Taxonomy & naming
Gymnotus choco was described in 2003 by James Albert, William Crampton and Javier Maldonado-Ocampo, part of a wave of species-level splits that broke the old, broadly-defined "Gymnotus carapo" into more than twenty distinct species across South and Central America. It belongs to the family Gymnotidae — the nakedback knifefishes and electric eels — within the order Gymnotiformes, the New-World electric knifefishes. Gymnotiformes are bony fish, but unusual ones: alongside catfishes and characins in the broader superorder Ostariophysi, they are set apart by a dedicated electric organ and a ribbon-like anal fin that does almost all the swimming.
The genus Gymnotus is the most geographically wide-ranging in the order and the only one to reach north of South America, with representatives up into southern Mexico. The Gymnotidae take their common name "nakedback knifefishes" from their scaleless skin and myogenic (muscle-derived) electric organs, which produce a pulse-type discharge rather than the continuous wave-type signal of ghost and glass knifefishes. The species epithet choco refers to the Chocó biogeographic region of Colombia's Pacific coast, where the fish is endemic. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the standing authority for the valid name.
Morphology
Gymnotus choco reaches about 10 in in total length, a solid mid-sized member of the genus. Like all banded knifefishes it has a stout, laterally compressed, elongate body tapering to a whip-like tail, no dorsal fin, and no true caudal fin — propulsion comes almost entirely from a very long anal fin (roughly 210 to 255 rays) that runs along most of the ventral length of the body and drives the fish forward or backward with equal ease. The skin is scaleless.
The species is best told apart from its many close relatives by its colour pattern: a series of oblique, pale yellowish bands with distinctly wavy margins cross the body, and toward the rear one to three dark, inverted Y-shaped markings sit against the paler ground colour. Along its flanks and head run rows of electroreceptive pores, and buried in the tail musculature is the electric organ itself — built from modified muscle cells (electrocytes) that fire in a coordinated pulse to generate the fish's weak electric field. No dorsal spines are present.
Habitat
This species is a narrow-range endemic of Colombia's Pacific slope, recorded from the Río Baudó — a short, fast-flowing Pacific-draining river in the Chocó — and from the Río Atrato, a larger river system that drains north into the Caribbean through the same biogeographically distinctive region. Both drainages sit in one of the wettest, most biodiverse lowland rainforest zones in the Americas.
Gymnotus choco is a benthopelagic freshwater fish, living and hunting near the bottom rather than in open water. Like other Gymnotus it is a facultative air-breather, able to gulp air at the surface to supplement its gills, which lets it tolerate the warm, oxygen-poor conditions typical of sluggish, vegetated river margins, flooded forest edges and swamp-like backwaters. Its electric sense is best suited to exactly this kind of dim, turbid, obstacle-filled water, where sight is of limited use and an electric field is a far more reliable way to navigate.
Feeding
Detailed dietary data specific to Gymnotus choco is sparse, but the genus is uniformly built as a nocturnal predator, and there is no reason to expect this species differs. Banded knifefishes hunt after dark using electrolocation to detect the weak electrical signatures of prey hidden in sediment, leaf litter or root tangles — a hunting style shared with the wider family and well documented in congeners such as Gymnotus carapo, which take benthic worms, aquatic insect larvae, small crustaceans and small fish.
As a stout-bodied, air-breathing predator of swamp and floodplain margins, G. choco most plausibly takes a similar range of invertebrate and small vertebrate prey, foraging methodically over the substrate rather than lying in ambush. Given the lack of species-specific feeding studies, this account should be read as inference from the genus rather than a confirmed record for the species.
Mating
As in other Gymnotiformes, the electric organ discharge in Gymnotus is not just a sensing tool but a communication channel: each species, and often each sex, carries a broadly recognisable discharge signature, and individuals can vary the timing and pattern of their pulses when they encounter a rival or a potential mate. Courtship in the genus is therefore understood to unfold partly as an electrical exchange in the dark, layered over physical approach and, in many Gymnotus, an aggressive, territorial temperament even outside spawning.
No courtship behaviour has been specifically documented for Gymnotus choco, and this account does not extend beyond that general pattern known for the genus. Data on this species' reproductive behaviour is sparse.
Breeding
Gymnotidae are unusual among fishes for showing genuine parental care, and the pattern documented in the well-studied congener Gymnotus carapo gives the best available proxy for this species: the male builds and guards a nest — often among roots, dense vegetation or a sheltered hollow — the female spawns a batch of small, transparent eggs into it (on the order of 100–200 in G. carapo), and the eggs hatch within a few days, with the male continuing to guard the eggs and then the free-swimming larvae until they can fend for themselves. Spawning in the genus is typically seasonal and cued to flooding.
There is no record of Gymnotus choco having been bred in captivity, and given its narrow, little-collected native range it is essentially unknown to the aquarium trade. The safest statement is that its breeding biology is presumed to follow the guarded-nest pattern typical of Gymnotidae, rather than one that has actually been observed in this species.
In the aquarium
Gymnotus choco is not an established aquarium fish; it is a narrow-range Colombian endemic with no meaningful presence in the trade, and this account is offered for the genus-typical husbandry principles that would apply if it were ever kept, not as a tested protocol. Like all Gymnotidae it is scaleless and correspondingly sensitive to ammonia and nitrate, and to copper-based or otherwise harsh medications, which should be avoided or used at reduced dose. Being a nocturnal, electrolocating predator, it would need a dimly lit tank with ample cover — roots, pipes and dense planting — and should not be trusted with small or bite-sized tankmates, which a Gymnotus will hunt after dark regardless of how placid it appears by day.
As a facultative air-breather from warm, often oxygen-poor floodplain water, it would tolerate a wide temperature range and modest water movement better than many aquarium fish, but given its rarity in the hobby and its conservation status, any interest in the genus is better directed toward the handful of Gymnotus species that are actually available and, ideally, captive-bred.
Conservation
Gymnotus choco is assessed by the IUCN Red List as Near Threatened, reflecting the narrow extent of its known range — restricted to the Río Baudó and Río Atrato drainages on Colombia's Pacific slope — which leaves it more exposed than the many Gymnotus species that range across whole continental basins. The Chocó region is a biodiversity hotspot under real and ongoing pressure from deforestation, mining and land-use change, the kinds of threats typically implicated in the decline of narrow-range Neotropical freshwater fishes.
There is no evidence this species is targeted by the aquarium trade or by fisheries; its conservation concern is a function of geography rather than direct exploitation. As with other single-region endemics in this order, protecting the integrity of its home rivers — water quality, flow and floodplain habitat in the Baudó and Atrato systems — is the relevant conservation lever, not anything the aquarium hobby can meaningfully affect.