Taxonomy & naming
Microsternarchus longicaudatus was described in 2024 in the journal Zootaxa, based on a single holotype (MPEG 14690, about 3.5 in total length) collected from Igarapé da Ponte, a clearwater stream near Juruti in Pará, Brazil, in the lower Rio Amazonas basin. It is only the third species recognized in the genus Microsternarchus, joining M. bilineatus and M. brevis. The genus sits in the family Hypopomidae — the bluntnose knifefishes — within the order Gymnotiformes, the New-World electric knifefishes. Gymnotiformes are bony fish, but unusual ones: nearly every species in the order produces a weak electric organ discharge (EOD) around its body, used for electrolocation in the dark and for communication with other individuals of the same species. Hypopomids like Microsternarchus produce a pulse-type discharge, delivered in discrete pulses rather than the continuous wave-type signal of families such as the ghost knifefishes.
The species name longicaudatus is Latin, from longus ('long') and caudatus ('tailed'), referring to the elongated caudal filament that distinguishes it from its congeners — proportionally the longest such filament recorded in the genus, at roughly 35–37% of total length. As with all Gymnotiformes, Microsternarchus has no relationship to the Asian featherback or clown knifefishes (Chitala, Notopterus); the similar 'knifefish' body plan is convergent, not a sign of kinship. Catalog of Fishes is the reference for the currently valid name; because the species was described in 2024, it has not yet been indexed by FishBase, and no IUCN Red List assessment yet exists for it.
Morphology
The holotype measures about 3.5 in total length (roughly 3 in standard length), making Microsternarchus longicaudatus a small member of an already small-bodied family. Like other hypopomids it has the classic bluntnose-knifefish build: a slim, laterally compressed body, a blunt, rounded snout rather than a drawn-out probe, no dorsal fin, and a long ribbon-like anal fin running along most of the underside that provides essentially all of its locomotion, allowing it to move forward and backward with equal ease. It is scaleless, like every member of the order.
The defining feature of the species is its tail: an elongated caudal filament, proportionally the longest known in the genus, extending well beyond the body. The description also notes irregular blotches beneath the eye (suborbital region) and small black spots scattered across the mid-dorsal body — a cryptic patterning consistent with a fish that spends its life hidden in leaf litter and shaded stream margins rather than swimming in the open. Running beneath the skin is the electric organ that generates its pulse-type discharge, paired with electroreceptor pores that let it read the field for prey and obstacles in the dark.
Habitat
The only confirmed locality for Microsternarchus longicaudatus is Igarapé da Ponte, a clearwater stream near Juruti, in the state of Pará, in Brazil's lower Amazon basin — the fish appears, on current evidence, to be a narrow-range endemic of this drainage. Clearwater igarapés of this kind are typically shallow, shaded forest streams with sandy or leaf-littered substrate, slow to moderate flow, and warm, soft, often slightly acidic water — very different chemically from the blackwater and whitewater rivers that dominate much of the wider Amazon, though still classic Neotropical stream habitat.
As with other Hypopomidae, this is almost certainly a nocturnal, bottom-associated fish that spends daylight hours buried or hidden among leaf litter, roots, or sand, becoming active after dark when it relies on its electric sense rather than eyesight to find its way and locate prey. No detailed field observations of its ecology have been published beyond the type description, so specifics of its microhabitat and behavior are inferred from the wider genus and family rather than documented directly for this species.
Feeding
No dedicated dietary study exists for Microsternarchus longicaudatus. Based on the well-documented feeding ecology of Hypopomidae generally, it is almost certainly an invertivore, foraging at night over sand and through leaf litter for small aquatic insect larvae, worms, and other benthic invertebrates, located largely by electrolocation rather than sight in the dim, shaded water it inhabits.
There is no information on feeding behavior in captivity, since the species is not known to be established in the aquarium trade. Data here is sparse, and any specific feeding guidance would be an extrapolation from related bluntnose knifefishes rather than a documented fact for this species.
Mating
Courtship and mate recognition in Hypopomidae are believed to be mediated substantially by the pulse-type electric organ discharge, which — as in other Gymnotiformes — is thought to carry information about a fish's species, sex, and individual identity, allowing potential mates to find and assess one another in the dark leaf litter and turbid margins these fish inhabit. No courtship behavior has been directly observed or described for Microsternarchus longicaudatus specifically.
Beyond that general pattern shared across pulse-type electric knifefishes, nothing further is documented for this species. Given it was described from a single holotype in 2024, essentially nothing is known about its reproductive behavior in the wild, and no captive observations exist.
Breeding
Breeding in Microsternarchus longicaudatus is entirely undocumented. The wider pattern in Hypopomidae and the related Rhamphichthyidae is poorly studied even for far better-known species: where anything is recorded, these small hypopomids are egg-layers that appear to scatter or deposit eggs among leaf litter, roots, or sand rather than building nests or forming lasting pairs, generally without meaningful parental care, and often tied to seasonal (wet-season) cues. Whether this applies to M. longicaudatus specifically is unconfirmed.
There are no captive-breeding reports for this species, unsurprising given its 2024 description and its absence from the aquarium trade. Data is sparse to the point of nonexistence here, and any account of a specific breeding protocol for this fish would be fabricated rather than sourced.
In the aquarium
Microsternarchus longicaudatus is not currently known to be in the aquarium trade; it was described from a single scientific holotype collected for taxonomic research, not from commercial collection. Any aquarium guidance here is therefore necessarily general, drawn from the husbandry needs of its close relatives in Hypopomidae and Rhamphichthyidae, the bluntnose and sand knifefishes, rather than from documented experience with this species itself.
Were it to appear in the hobby, it would need the same careful handling those relatives require: warm, soft, dimly lit water with a fine sand substrate and ample leaf litter or cover to hide in by day, since it is a nocturnal, cover-dependent fish. Being scaleless, like all Gymnotiformes, it would be sensitive to ammonia and nitrate and to copper-based or harsh medications, which should be avoided or used at reduced doses. Tankmates would need to be small, peaceful, and not prone to picking at a delicate, slow-moving fish that spends its day buried or hidden. At present, the responsible position is that this species should be left to scientific collection and study rather than sought for the trade.
Conservation
Microsternarchus longicaudatus has not been assessed by the IUCN Red List; its conservation status is Not Evaluated (NE), a reflection of how recently it was described (2024) rather than any judgment about its population trend. It is currently known only from its type locality, Igarapé da Ponte near Juruti, Pará, in Brazil's lower Amazon basin, making it — on present evidence — a narrow-range, single-stream endemic.
Species with this kind of restricted distribution in the lower Amazon are potentially vulnerable to the pressures affecting the region generally: deforestation, land-use change, and water-quality degradation in and around small forest streams like igarapés. Juruti itself is an area with active bauxite mining, which places additional pressure on local waterways, though no study has specifically linked this species' status to that activity. Without further survey work, its actual range and vulnerability remain unknown, and any stronger conservation claim would go beyond what the evidence currently supports.