Taxonomy & naming
Eigenmannia sirius was described by Peixoto & Ohara in 2019 and 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 are defined by an electric organ used for electrolocation and communication and by a single long anal fin that replaces the normal fin plan of most fish. Despite the shared common name, they are not related to the Asian featherback and clown knifefishes (Chitala, Notopterus); the resemblance is convergent body shape, not kinship.
Within Eigenmannia, the species was diagnosed from its congeners chiefly by a superior midlateral stripe and, among the E. trilineata species-group, by a subterminal mouth that separates it from most relatives except E. besouro, E. correntes, E. vicentespelaea and E. waiwai. The species epithet sirius refers to the star Sirius (alpha Canis Majoris), which appears on the state flag of Mato Grosso, Brazil, tying the name to the type locality. The Catalog of Fishes is the authority for the valid name.
Morphology
Eigenmannia sirius is a small- to medium-sized glass knifefish, reaching only about 5 in in total length in the specimens on which it was described — modest even by the standards of a genus known for staying compact. Like other Eigenmannia it is semi-transparent, with the body's electric organ and internal anatomy visible through translucent flanks, and it lacks a dorsal fin and a distinct tail fin; propulsion comes from a long ribbon-like anal fin run in continuous waves along the underside of the body, letting the fish glide forward or reverse with equal ease.
The species carries the electric organ typical of its family, running most of the length of the body, and the sensory pores across the skin that read the field it generates. Its discharge is monophasic and wave-type, reported in the 100–780 Hz range — a steady, continuous hum rather than the intermittent pulses used by some other gymnotiform lineages. The skin is scaleless, a trait shared across the order that has real consequences for how the fish must be kept and medicated.
Habitat
The species is known only from the río Mutum, a tributary of the upper río Juruena within the río Tapajós basin near Comodoro, Mato Grosso, Brazil, at an elevation of about 1647 ft. Specimens were collected among root masses and submerged aquatic vegetation in clearwater habitat — the kind of structurally complex, slow-to-moderate flow typical of Sternopygidae, which favour slow-flowing streams, river edges, deep channels and floodplain margins rather than open, swift water.
As with other glass knifefishes, this is warm, soft to moderately mineralised Neotropical water, and the dim or shaded conditions among roots and plants suit an animal that leans on electrolocation rather than eyesight. No population data beyond the type locality has been published, and the species should be treated as a narrow-range single-river endemic until further surveys say otherwise.
Feeding
Direct dietary data for Eigenmannia sirius is sparse, but the original description characterises it as a small- to medium-sized omnivore with insectivorous leanings, consistent with the feeding ecology documented across the genus. Like other glass knifefishes it is a nocturnal micro-predator, most active after dark, using its electric sense to locate small invertebrates and other prey items hidden among roots, leaf litter and vegetation where vision would be of little use.
In the aquarium, glass knifefish of this genus are typically fed a variety of small meaty foods — bloodworm, brine shrimp, small chopped invertebrates and sinking prepared foods — offered in the evening or after lights-out to match their natural activity pattern. As with its better-known relatives, food small enough to be swallowed should be treated as prey rather than a safe tankmate.
Mating
No courtship account specific to Eigenmannia sirius has been published, but the genus is the classic model for wave-type electric communication: each fish's electric organ discharge carries a signature frequency, and in Eigenmannia this signal is central to how individuals recognise sex and readiness to spawn and how they avoid jamming one another's electrolocation when discharge frequencies are close — the well-studied jamming avoidance response. Courtship in this genus is understood to unfold substantially through this electrical channel, alongside physical approach in cover among roots and plants.
Beyond the electrical signalling, little is documented for this species' pairing or spawning behaviour, and the practical honest statement is that its reproductive biology in the wild remains largely unrecorded outside the type description.
Breeding
No captive-breeding account for Eigenmannia sirius has been published; the species was described from wild-collected specimens only, and it is best treated as unbred or essentially undocumented in the aquarium trade. Reproductive biology in Sternopygidae generally, drawn from the well-studied congener E. virescens, is egg-laying: females scatter or deposit some 100–200 adhesive eggs per clutch among aquatic plant leaves and root masses, with batch spawning repeated every few days under favourable conditions and reproductive peaks tied to rising-water, flood-season conditions — warm water and abundant fresh vegetation.
Parental care in this pattern is minimal to absent; eggs are left among the vegetation rather than guarded. Eigenmannia as a genus breeds more readily in captivity than many other gymnotiforms, but there is no specific record of E. sirius having been spawned outside its native río Mutum, so this should be read as data sparse rather than as an established protocol for the species.
In the aquarium
Eigenmannia sirius is not established in the aquarium trade; its narrow single-river range and recent description mean it is essentially unavailable to hobbyists, and any specific husbandry guidance for this species specifically would be speculation. What can be said with confidence comes from the genus: glass knifefish are semi-transparent, comparatively peaceful, and among the easier gymnotiforms to keep, often doing well in a small shoal of several individuals rather than alone, provided tankmates are too large to be preyed on and not prone to nipping the long anal fin.
As with all gymnotiforms, being scaleless makes the group sensitive to ammonia and nitrate and intolerant of copper-based medications, which should be avoided, with other treatments dosed cautiously. A dim, quietly-flowing tank with roots, plants or other cover to shelter in by day, and warm, soft, near-neutral water, mirrors the río Mutum habitat this species was collected from. Given its status as a newly described, single-locality endemic, any specimens that did reach the trade should be treated with particular conservation caution rather than as a routine community fish.
Conservation
Eigenmannia sirius is assessed as Least Concern on the IUCN Red List. That assessment sits somewhat uneasily alongside its natural history: the species is currently known only from its type locality in the río Mutum, a tributary of the upper río Juruena in the río Tapajós basin, Mato Grosso, Brazil — a genuinely narrow range for a fish with no confirmed records elsewhere.
Single-river endemics of this kind are generally the most exposed members of otherwise secure genera, vulnerable to localised habitat change, water-quality degradation, deforestation and hydrological disruption within their one known drainage even when the wider family faces no broad threat. With so little published beyond the original description, the most defensible position is that this species' status should be watched as further surveys of the upper Tapajós basin become available, rather than treated as settled.