Taxonomy & naming
Gymnotus diamantinensis was described by Campos-da-Paz in 2002 and belongs to the family Gymnotidae, within the order Gymnotiformes — the New-World electric knifefishes. The genus Gymnotus is the banded-knifefish group: stout, boldly barred, pulse-type electric fish that includes the widespread Gymnotus carapo species complex, from which many range-restricted species such as this one have since been split. Gymnotus is also the only genus of gymnotiform to reach Central America, its range extending north from South America all the way to southern Mexico, though G. diamantinensis itself is a strictly South American, single-basin endemic.
The epithet diamantinensis points to the Chapada dos Guimarães / Diamantino area of Mato Grosso, in the upper Rio Arinos drainage where the species was collected. As with all Gymnotiformes, these are bony fish but unusual ones — defined by a myogenic (muscle-derived) electric organ and a ribbon-like anal fin used for locomotion — and they are entirely unrelated to the Asian featherback and clown knifefishes (Chitala, Notopterus), which merely share the informal name "knifefish" by convergence of body shape. The Catalog of Fishes is the authority for the valid name.
Morphology
Gymnotus diamantinensis reaches a modest maximum of about 5 in total length, small for the genus. FishBase describes the anterior body carrying dark blotches on a brownish-gray background, giving way along the posterior half to the alternating dark and light bands typical of the genus's common name. The anal fin is very long, with 194–211 soft rays recorded, and provides essentially all of the fish's propulsion, undulating in travelling waves that let it move forward or backward with equal ease. There is no dorsal fin and no conventional tail fin.
The skin is scaleless and smooth, running the length of the body over a myogenic electric organ that produces a weak, pulse-type discharge — the field the fish uses to electrolocate prey and obstacles in the dark, sediment-laden water it inhabits, and to communicate with other Gymnotus. Like other members of the genus it is a facultative air-breather, able to gulp air at the surface when dissolved oxygen runs low, an adaptation to the seasonal, sometimes stagnant floodplain and river-margin habitats Gymnotus favours.
Habitat
The species is known only from the Tapajós River basin, specifically the upper Rio Arinos drainage in Mato Grosso State, Brazil — a tropical freshwater, pelagic habitat within that river system. This narrow known range makes it a basin endemic rather than a wide-ranging species like its relative Gymnotus carapo.
No specific temperature or pH data for the species has been recorded in the available sources, but as with other Gymnotus it can be expected to live in warm, Neotropical river and floodplain water, likely soft and acidic to near-neutral in character, and often turbid or shaded — conditions under which electrolocation matters far more than eyesight. Its facultative air-breathing suggests it is equipped to tolerate the warm, oxygen-poor backwaters and seasonal floodplain margins that many Gymnotus species use as refuge during the dry season.
Feeding
Detailed dietary data for Gymnotus diamantinensis has not been documented, but the genus Gymnotus is uniformly made up of nocturnal predators. They hunt after dark using electrolocation to detect aquatic insect larvae, worms, small crustaceans and other invertebrates hidden in sediment or vegetation, and larger individuals will also take small fish. There is nothing in the record to suggest G. diamantinensis, small even by the standards of the genus, departs from this pattern.
In captivity, close relatives take standard meaty foods — bloodworm, chopped earthworm, blackworm and frozen or live invertebrate foods — offered in the evening or after lights-out when the fish becomes active. Given its predatory habits, any tankmates would need to be too large to be eaten and secure from a fish that hunts by feel rather than sight.
Mating
No courtship or spawning behaviour specific to Gymnotus diamantinensis has been documented in the available sources. In gymnotiforms generally, the electric organ discharge functions as more than a sensing tool: it carries information about identity, sex and reproductive state, and courtship in the genus is understood to involve an exchange of electric signals between prospective mates alongside physical approach, though data for this exact species is lacking.
Gymnotidae as a family are unusual among fishes in showing genuine parental care, and Gymnotus in particular is known for male nest-building and guarding — traits that likely extend to G. diamantinensis, but which have not been directly observed or reported for it. Data remains sparse for this narrow-range species.
Breeding
Breeding has not been documented for Gymnotus diamantinensis specifically, in the aquarium or in the wild. What is known comes from the genus: male Gymnotus build and guard a nest, often a floating mass of foam or bubbles, or a sheltered hollow among vegetation and roots, into which the female spawns; the male then guards the eggs and larvae, with spawning typically cued to seasonal flooding.
Given the complete absence of species-specific breeding records, it would be dishonest to describe a protocol for G. diamantinensis. The most accurate statement is that this remains a rarely bred or entirely unbred species in captivity, and any aquarist attempting it would be working from the general Gymnotus pattern of male-guarded nests rather than from any account specific to this fish.
In the aquarium
Gymnotus diamantinensis is essentially unknown in the aquarium trade — its restricted Tapajós/upper Rio Arinos range and Data Deficient status mean it is not a fish most hobbyists will ever encounter, and there are no established husbandry accounts to draw on. Anyone keeping it would need to apply general Gymnotus and gymnotiform practice: a dimly lit tank with ample cover such as pipes, roots or dense planting for daytime retreat, since these are nocturnal predators uncomfortable and inactive under bright light.
As a scaleless fish it would be expected to share the sensitivity typical of the order — vulnerable to ammonia and nitrate, and to copper-based or otherwise harsh medications, which should be used at reduced doses if at all. Tankmates would need to be too large to be eaten, since Gymnotus are not community-safe with small, bite-size fish or shrimp. Given how little is documented about this species, keeping it well would rest more on genus-level caution than on any specific care sheet, and captive-bred stock does not appear to exist.
Conservation
Gymnotus diamantinensis is assessed by the IUCN as Data Deficient, last evaluated on 7 November 2018 — there is not enough information available to determine its extinction risk with confidence. This status is common among narrow-range gymnotiforms that have been described relatively recently and studied little since.
As a single-basin endemic restricted to the Tapajós River system's upper Rio Arinos drainage in Mato Grosso, the species would be expected to face the same pressures as other range-limited Amazonian basin fish: habitat alteration, deforestation, and the cumulative effects of agricultural and mining development across Mato Grosso. No population trend or specific threat has been documented for this species in particular, and it would be an overstatement to assign it a threat level the data does not support — Data Deficient is the accurate and honest classification until further survey work is done.