Taxonomy & naming
Eigenmannia desantanai was described in 2015 by Peixoto, Dutra & Wosiacki, one of several Eigenmannia species split out in the years since the genus was recognised as far more diverse than its handful of classically-known members suggested. It belongs to the family Sternopygidae, the glass and rattail knifefishes, within the order Gymnotiformes — the New-World electric knifefishes, a group of bony fish defined less by a single ancestor everyone pictures and more by two shared, unusual traits: an electric organ running the length of the body, and a long ribbon-like anal fin that does all the swimming in the near-total absence of a dorsal fin or a conventional tail.
The genus name Eigenmannia honours the ichthyologist Carl H. Eigenmann, a major figure in the early study of South American freshwater fishes; desantanai is a patronym, as is typical for recently described Sternopygidae named for a collector or colleague. 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 in body shape is convergent, not a sign of kinship. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the authority for the valid name.
Morphology
Eigenmannia desantanai has the classic glass-knifefish build: a long, laterally compressed, almost ribbon-shaped body that tapers to a fine point at the tail, with no dorsal fin and no distinct caudal fin. Locomotion comes from a very long anal fin running along most of the ventral edge of the body — FishBase records 170 to 198 soft rays in that fin alone — which throws travelling waves to drive the fish forward or backward with equal facility. Like other Eigenmannia the body is semi-transparent to olive-yellow, thin enough in life that internal organs and the swim bladder can be seen through the flank, which is where the genus's "glass knifefish" nickname comes from.
Adults reach a maximum recorded total length of about 7.5 in, modest even by glass-knifefish standards. The species is scaleless, like all gymnotiforms, which leaves the skin more exposed to water quality and medication than a typical scaled fish. Running beneath the skin is the electric organ, paired with an unusually well-developed array of electroreceptors and, in Sternopygidae, enlarged cephalic lateral-line canals that help the fish read the electric field it generates — the sensory basis for hunting and orienting in the dark, turbid water it inhabits.
Habitat
The species is known from the Rio Cuiabá and the wider upper Rio Paraguay basin in Mato Grosso, Brazil — the northern Pantanal, one of the largest freshwater wetlands on Earth. Field surveys covering 2009–2011 sampled it from floodplain river channels in this system, a benthopelagic fish of flowing, seasonally flooding water rather than still backwaters. As a narrow-range basin endemic, its confirmed distribution is far smaller than that of widespread genus relatives such as Eigenmannia virescens.
The Pantanal's rivers are warm, turbid and structured by a strong flood pulse: water levels and temperatures rise through the wet season and recede in the dry season, and that rhythm governs the fish's biology. Like other Sternopygidae, E. desantanai is a nocturnal micro-predator, active after dark and relying on its electric sense rather than eyesight in water where visibility is limited. Because it is scaleless, it is also sensitive to ammonia and nitrate, a trait shared across the family.
Feeding
A dedicated feeding-ecology study from the Rio Paraguay system (338 specimens examined, split between the Amonguijá and Paraguay rivers) found E. desantanai taking a broad diet of at least 21 identified food items, dominated by chironomid (midge) larvae, cladocerans and ostracods, alongside unidentifiable remains of already-digested prey. This marks it as a benthic and near-bottom forager taking small invertebrates rather than a fish-eating predator, consistent with its modest adult size.
Foraging activity is concentrated at dusk and after dark, in keeping with the nocturnal, electrolocation-driven hunting typical of the family — the fish reads the weak electric distortions produced by small prey hidden in sediment or vegetation rather than searching visually. The study also found diet composition shifted with the hydrological season, between the two river systems sampled, and across the fish's own life stages, underlining how tightly this species' feeding is tied to the Pantanal's flood-pulse cycle. No aquarium feeding data specific to this species exists; care would extrapolate from the wider genus, favouring small live and frozen invertebrate foods offered after dark.
Mating
No courtship behaviour has been documented for E. desantanai specifically, but Eigenmannia is one of the best-studied genera in gymnotiform electrophysiology, and the wave-type discharge that defines the genus is central to how these fish are believed to find and assess mates. Each fish emits a continuous, near-sinusoidal electric organ discharge at a signature frequency that varies with sex and individual identity; when two fish's discharges come too close in frequency they perform the jamming avoidance response, shifting their own frequency apart to keep reading the environment clearly — a phenomenon first characterised in this genus and now a textbook case in sensory neuroscience.
That same frequency information plausibly doubles as a social signal, letting fish assess sex, size, and reproductive condition of others from a distance, in the dark, without vision. Beyond this electrical signalling, nothing specific to E. desantanai's approach or pairing behaviour has been published; the life-history study from the Rio Cuiabá focused on reproductive timing and fecundity rather than courtship.
Breeding
The one dedicated life-history study of this species, covering the southern Pantanal population from 2009 to 2011, describes a batch-spawning strategy with relatively low fecundity per spawning event and an extended reproductive period rather than a single sharp season. Reproductive activity peaked during the rising-water and flood phases of the Pantanal's hydrological cycle, when temperatures were high and increasing — consistent with the broader Sternopygidae pattern of spawning triggered by the onset of the wet season. The authors characterise the species as an equilibrium-strategy spawner, investing in repeated, moderate spawning events over an extended window rather than one large synchronized event.
As in other Sternopygidae, eggs are adhesive and, based on the breeding biology known for the well-studied congener Eigenmannia virescens, are likely scattered among floating plant roots or submerged vegetation with no meaningful parental care afterward. E. desantanai itself has never been reported bred in captivity, and given how narrow its known range is and how recently it was described, aquarium breeding data for this exact species should be treated as effectively nonexistent — data sparse rather than assumed absent.
In the aquarium
Eigenmannia desantanai is not established in the aquarium trade; nothing in the available literature describes it being kept or bred by hobbyists, and any care approach has to be extrapolated cautiously from its much more commonly kept relative, the glass knifefish Eigenmannia virescens — one of the easier gymnotiforms to keep, doing well in a small shoal of its own kind. Anyone attempting E. desantanai specifically should expect a genuinely obscure, specialist fish and treat every husbandry detail as inferred rather than confirmed for this species.
General Sternopygidae care principles should still apply: a dim, quiet tank with plenty of cover (roots, plants, tubes) to shelter in by day, since these are nocturnal micro-predators most active after dark; warm, soft, near-neutral to slightly acidic water reflecting its Pantanal floodplain origin; and caution with tankmates, since anything bite-sized may be hunted at night even if the fish looks placid during the day. Being scaleless, it would be expected to share the family's sensitivity to ammonia, nitrate and copper-based or harsh medications, which should be used at reduced doses. Given the total absence of an established aquarium history, wild collection pressure on such a narrow-range species is a real concern, and this is not a fish to seek out casually.
Conservation
Eigenmannia desantanai has not been formally assessed by the IUCN Red List; FishBase lists its status as Data Deficient. This is the default position for the great majority of recently described, narrow-range Sternopygidae, most of which have never been the subject of a dedicated conservation assessment.
What is known is that the species' confirmed range is limited to the Rio Cuiabá and neighbouring channels of the upper Rio Paraguay basin in the Pantanal — a single-basin endemic rather than a continent-wide species like some of its genus relatives. Basin endemics of this kind are inherently more exposed to localized pressures — changes to the Pantanal's flood pulse from damming or land-use change upstream, agricultural runoff, and habitat degradation within the wetland — even though no specific population decline has been documented for this species. Given the data gap, the honest summary is that E. desantanai's conservation status is simply unknown, and that its narrow range makes it a candidate worth watching rather than a species known to be at risk.