Taxonomy & naming
Gymnotus jonasi was described by James Albert and William Crampton in 2001, one of five new Gymnotus species the pair split out of a single densely packed floodplain assemblage near Tefé in the Brazilian Amazon, distinguished largely by differences in their electric organ discharges and ecology alongside anatomy. It belongs to the family Gymnotidae — the banded knifefishes and, in the same family, the electric eel — within the order Gymnotiformes, the New-World electric knifefishes. Gymnotiformes are bony fish, but unusual ones: defined by an electric organ used for electrolocation and communication, and by a single long anal fin that provides essentially all of their propulsion in place of a normal tail.
The species epithet honours Jonas Alves de Oliveira, a Brazilian naturalist associated with the Mamirauá Sustainable Development Reserve near Tefé, where the type material was collected. Within Gymnotus, G. jonasi is placed in the subgenus Tigrinus. Despite the shared common name "knifefish", Gymnotiformes are not related to the Asian featherback and clown knifefishes (Chitala, Notopterus) — those are bonytongues, and the resemblance is convergent, not a sign of kinship. The Catalog of Fishes is the authority for the valid name.
Morphology
Gymnotus jonasi is a modestly sized member of a genus known for stout, boldly barred bodies — Gymnotus species typically show dark, tiger-like crossbands over a paler ground colour, and the pattern is one of the few external cues that helps separate the many similar-looking species sharing the same floodplains. Like all gymnotiforms it has no dorsal fin and no functional caudal fin; movement comes from a long anal fin run in a travelling wave along the underside of the body, which lets it move backward as readily as forward through tangled roots and vegetation. The body is scaleless, a trait shared across the order that leaves the skin more exposed and makes the fish sensitive to water quality and to certain medications.
Running the length of the body is the electric organ that gives the group its name, paired with electroreceptor pores across the skin that read the field it generates. G. jonasi produces a PULSE-type discharge — brief, distinct pulses rather than the continuous wave of the ghost knifefishes — a signal whose timing and waveform differ subtly between the many sympatric Gymnotus species at Tefé and appears to help them recognise their own kind in the dark. Reaching a modest maximum size for the genus, it is built for life within dense root mats rather than open water.
Habitat
Gymnotus jonasi is known from the Amazon River basin in Brazil, with its type locality at Cano Lago Rato, near the confluence of the Rio Solimões and Rio Japurá in the Tefé region — an area that supports an unusually rich sympatric assemblage of Gymnotus species, eleven of them recorded within the same stretch of floodplain. It lives specifically among the root masses of floating meadows in whitewater floodplain habitat, and notably has not been recorded from flooded forest, unlike some of its relatives that partition the same landscape by habitat type.
This is warm, turbid, nutrient-rich floodplain water, seasonally flooded by the sediment-laden "whitewater" of the Solimões system, with root tangles offering both cover and hunting ground. Adults appear to breed and forage in the same root-mass habitat year-round rather than moving to separate nursery grounds. As in other Gymnotus, low-oxygen conditions in these dense floodplain waters are manageable thanks to facultative air-breathing, and darkness or turbidity within the root mats is where the electric sense does its real work, since eyesight is of limited use there.
Feeding
Gymnotus jonasi feeds on small aquatic organisms, with chironomid (non-biting midge) larvae identified as a particular favourite in the root-mass habitat it occupies — a diet typical of the genus, which hunts small invertebrates hidden in leaf litter, root tangles and sediment. As a nocturnal predator it likely relies heavily on electrolocation to find prey concealed in these structurally complex microhabitats, sensing the electrical disturbance prey creates within its self-generated field rather than hunting primarily by sight.
Little further ecological detail on foraging behaviour is documented for this specific species beyond its association with root-mass invertebrates; broader Gymnotus biology suggests an opportunistic predator of aquatic insect larvae, worms and small crustaceans, foraging actively through cover after dark rather than lying in ambush.
Mating
As in other Gymnotus, the electric organ discharge in G. jonasi is understood to carry information beyond navigation — signalling species identity and, in the genus generally, sex — and the unusually diverse, sympatric Gymnotus assemblage at Tefé is itself thought to be maintained partly through the fine differences between species' discharges, which likely help individuals recognise and choose mates of their own kind in a crowded, multi-species root mat. Courtship specifics for G. jonasi are not documented in the available literature; data sparse.
Gymnotus as a genus shows parental care, unusual among fishes generally, with breeding tied to seasonal, flood-driven cycles in floodplain habitats. Beyond that general pattern, no species-specific courtship account exists for G. jonasi.
Breeding
Breeding biology has not been documented in detail for Gymnotus jonasi specifically; data sparse. In Gymnotus generally, the male builds and guards a nest — often a sheltered hollow among vegetation or a mass of floating material — into which the female spawns, with the male then guarding the eggs and larvae, a genuinely unusual degree of parental investment for the order. Spawning in the genus tends to be seasonal and tied to the flood cycle of the floodplain habitats these fish occupy, which fits the whitewater floodplain setting where G. jonasi lives.
There is no record of this species being bred in captivity; it is not established in the aquarium trade at all. Prefer the genus-level pattern as a guide to what likely happens rather than treating it as confirmed for this species.
In the aquarium
Gymnotus jonasi is not a species found in the aquarium hobby — it is a scientifically described floodplain endemic known from research collections rather than trade, and no husbandry information exists for it specifically. General Gymnotus care principles would apply if a wild-collected specimen were ever kept: these are nocturnal, territorial predators best not housed with bite-sized tankmates, since they hunt by electrolocation after dark; being scaleless, they require careful attention to water quality and reduced doses of copper-based or otherwise harsh medications; and floating cover or root-like structure mimicking the tangled habitat they evolved in would likely reduce stress.
Because the species is undescribed in aquarium literature and effectively unavailable in the trade, none of this can be stated as established practice — it is inference from genus-level biology, not species-specific experience.
Conservation
Gymnotus jonasi is assessed as Least Concern on the IUCN Red List, an assessment dated 13 August 2020. It is known only from the Amazon River basin in Brazil, with its core range centred on the species-rich Tefé floodplain assemblage, but the Least Concern rating reflects both the extent of suitable floodplain habitat within the wider Amazon basin and the lack of evidence for significant population decline, rather than an exhaustive range survey.
As with many narrow-range Amazon floodplain fish, the practical pressures on this kind of habitat are broader Amazon deforestation, changes to flood regimes, and pollution, though no species-specific threat has been documented for G. jonasi. It is not part of the aquarium trade, so collection pressure from the hobby is not a relevant factor for this species.