Taxonomy & naming
Gymnotus henni was described in 2003 by James Albert, William Crampton and Javier Maldonado-Ocampo, one of seven new Gymnotus species named in the same paper as part of a broader revision of the genus using electric organ discharge waveforms alongside traditional anatomy. It belongs to the Gymnotus carapo species-group, a complex of similar-looking banded knifefishes that were long treated as a single widespread species before genetic and electrophysiological work showed the "complex" to be dozens of geographically narrow, distinct species. The species epithet honours Arthur W. Henn, an early-twentieth-century American ichthyologist and a pioneer collector of Neotropical fishes, whose specimens helped lay the groundwork for later Gymnotus taxonomy.
The genus sits in the family Gymnotidae, within the order Gymnotiformes — the New-World electric knifefishes. These are bony fish, but unusual ones: nearly every species produces a weak electric organ discharge (EOD) that forms a private electric field around the body, used for electrolocation of prey and obstacles in dark water and for communication between individuals of the same species. Gymnotus produces a pulse-type discharge, delivered as discrete, well-spaced pulses rather than the continuous hum of the ghost knifefishes (Apteronotidae). Despite the shared common name, Gymnotiformes are not related to the Asian featherback and clown knifefishes (Chitala, Notopterus); the resemblance is convergent, not a sign of kinship. The Catalog of Fishes (Eschmeyer, CAS) is the taxonomic authority for the valid name.
Morphology
Like other members of its genus, Gymnotus henni has the classic banded-knifefish build: a stout, laterally compressed, blade-shaped body with a brown ground colour broken by pale yellowish blotches on the head and a series of bands running down the flanks and body. FishBase records the species with a high anal-fin ray count, roughly 223 to 275 soft rays, reflecting the enormously elongated anal fin typical of the family — this single fin, thrown into a travelling wave, is the fish's main means of propulsion and lets it swim backward as readily as forward. There is no dorsal fin and no distinct tail fin in the ordinary sense.
The reported maximum size is around 12 in standard length. As in other gymnotiforms, the skin is scaleless, and an electric organ runs much of the length of the body beneath the muscle, paired with electroreceptor pores scattered over the skin that pick up distortions in the fish's own field. Data on external sexual differences are sparse for this species specifically; Gymnotus in general show little obvious dimorphism outside of breeding condition.
Habitat
Gymnotus henni is a narrow-range endemic of the Pacific slope of Colombia, recorded from the Río Calima drainage in Valle del Cauca department and the Río Juradó in Chocó department, with additional records extending into the Chucunaque–Tuira basin of Panama near the Colombian border. This places it among the relatively small number of Gymnotus species found outside the Amazon–Orinoco heartland — the genus is the only one in the order to reach Central America, ranging as far north as southern Mexico, and G. henni sits near the northern, Pacific-coast edge of that spread.
As a benthopelagic freshwater species it favours slow floodplain channels, swampy margins and vegetated backwaters rather than fast open river channels. Gymnotus as a genus is a facultative air-breather, gulping air at the surface to supplement gill respiration, which lets it survive in warm, oxygen-poor swamp water that would exclude many other fish — a trait shared across the genus and consistent with the sluggish, seasonally flooded habitats it is associated with on Colombia's Pacific coast.
Feeding
Gymnotus species are nocturnal predators that hunt after dark using electrolocation to detect aquatic insects, crustaceans and small fish hidden in sediment, leaf litter or among roots — exactly the conditions of murky floodplain water where vision is of little use. Detailed dietary data specific to G. henni have not been published, but the genus as a whole is consistently carnivorous and opportunistic, taking whatever suitably sized invertebrate and small vertebrate prey it can locate.
In line with its close relatives, G. henni should be assumed to be an active, probing nighttime hunter rather than a passive grazer, foraging along the bottom and through cover for animal prey located by the distortions it senses in its own electric field.
Mating
Courtship and spawning behaviour have not been documented for Gymnotus henni specifically; what is known comes from the genus and family more broadly. Gymnotidae are unusual among fishes for showing genuine parental care, and electric organ discharges are known to carry information used in courtship and rivalry across the group — each individual's pulse pattern can signal identity, sex and reproductive state to other knifefish sharing the same water. Reproduction in Gymnotus is understood to be seasonal, cued to the flooding cycle that opens up the swampy, vegetated margins the fish favours.
Data for this species are sparse, and nothing specific to G. henni's courtship sequence has been published; the safest statement is that it likely follows the pattern documented in better-studied Gymnotus, with electric signalling forming part of the approach between the sexes.
Breeding
Breeding biology in Gymnotus henni is essentially undocumented, and the species has no established aquarium breeding history. In the genus generally, the male builds and guards a nest — often a floating mass of foam or bubbles, or a sheltered hollow among vegetation and root tangles — into which the female spawns; the male then guards the eggs and larvae, a genuinely unusual degree of parental investment for a fish of this kind.
Whether G. henni follows this pattern exactly has not been confirmed in the literature available, but it is consistent with what is known of its close relatives in the G. carapo species-group. The species should be treated as essentially unbred in captivity; nothing here should be read as a tested protocol.
In the aquarium
Gymnotus henni is not an established aquarium fish and is rarely, if ever, seen in the trade — most of what can be said about keeping it is extrapolated from the genus. Like all Gymnotus it is scaleless, which means real care around water quality and medication: ammonia and nitrate must be kept low, and copper-based treatments should be avoided or used at reduced strength. It is also a nocturnal predator, so tankmates need to be too large to be eaten and the fish itself is not a candidate for a community of small, bite-sized fish or shrimp, which it will hunt after dark by electrolocation.
As a facultative air-breather from warm, low-oxygen floodplain water, it would need a warm, dimly lit tank with plenty of cover — root tangles, pipes or dense planting — to feel secure by day, along with access to the surface. Given its status as a narrow-range wild species with essentially no aquarium track record, any specimen encountered in the hobby should be assumed wild-caught, and kept accordingly with conservative stocking and careful acclimation.
Conservation
Gymnotus henni is assessed by the IUCN Red List as Vulnerable, reflecting its very restricted range: it is known only from the Calima and Juradó river drainages on Colombia's Pacific slope, plus a limited extension into the neighbouring Chucunaque–Tuira basin in Panama. A species confined to a small number of river systems is inherently exposed to habitat degradation, deforestation, pollution and changes in flow within those specific drainages, even where no single acute threat has been documented in the available sources.
Because its entire known distribution sits within just two or three river basins, any future development, water-quality decline or habitat loss in that part of Colombia's Pacific coast has an outsized effect on the species relative to a wide-ranging Gymnotus. It is not a fish under heavy documented pressure in the sources reviewed here, but its Vulnerable listing is a direct consequence of that narrow, single-region distribution rather than of any single catastrophic threat.