Taxonomy & naming
Porotergus sambaibensis was described in 2025 by Mendonça, Peixoto, Chamon, Akama & de Santana in the Journal of Fish Biology, as part of a molecular phylogenetic study of the genus Porotergus (Gymnotiformes: Apteronotidae) using a concatenated multilocus dataset. The species sits in the family Apteronotidae, the ghost knifefishes, within the order Gymnotiformes — the New-World electric knifefishes, a group of bony fish only distantly related to the rest of the aquarium-fish fauna and defined by their electric organs and ribbon-like anal fin.
The molecular data placed Porotergus sambaibensis as the closest relative of P. gimbeli, with a genetic divergence of 3.7% from that species and 10.3% from P. duende, supporting it as a distinct, newly recognised lineage rather than a variant of either. The species epithet sambaibensis refers to the type locality, the Pedral da Sambaíba ("sambaíba" meaning roughly "rocky bank" in Portuguese) on the Javaés River — so far the only place the species is known from. As with all Gymnotiformes, Porotergus is unrelated to the superficially similar Asian featherback and clown knifefishes (Chitala, Notopterus); the resemblance is convergent body shape, not kinship. The Catalog of Fishes is the authority for the valid name.
Morphology
The holotype measures 4.5 in total length, with the type series ranging from 2–4.5 in TL — a small to modest-sized ghost knifefish by the standards of the family. Like its relatives it is deep-bodied and compressed, with no dorsal fin and no true tail fin; the original description records a dark brown trunk against a noticeably clearer belly, with dark-brown fin rays. Diagnostic features used to separate it from its congeners include two rows of dentary teeth, five premaxillary teeth, an unossified second basibranchial, and a hyomandibula bearing two prominent dorsal foramina — technical, skeletal characters typical of the fine-grained work needed to split closely related Porotergus species.
Anal-fin ray counts run from 146 to 160 (153 in the holotype); this long anal fin is the ghost knifefish's main propulsive organ, rippled in travelling waves to move the fish forwards or backwards with equal ease. As in all Apteronotidae, the body carries an electric organ derived from modified nerve cells — capable of the fast, wave-type discharges characteristic of the family — and the skin is scaleless. The original description does not report data on sexual dimorphism.
Habitat
Porotergus sambaibensis is known only from its type locality, the Pedral da Sambaíba near Pium, Tocantins State, Brazil, in the Javaés River — a tributary of the Araguaia River within the greater Amazon basin. The site is described as a rocky, crevice-riddled stretch of river, and popular coverage of the discovery describes the fish as lurking in rock crevices, consistent with the secretive, cover-seeking habits typical of ghost knifefishes.
As with other Apteronotidae, a dim, structurally complex environment — here rock crevices rather than roots or leaf litter — suits a fish that navigates and hunts largely by electrolocation rather than sight. No temperature or pH data were reported in the description, and the species' broader habitat requirements beyond the type locality are not yet documented; whether it occurs more widely in the Araguaia–Tocantins drainage or is truly confined to this one rocky reach is unknown.
Feeding
No diet data have been published for Porotergus sambaibensis. Based on its family, it is reasonable to expect a nocturnal carnivore that hunts small invertebrates — aquatic insect larvae, worms and small crustaceans — using electrolocation to detect prey hidden in sediment or crevices after dark, as is well documented in other ghost knifefishes such as the black ghost. This should be read as an inference from the family rather than a confirmed finding for this species; the original description does not report feeding observations.
Mating
Nothing specific to Porotergus sambaibensis has been published on courtship or spawning behaviour. In ghost knifefishes generally, the wave-type electric organ discharge doubles as a social signal carrying information about a fish's identity, sex and reproductive readiness, and in the best-studied Apteronotidae, courting fish shift their discharge frequencies in a kind of electrical duet. Whether this species shows the same pattern has not been documented, and this remains an inference from the family rather than a direct observation.
Breeding
Breeding biology in Porotergus sambaibensis is undocumented; the species was only described in 2025 from a small type series collected at a single locality, and no captive or field reproductive data exist. Ghost knifefishes as a family are egg-layers that scatter eggs among cover with no substantial parental care, and they are rarely bred even in aquaria for the well-known species — this newly described, non-trade species is data sparse on breeding, and nothing should be assumed beyond the general pattern for Apteronotidae.
In the aquarium
Porotergus sambaibensis is not established in the aquarium trade; it is a newly described species known from a single Amazonian type locality, not a fish that hobbyists are likely to encounter. Like all Apteronotidae it would be expected to be scaleless and therefore sensitive to ammonia, nitrate and copper-based medications, and — as a fish that relies on electrolocation in the dark — to need dim lighting and secure cover if it were ever kept. None of this is confirmed for the species specifically; it is included here only as context from its family, not as care guidance drawn from experience with this fish.
Conservation
Porotergus sambaibensis is assessed as Data Deficient, reflecting how recently it was described and the very limited collection effort reported around its single known locality on the Javaés River. As a narrow-range endemic apparently confined to one rocky stretch of river within the Araguaia–Tocantins drainage of the Amazon basin, it fits the pattern seen elsewhere in Gymnotiformes where single-river endemics are the most vulnerable to habitat change, damming and pollution, even though no specific threats have yet been documented for this species. Further survey work in the surrounding Araguaia system would be needed to establish whether its range extends beyond the type locality and to support a more complete conservation assessment.