the aquarium hobby and the sources reviewed contain no husbandry record for it. Based on the genus, any specimen encountered would need the same core provisions as other banded knifefishes: a dim, well-covered tank offering caves, pipes or dense root tangles to retreat into by day, since these are nocturnal predators active mainly after dark; scrupulous water quality, since the species is scaleless and, like its relatives, likely sensitive to ammonia, nitrate and to copper-based or harsh medications, which should be avoided or dosed conservatively.
Given its size — up to roughly 11 in total length — and predatory nocturnal habits, it would not be safe with small tankmates, and territorial behaviour typical of Gymnotus toward its own kind should be assumed absent contrary evidence. As a narrow-range Bahia endemic with no trade history, prospective keepers should treat any acquisition with particular caution regarding provenance, and captive-bred stock is not known to exist for this species.
Conservation
Gymnotus bahianus is assessed by the IUCN Red List as Least Concern, an assessment made in November 2018. FishBase further notes high resilience with a minimum population doubling time under fifteen months, and low vulnerability to fishing pressure, factors consistent with a Least Concern rating despite the species' narrow known distribution.
That said, the species is currently documented only from the Rio Almada basin near Ilhéus, with related Gymnotus species occupying other, similarly small Atlantic Forest coastal basins in Bahia — a pattern of narrow-range endemism across the region that leaves these fish exposed to habitat degradation, deforestation and water-quality pressures specific to their individual river systems, even where the current Red List status is not elevated. The Least Concern rating reflects the assessment available in the cache and should not be read as insulating this basin from those broader Atlantic Forest pressures.