Taxonomy & naming
Microsternarchus bilineatus was described by Fernández-Yépez in 1968 and belongs to the family Hypopomidae — the bluntnose knifefishes — within the order Gymnotiformes, the New-World electric knifefishes. Gymnotiformes are bony fish but an unusual lineage among them, set apart by a dedicated electric organ and a single long ribbon-like anal fin used for propulsion in place of a normal tail. The Hypopomidae are among the smallest-bodied members of the order, and Microsternarchus sits within the tribe Microsternarchini inside that family.
The genus name reflects the fish's build: a small ("micro-") relative of Sternarchus-type knifefishes. A 2015 redescription in the Proceedings of the Academy of Natural Sciences of Philadelphia designated a neotype for the species, fixed its type locality in the Orinoco River, and confirmed its placement in Hypopomidae — while also flagging that specimens identified as "M. bilineatus" across its wide Neotropical range (Orinoco, Amazon, Guiana Shield) may represent more than one cryptic lineage, a common situation in small, poorly-studied Gymnotiformes. A 2016 cytogenetic study of Negro River material further supports the species' validity with a reported 2n=40 karyotype. As with all Gymnotiformes on this site, the Catalog of Fishes is treated as the authority for the valid name; no English common name is in wide use, though the Portuguese "Sarapó" is applied to it and to related small knifefishes.
Like every member of its order, Microsternarchus bilineatus is unrelated to the Asian featherback and clown knifefishes (Chitala, Notopterus) despite the shared "knifefish" name — those are bonytongues from an entirely separate lineage, and the resemblance between the groups is convergent body shape, not kinship.
Morphology
This is a small fish even by Hypopomidae standards, reaching a maximum of about 4.5 in in total length. It has the compressed, blade-like knifefish body plan in miniature: no dorsal fin, no true tail fin, and a long anal fin running most of the underside that ripples in travelling waves to drive the fish forward or backward with equal ease. The body is scaleless and soft, a feature shared across the bluntnose knifefishes, and the snout is blunt and rounded rather than drawn into the long probing tube seen in the related sand knifefishes (Rhamphichthys, Gymnorhamphichthys).
Running the length of the body is the electric organ, paired with electroreceptor pores across the skin that read the field it produces. In Hypopomidae, including Microsternarchus, this discharge is PULSE-type: a biphasic pulse repeated at a slow rate of roughly 3–16 Hz, quite unlike the continuous high-frequency hum of a wave-type knifefish such as the black ghost. That slow, ticking discharge is well suited to a cryptic, low-energy lifestyle spent mostly motionless in cover. Being scaleless and soft-bodied, the species is delicate to handle and, like its relatives, sensitive to poor water quality.
Habitat
Microsternarchus bilineatus is Neotropical, documented from the upper Orinoco and Rio Negro basins and more broadly across Amazonian and Guiana Shield drainages in South America. It is a fish of soft, blackwater and clearwater tributaries rather than the main river channels, typically found among leaf litter, submerged roots and fine sediment at the margins of streams and flooded forest.
Like other small Hypopomidae, it is cryptic and largely nocturnal, lying up by day within leaf-litter banks and becoming active after dark. This is dim, tannin-stained, structurally complex water where vision is of little use, which is exactly the setting in which a pulse-type electric sense is most valuable — sensing nearby objects and prey by touch-at-a-distance rather than by sight.
Feeding
Direct dietary data for Microsternarchus bilineatus specifically is sparse, but the Hypopomidae as a group are nocturnal carnivores that forage over and within leaf litter and soft sediment for small invertebrates — aquatic insect larvae, worms and tiny crustaceans — locating them chiefly by electrolocation in the dark rather than by sight. As a small-bodied species it is likely limited to correspondingly small prey items.
In the rare event of aquarium care, small Hypopomidae are generally kept on live or frozen foods sized for a fish of a few centimetres — bloodworm, micro-worms, small crustaceans such as daphnia or brine shrimp, and similar meaty foods offered after dark, when the fish is naturally active.
Mating
Courtship and reproduction in Microsternarchus bilineatus are not documented in detail in the available literature. What can be said with confidence, extrapolating from the wider Hypopomidae and Gymnotiformes, is that the pulse-type electric organ discharge is likely to play a role in social signalling and courtship, since pulse frequency and waveform in this family carry information about individual identity and reproductive state, much as they do in better-studied gymnotiforms. Beyond that electrical signalling, no specific courtship behaviour has been described for this species — this is a case where the honest answer is that data are sparse.
Breeding
Breeding biology in Microsternarchus bilineatus is poorly documented, in keeping with the wider pattern in small Hypopomidae. Where reproduction has been studied in related bluntnose knifefishes, it is understood to be egg-laying, with eggs deposited or scattered among leaf litter, roots or fine sediment rather than guarded in a nest, generally tied to seasonal (wet-season) conditions, and with little or no parental care afterward. No aquarium spawning of Microsternarchus bilineatus is documented — the species is essentially absent from the hobby, known mainly from field collections and museum material rather than captive populations.
In the aquarium
Microsternarchus bilineatus is not an established aquarium fish; it is essentially unavailable in the trade and what follows is inferred from its biology and from care of similar small Hypopomidae rather than from a developed keeping tradition. Any specimen that did reach a tank would need the same core provisions as other bluntnose and sand knifefishes: warm, soft, near-neutral to acidic water, dim lighting, and a deep layer of leaf litter or fine, soft substrate to burrow and hide in during the day, since these are nocturnal fish that rely on cover to feel secure.
As with all Gymnotiformes, the species is scaleless, which makes it unusually sensitive to ammonia and nitrate and to harsh medications — copper-based treatments should be avoided outright and other doses reduced. Given its small size and delicate, cryptic habits, it would be best suited to a quiet, species-appropriate setup with peaceful, similarly small tankmates rather than a general community tank, and captive-bred stock (were it ever to become available) would always be preferable to wild collection for a species this poorly known.
Conservation
Microsternarchus bilineatus is assessed as Least Concern on the IUCN Red List, reflecting a wide distribution across the Orinoco, Amazon and Tocantins basins that mitigates immediate extinction risk. Population trend data are sparse, and the assessment itself notes the limits of current knowledge about the species' status through its range.
As with many small, cryptic Neotropical fish, the main long-term pressures are indirect: habitat change from deforestation, water pollution, and disturbance of leaf-litter and blackwater habitats within its basins, rather than any targeted collection pressure, since the species has no meaningful presence in the aquarium trade. Taxonomic work suggesting cryptic lineage diversity within what is currently treated as a single wide-ranging species is also a reminder that conservation status assigned to the nominal species may not evenly reflect the situation of any one local population.