Taxonomy & naming
Eigenmannia besouro was formally described by Peixoto & Wosiacki in 2016 in Zootaxa, as a new member of the Eigenmannia trilineata species-group — a cluster of superficially similar glass knifefishes that had long been confused with one another before careful comparative work began separating them by mouth position, fin-ray counts, dentition and stripe pattern. It belongs to the family Sternopygidae within the order Gymnotiformes, the New-World electric knifefishes, and like every gymnotiform it is defined less by outward shape than by an electric organ and a weak, continuous electric field it uses to sense its surroundings — a trait this species shares with its relatives and that sets the whole order apart from ordinary bony fishes.
The type locality is the rio São Francisco basin in Bahia, northeastern Brazil, and the species epithet is a tribute rather than a description: "besouro" is Portuguese for "beetle," and honors Manoel Henrique Pereira — known as Besouro Mangangá — a legendary capoeira fighter from Bahia, the same state where the fish was collected. Sternopygidae's glass and rattail knifefishes are not related to the Asian featherback and clown knifefishes (Chitala, Notopterus) that share the common name "knifefish"; the resemblance is convergent body shape, not kinship.
Morphology
Eigenmannia besouro has the classic glass-knifefish build: a long, laterally compressed, ribbon-like body that tapers steadily to a fine point at the tail, with skin translucent enough in life to show a hint of the internal anatomy through it — hence "glass knifefish" for the genus generally. There is no dorsal fin and no separate tail fin; locomotion comes entirely from a very long anal fin running most of the length of the underside, thrown into a continuous travelling wave that lets the fish glide forward, hover, or reverse with equal precision. A pale midlateral stripe runs the length of the body, and the original description used its point of origin, together with a subterminal mouth and a specific count of pectoral-fin rays (ii,13–14) and premaxillary/dentary tooth pattern, to separate E. besouro from its close relatives in the trilineata group.
The species is scaleless, like all gymnotiforms, and carries the electric organ running down the trunk alongside the sensory pores that read the field it produces. Eigenmannia's discharge is wave-type — a continuous, quasi-sinusoidal signal in the roughly 200–700 Hz range rather than the intermittent pulses of families like Gymnotus — and it is this steady hum, not eyesight, that does most of the work of sensing prey, obstacles and other fish in the dark. Adults reach a modest maximum size of around 6 in total length, in keeping with the genus's generally small, easy-to-house build.
Habitat
Eigenmannia besouro is known only from the rio São Francisco basin in Bahia state, northeastern Brazil — a large, geographically isolated river system that drains much of the interior of the Brazilian Northeast independently of the Amazon. That narrow-range endemism is itself the species' defining ecological fact: unlike the broadly distributed Eigenmannia virescens complex once thought to span most of tropical South America, E. besouro appears to be confined to this one basin, a pattern increasingly recognized across the trilineata species-group as taxonomists resolve what used to be treated as a single wide-ranging species into several narrower ones.
Like other Sternopygidae, it is a fish of flowing water — moving channels and river margins rather than still backwaters — where turbidity or shade keeps light levels low and electrolocation does the work that vision cannot. Warm, soft to moderately mineralised, near-neutral water is typical of the family, and tannin-stained or dimly lit conditions suit its electric sense; sudden, harsh light and glare are stresses to be avoided in the aquarium.
Feeding
As with other glass knifefishes, Eigenmannia besouro is a nocturnal micropredator, working over the bottom and among roots and vegetation after dark to find small aquatic invertebrates — insect larvae, worms and tiny crustaceans — largely by electrolocation rather than sight. It is not an ambush predator; it actively patrols its territory, probing the electric field it generates for the tiny distortions a hidden prey item causes.
In the aquarium, Eigenmannia species readily take small meaty foods — bloodworm, brine shrimp, chopped worms and suitably sized sinking pellets — and because they are peaceful toward anything too large to swallow, they do well feeding alongside similarly sized tankmates rather than needing to be fed in isolation. Feeding in the evening, when the fish naturally becomes active, gets the best results.
Mating
Electric signalling is central to courtship in Eigenmannia. The genus's wave-type discharge carries a fixed frequency that differs between males and females, and this sex-specific signature — together with the jamming-avoidance response that lets two nearby fish shift their frequencies apart to avoid interfering with each other's sensing — is thought to play a direct role in mate recognition and courtship interactions in the dark. Much of what is understood about this signalling comes from laboratory electrophysiology on Eigenmannia generally rather than from field observation of E. besouro specifically.
Beyond the electrical courtship, detailed accounts of pairing and spawning behaviour in this species are not documented; data are sparse for this narrow-range, recently described species, and what is known about mating leans heavily on the wider genus.
Breeding
Eigenmannia as a genus breeds more readily in captivity than most gymnotiforms, and species in the trilineata group are known to spawn adhesive eggs scattered among plant roots or floating vegetation, typically under warm, soft-water conditions associated with seasonal high water. Parental care is minimal to absent — eggs are left among the vegetation rather than guarded.
Specific captive-breeding reports for Eigenmannia besouro itself are not documented; given how recently the species was described and its restricted range, its breeding biology in the aquarium remains largely unrecorded, and the general pattern for the genus is the best available guide rather than a confirmed protocol for this species.
In the aquarium
Glass knifefish in general, and Eigenmannia in particular, are considered among the more approachable gymnotiforms for the aquarium — modest in size, generally peaceful, and one of the few genera in the order that does well kept in a small shoal rather than alone. A tank with gentle flow, soft furnishings such as fine-leaved plants or root tangles to shelter among by day, and subdued lighting suits their nocturnal, electrolocation-driven habits; harsh overhead light and a bare, glaring tank are stressors.
Being scaleless, E. besouro needs the same caution over water quality and medication that applies across the order: no copper-based treatments, reduced doses of anything else, and consistently low ammonia and nitrate in mature, well-filtered water. As a nocturnal predator it should not be trusted with fish or shrimp small enough to be mistaken for prey after dark, even though it is otherwise a peaceable tankmate for similarly sized fish. Given its narrow natural range and recent description, wild-collected stock — where it appears in the trade at all — should be favored only from traceable sources, and captive-bred fish preferred as they become available.
Conservation
Eigenmannia besouro is assessed by the IUCN as Near Threatened, a status that reflects its restriction to a single river basin — the rio São Francisco in Bahia, Brazil — rather than any immediate population collapse. Single-basin endemics of this kind are inherently vulnerable: the entire species depends on the health of one river system, and pressures such as damming, water abstraction, pollution and habitat alteration in the São Francisco basin have the potential to affect the whole population at once, in a way that would not threaten a species distributed across several independent drainages.
As a recently described species with a narrow range, E. besouro is a useful reminder that many gymnotiforms once assumed to be widespread and secure are, on closer taxonomic examination, several separate narrow-range species — each individually more exposed to habitat change than the old, lumped "glass knifefish" was assumed to be.