Taxonomy & naming
Eigenmannia catira was described by Cardoso & Dutra in 2023 from specimens collected in the upper rio Paraná basin — the holotype comes from Córrego do Schmidt, with the type series drawn from the upper rio Grande and its tributaries in São Paulo state, Brazil. It belongs to the family Sternopygidae (the glass and rattail knifefishes) within the order Gymnotiformes, the New-World electric knifefishes, and more specifically it is placed in the Eigenmannia trilineata species-group, a cluster of superficially similar glass knifefishes distinguished mainly by fine anatomical and genetic characters rather than obvious external differences.
Gymnotiformes are bony fish, but unusual ones: nearly every species in the order generates a weak electric organ discharge (EOD) around its body, used for electrolocation in the dark, silty rivers where eyes are of limited use, and for communication between individuals. Eigenmannia is a classic wave-type genus — its members produce a continuous, near-sinusoidal discharge rather than the intermittent pulses of some other knifefish lineages — and the genus is the textbook example neuroscientists use to study the jamming avoidance response, the reflex by which two nearby fish shift their discharge frequencies apart to avoid interfering with each other's electrolocation. Despite the shared common name, gymnotiforms are not related to the Asian featherback and clown knifefishes (Chitala, Notopterus); the resemblance is convergent, not a sign of kinship. The Catalog of Fishes (Eschmeyer, CAS) is the authority for the valid name.
Morphology
As a member of the Eigenmannia trilineata group, E. catira has the classic "glass knifefish" build: a long, laterally compressed, semi-transparent body tapering to a fine point at the tail, with no dorsal fin and no true caudal fin. Locomotion comes almost entirely from a very long anal fin running most of the length of the underside, thrown into travelling waves that let the fish glide forward or backward with equal control — useful for backing out of the tight cover it hunts among. The largest specimen examined in the type series measured 8.5 in in standard length, roughly 8–8.5 in total length, making it a mid-sized member of the genus.
Cytogenetic work on populations from the Iguatemi River, in the same upper Paraná drainage, found a diploid chromosome number of 2n = 36 (2 metacentric, 10 submetacentric, 8 subtelocentric and 16 acrocentric chromosomes), with no heteromorphic sex chromosomes detected across the sexed specimens. As in other Eigenmannia, the electric organ runs the length of the body beneath the skin, and the skin itself is scaleless — a detail with real consequences for how the fish must be kept and treated for disease.
Habitat
Eigenmannia catira is known from tributary streams of the upper rio Paraná basin in São Paulo state, southeastern Brazil, including the Iguatemi River drainage in neighboring Mato Grosso do Sul — a narrow-range, single-basin distribution typical of many recently described glass knifefishes. As with its relatives, it is a fish of warm, soft, acidic to near-neutral Neotropical fresh water, generally in the range of 23–28 degrees Celsius with a pH around 6.0–7.0 and low mineral hardness, in flowing streams and rivers rather than still water.
Glass knifefishes are active mainly after dark and by day tend to shelter among roots, submerged vegetation and other cover in dim or turbid water — precisely the low-visibility conditions in which an electric sense pays off over eyesight. Specific habitat data for E. catira beyond its collecting localities is sparse; most of what can be said about its day-to-day ecology is extrapolated from better-studied congeners such as E. virescens, which occupies similar tannin-stained, sluggish-to-moderate-flow water across a much wider South American range.
Feeding
No feeding data specific to Eigenmannia catira has been published; the original description is focused on taxonomy, morphology and cytogenetics rather than ecology. Based on the genus as a whole, it is almost certainly a nocturnal micro-predator, working over the substrate and among roots after dark for small aquatic invertebrates — insect larvae, worms, tiny crustaceans and similar prey — located largely by electrolocation rather than sight, in water where visibility is often poor.
In the closely related and much better documented E. virescens, this translates in captivity to a fish that readily takes small live and frozen foods — bloodworm, brine shrimp, chopped worms — offered in the evening when the fish becomes active. There is no reason to expect E. catira to differ meaningfully, though this is inference from the genus rather than a documented fact about the species itself.
Mating
Nothing has been published specifically on courtship behavior in Eigenmannia catira. Across the genus, however, the wave-type electric discharge is central to social and reproductive signalling: Eigenmannia species show sex-specific discharge frequencies, and the same jamming avoidance response that lets two nearby fish separate their electrolocation signals is thought to double as a channel for recognizing and approaching potential mates in the dark. Courtship in glass knifefishes is therefore understood, where it has been studied at all, as substantially an electrical exchange layered over physical approach, rather than visual display.
Beyond that general picture drawn from the genus, data on E. catira's own mating behavior is sparse to nonexistent — it is a species known so far chiefly from museum specimens collected for taxonomic and cytogenetic study, not from behavioral observation in the field or in captivity.
Breeding
Eigenmannia catira has no documented breeding record, in the wild or in captivity; it is a recently described species not established in the aquarium trade. Sternopygidae as a family are egg-layers, and the best-known congener, E. virescens, spawns adhesive eggs scattered among floating plant roots and fine vegetation, with little or no parental care afterward — a pattern reasonably assumed to extend to E. catira, though it has not been confirmed for this species.
Glass knifefishes in the genus Eigenmannia are, as a group, bred in captivity somewhat more readily than most gymnotiforms, but that history belongs to the widely traded E. virescens rather than to E. catira, which for now remains a taxonomically and cytogenetically documented species with essentially no reproductive data of its own.
In the aquarium
Eigenmannia catira is not established in the aquarium hobby; it was described in 2023 from a localized part of the upper Paraná basin and, so far as available sources show, is not currently offered in the trade. Any aquarium guidance has to be extrapolated from its genus, with the caveat that this species itself is essentially untested in captivity.
Glass knifefishes in general are considered one of the more manageable entry points into keeping electric knifefish, doing well in a small shoal of their own kind in a well-covered, dimly lit tank with soft, warm, slightly acidic water. Being scaleless, they need careful medication — no copper-based treatments, and reduced doses of anything else — along with clean water and low nitrate, since scaleless skin is unusually permeable and sensitive. As nocturnal micro-predators they are not safe with fish or shrimp small enough to be hunted after dark, and they appreciate plenty of cover such as roots, pipes or dense planting to retreat into by day. Should E. catira ever reach the trade, these general Eigenmannia husbandry principles are the reasonable starting point; captive-bred stock should always be favored over wild collection from its narrow native range.
Conservation
Eigenmannia catira has not yet been formally assessed by the IUCN Red List; its conservation status is Data Deficient in practice, reflecting how recently it was described and how little is known of its population size, trend or full range beyond the upper rio Paraná tributaries where it has been collected.
As a species known only from a limited area of the upper Paraná basin in São Paulo state and adjoining Mato Grosso do Sul, it shares the vulnerability common to narrow-range Neotropical stream fishes: land-use change, damming and water-quality degradation in tributary streams of an intensively developed agricultural region are the plausible pressures, though none has been specifically quantified for this species. Its formal description is recent enough that a proper conservation assessment, grounded in actual survey data across its known range, is still pending.