Taxonomy & naming
Microsternarchus brevis was described by Cox Fernandes, Nogueira, Williston & Alves-Gomes in 2015 and belongs to the family Hypopomidae, the bluntnose knifefishes, within the order Gymnotiformes — the New-World electric knifefishes. It sits in the tribe Microsternarchini within that family. Gymnotiformes are a genuinely distinct branch of bony fish, defined less by outward shape than by an electric organ and a single long, ribbon-like anal fin that runs most of the length of the underside; nearly every species in the order produces a weak electric organ discharge (EOD) used to sense its surroundings and to communicate, a trait this small species shares in full despite its size.
The genus name Microsternarchus reflects the fish's small electric organ (from Greek roots referring to "small" and the sternarchid-type organ), while the species epithet brevis is Latin for "short," describing its small overall size and short body relative to other hypopomids. The Gymnotiformes are entirely unrelated to the superficially similar Asian featherback and clown knifefishes (Chitala, Notopterus), which are bonytongues in a different order altogether — the resemblance is convergent, not a sign of kinship. The Catalog of Fishes is the authority for the valid name.
Morphology
Microsternarchus brevis reaches only about 2 in in total length, making it the smallest described species in the entire order Gymnotiformes. Like other hypopomids it is elongated and laterally compressed, scaleless, and lacks a dorsal fin and a conventional tail fin; locomotion comes from undulations of the long anal fin running beneath the body, letting the fish move forward or backward with equal ease over the substrate it forages across.
As in all Gymnotiformes, an electric organ runs along the body and is paired with electroreceptor pores scattered over the skin, together forming the electrosensory system the fish relies on in place of vision in its dark, leaf-litter habitat. Hypopomids, including Microsternarchus, generate a multiphasic pulse-type discharge rather than the continuous wave-type signal of families like the ghost knifefishes — a slower, intermittent pulsing pattern suited to short-range electrolocation. Sexes are not documented as externally distinguishable in this species.
Habitat
Microsternarchus brevis is known only from the lower Rio Negro drainage in Amazonas State, Brazil — its type locality is a small forest stream, Igarapé Jaradi, near Santa Isabel do Rio Negro. Available records place it within this Amazon basin tributary system, and it appears to be a narrow-range, single-river endemic rather than a widely distributed species.
It lives in shallow freshwater creeks and streams typical of the Rio Negro system: soft, strongly acidic, tannin-stained blackwater running through dense leaf litter, submerged roots and vegetation, with a soft or sandy bottom. Water there runs warm (around 77–79 °F) and very acidic (pH roughly 4.0–5.0) — among the softest, most acidic conditions any aquarium fish is drawn from. As with other Gymnotiformes, that gloom and turbidity is exactly where an electric sense earns its keep, letting the fish find its way and find food where eyesight would be nearly useless.
Feeding
Direct dietary studies of Microsternarchus brevis are not available, but its ecology points clearly to a nocturnal, benthic invertebrate forager, consistent with other small Hypopomidae. It most likely takes small aquatic insect larvae, worms and tiny crustaceans picked out of leaf litter, sand and root tangles after dark, located by electrolocation rather than sight in the dim, stained water it inhabits.
No aquarium feeding records exist for this species given its rarity in the trade; care notes for related bluntnose knifefishes suggest small live or frozen foods — bloodworm, micro-worms, small crustaceans — offered after dark would suit its natural foraging pattern, but this should be treated as an inference from its relatives rather than a documented protocol for the species itself.
Mating
The courtship and mating behaviour of Microsternarchus brevis has not been documented in detail. In pulse-type Hypopomidae generally, the electric organ discharge doubles as a social signal alongside its sensing role, with discharge waveform and timing varying by species and sex and likely playing a part in how individuals recognize potential mates and rivals in the dark. It is reasonable to expect a similar electrical component to courtship in this species, but this is inferred from the family rather than observed directly in M. brevis.
Data on this species' reproductive biology is sparse, and no specific courtship account has been published to date.
Breeding
Breeding biology in Microsternarchus brevis is poorly documented, and no captive-breeding records are known. Based on the general pattern in small Hypopomidae, it is presumed to be an egg-layer that scatters or deposits eggs among leaf litter, roots or sand rather than building a nest or forming a lasting pair bond, likely timed to seasonal wet-period conditions in its home streams, with little or no parental care afterward.
Given how rarely this species is even seen in the hobby, there is no aquarium spawning account to draw on. The honest position is that its breeding mode follows the hypopomid pattern by inference, not direct observation, and remains data sparse.
In the aquarium
Microsternarchus brevis is essentially unknown in the aquarium trade — sourcing is sparse to nonexistent, and almost nothing has been published on keeping it in captivity. What can be said follows from its documented wild conditions and its family: it would need warm (around 77–79 °F), very soft, strongly acidic water (pH roughly 4.0–5.0), a soft sand or leaf-litter substrate, and dense cover from roots, driftwood and plants to feel secure, with subdued or no lighting given its nocturnal habits.
Being scaleless, like every Gymnotiform, it would be especially sensitive to ammonia and nitrate and to copper-based or otherwise harsh medications, which should be avoided or heavily reduced if ever needed. Its tiny size at just over 2 in makes it fundamentally different to keep from the larger, better-known knifefishes of the hobby — it would need small, similarly gentle tankmates, if any, rather than the spacious predator-proof setup a black ghost or Gymnotus demands. Given the near-total absence of husbandry information, this is not a species to acquire casually; it belongs with specialists prepared to treat its care as an open question.
Conservation
Microsternarchus brevis is assessed by the IUCN Red List as Least Concern, with the assessment dated 2022. Available sources describe its population as stable, without specific documented threats, though as a species known only from the lower Rio Negro drainage it is inherently a narrow-range endemic, the kind of distribution that is more exposed to localized pressures — habitat change, pollution or deforestation in its home tributaries — than a widely distributed species would be.
The Least Concern rating should be read against that geographic narrowness: little population monitoring specific to this small, cryptic, recently described fish appears to exist, and its status could shift quickly if conditions in the lower Rio Negro system changed. For now, no elevated conservation concern is documented.