Taxonomy & naming
Gymnotus arapiuns was described in 2020 by Kim, Crampton and Albert as one of two new species erected in a paper on the subgenus Lamontianus and the historical biogeography of Gymnotus in Brazil. It belongs to the family Gymnotidae — the naked-back knifefishes — within the order Gymnotiformes, the New-World electric knifefishes. The genus name Gymnotus comes from Greek gymnos, "bare" or "naked", and notos, "back", referring to the complete absence of a dorsal fin; the family name Gymnotidae carries the same meaning. The species epithet arapiuns is a toponym, taken directly from the Rio Arapiuns, the blackwater river in the Tapajós drainage from which the type series was collected.
The holotype (ZUEC 16727) measures 6 in total length. Gymnotus is placed with the electric eel, Electrophorus, in Gymnotidae, but the two are very different animals in practice: Electrophorus is the order's one strongly electric, dangerous member, while Gymnotus species, arapiuns included, are weakly electric like the rest of the order. As with all Gymnotiformes, the defining trait is an electric organ discharge (EOD) used for electrolocation in murky or dark water where vision is of limited use, and for communication between individuals. Gymnotus arapiuns is a pulse-type discharger, the type of EOD characteristic of the genus. The Catalog of Fishes is the authority for the valid name.
Morphology
As in other Gymnotus, the body is stout and laterally compressed, patterned with bold dark bars or bands over a paler ground colour — the source of the group's common name, banded knifefish. There is no dorsal fin and no distinct tail fin; locomotion is carried out by a single long anal fin running most of the length of the underside, thrown into travelling waves that push the fish forward or backward with equal facility. The skin is entirely without scales.
Running along the body is the electric organ that generates the species' pulse-type discharge, paired with electroreceptor pores across the skin that read the resulting field. The type specimens reach about 6 in total length, modest for the genus, which includes species considerably larger. Detailed comparative morphology (fin-ray counts, body proportions, pigmentation pattern) is what separates G. arapiuns from its close relatives within the subgenus Lamontianus in the original description; data beyond the holotype measurements are sparse in the sources available here.
Habitat
Gymnotus arapiuns is known from the Rio Arapiuns, a blackwater tributary that joins the lower Rio Tapajós in the Brazilian state of Pará, part of the wider Amazon basin. It appears to be a narrow-range endemic of Brazilian Shield blackwater systems, alongside a closely related species described in the same paper from the Rio Aripuanã. Like other Gymnotus, it is expected to live over soft substrates among leaf litter, roots and submerged vegetation in still or slow-flowing, acidic, tannin-stained water.
Gymnotus as a genus favours sluggish, turbid, dimly lit water — swamps, floodplain pools and quiet river margins — and several species tolerate low-oxygen conditions by taking air at the surface. Blackwater habitats such as the Arapiuns are naturally soft, acidic and stained the colour of tea, conditions under which an electric sense is a far more useful tool than eyesight. Beyond the type locality, published habitat detail specific to this species is limited.
Feeding
Dedicated dietary data for Gymnotus arapiuns were not found in the available sources. The genus as a whole is made up of nocturnal predators: tough, opportunistic hunters of aquatic insect larvae, worms, small crustaceans and small fish, located after dark largely by electrolocation rather than sight. Several Gymnotus, including the well-studied G. carapo, are facultative air-breathers, an adaptation that lets them exploit oxygen-poor swamp and floodplain water where prey may be abundant but competition from other predators is reduced.
In the absence of species-specific feeding studies, the safest statement is that G. arapiuns most likely shares this predatory, electrolocation-driven foraging strategy with its close relatives, hunting at night over soft bottoms and among cover rather than in open water.
Mating
Courtship and reproductive behaviour have not been documented specifically for Gymnotus arapiuns; nothing on this point appears in the available sources. In Gymnotus generally, the electric organ discharge doubles as a social signal — pulse pattern and frequency are species-specific and differ between the sexes, and are used for territorial signalling and, presumably, in the approach and recognition between mates that precedes spawning. Given how little is published on this species beyond its original description, any account of courtship in G. arapiuns specifically would be extrapolation from the genus rather than direct observation, and is best left as data sparse.
Breeding
No breeding record exists for Gymnotus arapiuns in the sources consulted, and the species has no known history in the aquarium trade. Gymnotidae as a family show real parental care, unusual among fishes generally: in well-studied Gymnotus such as G. carapo and G. mamiraua, males build and guard a nest — a floating mass of foam or bubbles, or a sheltered depression among vegetation or bottom debris — into which the female spawns, with the male then guarding the eggs and larvae until they disperse; spawning is typically seasonal and tied to flooding. It is reasonable to expect G. arapiuns follows the same general pattern of male-guarded nesting typical of its genus, but this has not been confirmed for the species itself, and it is not known to have been bred in captivity at all.
In the aquarium
Gymnotus arapiuns has no established presence in the aquarium hobby; it is a recently described, narrow-range species known essentially only from its type series, and none of the sources reviewed here describe it being kept. What follows is general guidance for Gymnotus as a genus, offered with the caveat that it is not species-specific. These are scaleless fish, so water quality has to be kept clean and stable, and medications — particularly copper-based treatments — must be used at reduced doses or avoided, as with all gymnotiforms. They are nocturnal predators that need dim lighting and secure cover such as pipes, roots or dense planting to feel settled during the day, and they are not community-safe with fish or shrimp small enough to be hunted after dark; Gymnotus can also be territorial toward their own kind and toward other electric fish, given how central electrocommunication is to their behaviour. Given how little is documented about this particular species, and the total absence of captive-bred stock, it is not a fish to recommend acquiring — any specimen in the trade would necessarily be wild-collected from a limited-range population, which cuts against responsible stewardship of a narrow-range endemic.
Conservation
Gymnotus arapiuns has not been formally assessed by the IUCN Red List; its conservation status is Data Deficient in practical terms, reflecting how recently it was described (2020) and how little survey work has followed. It is known only from the Rio Arapiuns, a blackwater tributary of the Rio Tapajós, and from what has been described as a similarly narrow range in a sibling species from the Rio Aripuanã — the pattern the original description highlights is one of local endemism within Brazilian Shield blackwater drainages rather than a broad Amazon-wide range.
As with other narrow-range single-basin endemics in the region, the Tapajós drainage faces real and ongoing pressures from deforestation, agricultural runoff, mining-related mercury contamination and hydroelectric development, any of which could affect a species with such a restricted known distribution. Without a formal assessment or population data, however, it would be overstating the evidence to characterise G. arapiuns as threatened; the honest summary is that its status is simply unknown, and that its narrow range makes it worth watching as regional environmental pressures continue.