Taxonomy & naming
Eigenmannia microstoma was first described by Danish zoologist Johannes Reinhardt in 1852 as Sternopygus microstomus, and was later moved into the genus Eigenmannia, where the Catalog of Fishes (Eschmeyer, CAS) lists it as the currently valid combination. It belongs to the family Sternopygidae — the glass and rattail knifefishes — within the order Gymnotiformes, the New-World electric knifefishes. Gymnotiformes are bony fish, but unusual ones: they lack a proper dorsal fin and tail fin, propel themselves with a single long anal fin, and nearly all of them generate a weak electric organ discharge (EOD) used for electrolocation and communication. Despite the shared common name, they are not related to the Asian featherback and clown knifefishes (Chitala, Notopterus); the knife-shaped body is a case of convergent evolution, not kinship.
Within Sternopygidae, Eigenmannia is the wave-type "hummer" genus best known to science through its close relative, the green glass knifefish (E. virescens), the classic subject of the jamming-avoidance-response studies that shaped modern electrophysiology. E. microstoma sits in what taxonomists call the trilineata species-group, diagnosed partly by vertebral counts, and cytogenetic work has confirmed it as a distinct karyotype closely allied to E. trilineata. The type locality is Lagoa Santa in Minas Gerais, Brazil, and the species is endemic to the São Francisco River basin — a comparatively narrow range for a genus otherwise distributed across much of tropical South America.
Morphology
Eigenmannia microstoma has the classic glass-knifefish silhouette: a long, laterally compressed, tapering body with no dorsal fin and no separate tail fin, propelled by a single greatly elongated anal fin that runs nearly the full length of the underside. The body is thin-skinned, scaleless, and semi-transparent in life, a quality that gives the genus its common name and lets the pale internal organs and the electric organ itself show faintly through the flank in good light. The mouth is notably small — a feature reflected in the species name microstoma — set low and slightly underslung, suited to picking small prey from the substrate and vegetation rather than taking large items.
Running the length of the body beneath the skin is the electric organ, generating a continuous, wave-type discharge, and scattered across the head and flanks are the electroreceptor pores that sense the field it creates and the way it is distorted by nearby objects. Diagnostic features separating E. microstoma from its close relatives are subtle and largely meristic — precaudal vertebral counts of 14 to 15 are cited as characteristic of its species-group — rather than anything visible to the aquarist. Sexes are not reliably distinguished by external appearance.
Habitat
Eigenmannia microstoma is a Neotropical freshwater fish restricted to the São Francisco River basin of eastern Brazil, one of South America's major river systems but a single basin rather than the continent-spanning range held by some of its relatives. Within that basin it is a fish of quiet, structurally complex water — deep ponds and backwaters with plant-rich substrates, undercut banks and floating meadows across the floodplain, the kind of dim, obstacle-filled habitat where electrolocation matters more than eyesight.
As with other glass knifefish, this is warm, still-to-slow-flowing Neotropical water, and the fish is active mainly after dark, spending daylight hours tucked among roots and vegetation. Because it is confined to a single river basin, its habitat requirements are tightly bound to the ecology of the São Francisco system, and data on its specific water chemistry preferences in the wild remain sparse in the literature.
Feeding
Direct dietary data for Eigenmannia microstoma is sparse, but its small mouth and its place within the glass knifefish genus point to a diet of small aquatic invertebrates — insect larvae, small crustaceans and other tiny prey picked from among plant roots and submerged vegetation. Like other Sternopygidae it is a nocturnal micro-predator, most active after dark, and locates prey largely by electrolocation, sensing the electrical signature of small animals hidden in cover rather than hunting by sight.
In the aquarium, glass knifefish in general take small meaty foods readily — bloodworm, brine shrimp, finely chopped invertebrate foods and appropriately sized frozen foods — offered in the evening or after lights-out to suit their nocturnal habits. Feeding data specific to E. microstoma has not been documented; care recommendations here follow the well-studied genus rather than the species itself.
Mating
As in other Eigenmannia, courtship in this species is presumed to be closely tied to its wave-type electric organ discharge. In the well-studied green glass knifefish, males and females carry sex-specific discharge frequencies and use the jamming-avoidance response — a reflex that shifts an individual's discharge frequency away from a neighbour's to avoid interference — as part of how they detect and approach one another in the dark. It is reasonable to expect similar electrical signalling in E. microstoma given the shared genus and discharge type, though this has not been directly documented for the species.
Specific courtship behaviour, spawning triggers and pairing behaviour of E. microstoma itself are not described in the available literature; this remains a data-sparse area for the species, and any account of its mating behaviour beyond the electrical signalling common to the genus would be speculation.
Breeding
Eigenmannia as a genus is bred in captivity more readily than most gymnotiforms, and the group's general method is instructive even though it has not been specifically reported for E. microstoma. Breeders simulate the onset of the rainy season with cool, soft, acidic water and a sustained drip or trickle of fresh water over several hours, which triggers spawning in successive pairings; eggs are adhesive and are laid, typically in the morning, among the roots of floating plants. Parental care is minimal to absent — eggs are simply left among the vegetation — and fry are raised on small live foods such as brine shrimp nauplii.
Whether E. microstoma itself has been bred in captivity is not documented in available sources; as a single-basin endemic rarely seen in the trade, it should be assumed poorly documented rather than assumed to follow the exact protocol used for its better-known relative, the green glass knifefish.
In the aquarium
Eigenmannia microstoma is essentially unknown in the hobby, and any aquarium guidance for it necessarily draws on the well-established care of its genus, particularly the widely kept green glass knifefish. Glass knifefish are considered one of the easier gymnotiforms to keep: they do well in a small shoal of their own kind, are peaceful toward fish too large to eat, and are less exclusively predatory than some other knifefish groups, though they remain nocturnal micro-predators that will take very small tankmates such as shrimp after dark.
Being scaleless, E. microstoma should be treated with the same caution as other gymnotiforms around medications — no copper-based treatments, and reduced doses of anything else — and given clean, mature water, since knifefish in general are sensitive to ammonia and nitrate. Dim lighting, dense planting or floating cover, and gentle water movement suit their nocturnal, electrosensory lifestyle. Given its extremely narrow natural range and scarcity in the trade, any specimens offered for sale should be treated as a conservation-sensitive fish, and captive-bred stock — where it can be found in the wider genus — is always preferable to wild collection.
Conservation
Eigenmannia microstoma is assessed as Data Deficient on the IUCN Red List, reflecting how little targeted survey and population data exist for this narrow-range endemic. Data Deficient is not a clean bill of health — it means the available data are insufficient to judge extinction risk one way or the other, a status shared by many gymnotiform species that are simply under-studied relative to their conservation needs.
Because E. microstoma is confined to a single river system, the São Francisco basin, it carries the vulnerability typical of single-basin endemics: any large-scale habitat change, damming, pollution or water diversion within that basin has an outsized effect on the species compared to a wide-ranging relative. The São Francisco has seen substantial hydrological modification from dams and water use over the past century, though the specific impact on this species has not been assessed. In the absence of population data, the most defensible conservation stance is caution — treating the species as potentially sensitive rather than assuming it is secure.