Taxonomy & naming
Gymnotus pantherinus was described by Franz Steindachner in 1908 and belongs to the family Gymnotidae, the banded knifefishes, within the order Gymnotiformes — the New-World electric knifefishes. Like all gymnotiforms it is a bony fish, but a strange one: the order is defined by an electric organ running the length of the body and by a single greatly elongated anal fin that does the work most other fishes split between a tail, a dorsal fin and paired fins. Gymnotus, the banded knifefishes, are the stocky, barred branch of the order and the only gymnotiform genus to range outside South America.
Despite sharing the common name "knifefish", Gymnotus and its relatives are not related to the Asian featherback and clown knifefishes (Chitala, Notopterus); those are bonytongues, and the resemblance to gymnotiforms is convergent body shape, not shared ancestry. Genetic work on Atlantic Forest coastal streams has shown that what has long been called Gymnotus pantherinus is not a single uniform lineage but a paraphyletic species complex, with at least five to six evolutionarily independent lineages nested within it — a pattern echoed in other Gymnotus species-complexes such as G. carapo. The Catalog of Fishes is the authority for the valid name.
Morphology
The panther knifefish has the classic Gymnotus build: a stout, laterally compressed, tapering body marked with dark, irregular bars and blotches over a paler ground colour, a pattern that gives the species its common and scientific names alike (pantherinus, 'leopard- or panther-like'). There is no dorsal fin and no separate tail fin; a long anal fin running most of the length of the underside provides propulsion, thrown into travelling waves that push the fish forward or backward with equal ease. FishBase records a maximum length of about 9.5 in total length, modest for the genus but typical of the pantherinus group.
Running beneath the skin along the body is the electric organ, paired with sensory pores that read the field it produces. Like other gymnotiforms the species is scaleless, which leaves it more exposed to water-quality stress and to certain medications than scaled fishes. The genome carries 52 chromosomes (32 metacentric, 18 submetacentric, 2 subtelocentric), a detail from the cytogenetic literature rather than anything visible to an aquarist. External sexual dimorphism is not well documented for this species.
Habitat
Gymnotus pantherinus is endemic to the coastal river drainages of Brazil's Atlantic Forest, recorded from Bahia south through the Tietê, Paraíba do Sul and Iguaçu basins to Santa Catarina; the type locality is Santos, in the state of São Paulo. Unlike most of its Amazonian and Orinocan relatives, it lives in a narrow coastal strip rather than a great interior river system, and much of that strip is heavily populated Atlantic Forest terrain.
Within these streams it favours riparian vegetation, burrows and marginal cover along coastal creeks, sheltering there through the day and moving out to feed at dusk and after dark. As a facultative air-breather, like other Gymnotus, it can tolerate the low-oxygen conditions common in dense marginal vegetation and slow floodplain water — an adaptation shared across the genus that lets it colonise habitats many fish cannot. Its electric sense does the work eyes would otherwise do in this dim, root-tangled, often turbid water.
Feeding
As in other Gymnotus, the panther knifefish is a nocturnal predator, emerging from its daytime shelter at dusk to hunt invertebrates and other small prey among the sediment and vegetation of its coastal stream habitat. Cache sources do not detail its diet at species level, but the genus as a whole takes aquatic insect larvae, worms, crustaceans and other benthic invertebrates, located largely by electrolocation rather than sight — a hallmark of the pulse-type gymnotiforms hunting in the dark.
In the aquarium it should be expected to behave the same way: a nocturnal, largely carnivorous feeder that takes meaty foods such as bloodworm, chopped earthworm and similar frozen or live fare, best offered as the lights go down. Being a Gymnotus, it is a genuine predator, not a scavenger, and its tank diet should reflect that rather than staple flake or pellet feeding alone.
Mating
Courtship in Gymnotus, as in other gymnotiforms, plays out substantially through electricity: the pulse-type electric organ discharge carries information about identity, sex and reproductive state, and related species use shifts in discharge pattern to recognise mates and rivals in the dark. Specific courtship behaviour has not been documented for G. pantherinus itself, but the pulse EOD typical of the genus is well characterised in the literature and almost certainly underlies mate recognition and approach in this species too.
Data on the finer detail of pairing and spawning triggers in G. pantherinus is sparse; what is known comes mostly by extension from better-studied relatives in the same species-group, including the seasonal, flood-associated spawning typical of Gymnotus generally.
Breeding
Breeding is not documented in detail for Gymnotus pantherinus specifically, but the genus Gymnotidae is unusual among fishes for showing genuine parental care, and the closely related G. refugio — a member of the same pantherinus species-group, also a coastal, high-latitude Atlantic Forest endemic — has been studied enough to sketch the likely pattern: males build a nest, often a depression or a shelter worked into vegetation, and guard the eggs and larvae after the female spawns into it, with relatively low fecundity and fractional spawning rather than one large clutch at once.
On that basis G. pantherinus is presumed to share the male-guards-nest pattern seen across Gymnotus, including G. carapo and G. mamiraua, but this has not been confirmed for the species itself and it is not a fish reported bred in home aquaria. The honest position is that its breeding remains poorly documented — likely a guarded-nest strategy by extension from its relatives, not a described protocol.
In the aquarium
Gymnotus pantherinus is rarely seen in the hobby, and husbandry guidance draws mainly on the wider genus. Like other banded knifefishes it is a nocturnal predator and should be given dim lighting and secure daytime cover — burrows, pipes, or dense planting and roots to hide among — with activity picked up after dark. It is not a community fish for small tankmates: Gymnotus are territorial, often aggressive toward their own kind, and will treat bite-size fish as prey.
Because it is scaleless, it needs the same caution other gymnotiforms demand: no copper-based medication, reduced doses of anything else, and clean water with well-managed ammonia and nitrate, since scaleless skin offers little protection against either. Warm water in the 73–82 °F range, soft to moderately mineralised and near-neutral to acidic, suits its Atlantic Forest coastal-stream origin. As a facultative air-breather it tolerates lower dissolved oxygen than many fish, but that is a fallback, not licence for poor water quality. Wild-caught stock is likely for a species this seldom traded; there is no established captive-breeding program to point to.
Conservation
Gymnotus pantherinus is assessed by the IUCN as Least Concern (2018), and FishBase records it as having low vulnerability to fishing with high resilience and a fast minimum population-doubling time. It is harmless to humans, and there is no evidence of population decline severe enough to warrant a higher threat category.
That said, the species is a narrow-range endemic restricted to Atlantic Forest coastal drainages — among the most heavily altered and urbanised landscapes in Brazil — and the recent finding that G. pantherinus is in fact a paraphyletic complex of five to six independent lineages means the Least Concern rating for the aggregate may mask more localised vulnerability in individual coastal river systems as those lineages are formally split out. Habitat degradation, pollution and urban development along the Bahia-to-Santa-Catarina coastal strip are the pressures most relevant to its long-term status, even though the current formal assessment is not one of concern.