Taxonomy & naming
Gymnotus capanema was described in 2012 by Milhomem, Crampton, Pierczeka, Shetka, Silva and Nagamachi, based on nine specimens — one male and eight females — collected at the type locality, with three additional museum specimens examined for comparison. It belongs to the family Gymnotidae within the order Gymnotiformes, the New World electric knifefishes, a lineage of bony fish defined by a specialized electric organ and a long ribbon-like anal fin rather than by any resemblance to the unrelated Asian featherback and clown knifefishes (Chitala, Notopterus), which sit in a completely different order.
The species epithet honors Capanema, the municipality in the state of Pará, eastern Amazonia, where the type series was collected. The original description leaned heavily on cytogenetics: G. capanema carries a diploid chromosome number of 2n=34 (20 metacentric/submetacentric plus 14 subtelocentric/acrocentric chromosomes), the lowest diploid count recorded in the genus Gymnotus at the time of description, alongside diagnostic external anatomy, pigmentation pattern and osteology. The Catalog of Fishes is the standing authority for the valid name.
Morphology
Gymnotus capanema is a stout, elongate, laterally compressed fish patterned with prominent dark bars, most strongly expressed over the anterior 60 percent of the body — the banding that gives the genus its common name. The holotype measured 7 in total length and the largest paratype 5.5 in TL, modest by the standards of the genus, whose larger relatives can exceed 23.5 in. As in all Gymnotidae, there is no dorsal fin and no distinct tail fin; locomotion comes from a single very long anal fin running beneath most of the body, thrown into travelling waves that let the fish glide forward or backward with equal ease.
The body is scaleless, and a weak electric organ extends along the ventral margin in hypaxial position, running much of the length of the fish. This organ produces a pulse-type electric organ discharge (EOD) — brief, discrete pulses rather than the continuous wave-type signal used by some other gymnotiform families — which the fish reads through electroreceptors distributed over its skin. That electric sense, not vision, is the primary way it perceives prey, obstacles and other fish around it in murky, low-light water.
Habitat
The species is known from small forest streams and floodplain-influenced waters of eastern Amazonia, specifically the Açaiteuazinho River and tributaries of the Rio Caeté near Bragança, in Pará state, Brazil — the same general area as the type locality of Capanema. This places it in the coastal-Amazonian drainages of northeastern Brazil rather than the main Amazon channel itself.
As with other Gymnotus, it likely inhabits shaded, structurally complex water — leaf litter, submerged roots and marginal vegetation in slow-flowing or floodplain-connected streams — where an electric sense is more useful than eyesight. Gymnotus as a genus is known for tolerating warm, low-oxygen conditions typical of swamps and seasonally flooded forest via facultative air-breathing at the surface, an adaptation that likely applies here, though this has not been documented specifically for G. capanema.
Feeding
Dedicated dietary data for Gymnotus capanema has not been published, but the genus as a whole is made up of nocturnal predators, and there is no reason to expect this species to differ. Related Gymnotus hunt after dark using electrolocation to detect insect larvae, small crustaceans, worms and small fish hidden in sediment, leaf litter and root tangles, rather than relying on sight.
Given its modest adult size — under 8 in total length by the type series — its natural diet is likely weighted toward smaller invertebrate prey than the larger Gymnotus species that regularly take small fish. Data on this point remains sparse for G. capanema specifically.
Mating
As in other Gymnotidae, courtship in Gymnotus is thought to be mediated substantially through the electric organ discharge, which carries information about a fish's species, sex and condition and is used both to attract mates and to establish territory against rivals in the dark. No courtship behavior specific to G. capanema has been documented in the literature reviewed here; what is known is inferred from the wider genus, in which parental care is unusual among fishes and centers on the male.
Breeding
Breeding in Gymnotus capanema itself is undocumented — it is known from only a small type series collected for taxonomic description, not from any life-history study. What can be said comes from the genus: Gymnotus species typically show a seasonal, flood-triggered spawning pattern in which the male builds and guards a nest, often a sheltered hollow or a mass of floating vegetation and bubbles, and the female deposits eggs into it; the male then guards the eggs and larvae, an unusual degree of parental investment for a fish family. Whether G. capanema follows this pattern precisely is not established, and the species should be treated as essentially unbred and unstudied in this respect.
In the aquarium
Gymnotus capanema is not established in the aquarium trade and there is no documented hobbyist husbandry for it; it is known only from scientific collection. Should material of this species ever reach aquarists, care would need to follow the general Gymnotus template: a dim, well-covered tank with plenty of hiding places, since these are nocturnal predators that spend daylight hours concealed and hunt by electrolocation after dark. Being scaleless, Gymnotus are sensitive to ammonia and nitrate and to copper-based or otherwise harsh medications, which typically need reduced dosing.
Gymnotus are also not community-safe with bite-size fish or shrimp, which they will hunt at night, and they are often territorial with their own kind, generally best kept singly or with ample space to establish separate territories. Given how little is known of this species, any aquarium context would currently be speculative rather than documented.
Conservation
Gymnotus capanema has not been formally assessed by the IUCN Red List; no published conservation status was found for the species. As a narrow-range endemic described from a small number of specimens collected near a single coastal-Amazonian locality in Pará, it fits the profile of the genus's more vulnerable members — species restricted to a single river system or small cluster of streams, where habitat change, deforestation and pollution in the Bragança region could plausibly affect it — though no targeted study of threats to this species is available. Most Gymnotus species overall are assessed as Data Deficient or Least Concern, with only range-restricted forms such as the Atlantic Forest endemic G. refugio rising to threatened categories; without a formal assessment, G. capanema's status is best treated as data deficient by inference from its extremely limited known range.