Taxonomy & naming
Brachyhypopomus arrayae was described by Crampton, de Santana, Waddell & Lovejoy in 2017 as part of a sweeping taxonomic revision that split the genus Brachyhypopomus into fifteen newly recognised species. It belongs to the family Hypopomidae, the bluntnose knifefishes, within the order Gymnotiformes — the New-World electric knifefishes. Like all gymnotiforms it generates a weak electric organ discharge (EOD) around its body, used for electrolocation in murky water and for communication with other knifefish; in Brachyhypopomus and its Hypopomidae relatives this discharge is pulse-type, delivered as discrete pulses rather than a continuous wave.
The species epithet honours Mariana Arraya, a Bolivian biologist who assisted the authors in collecting the type series in Bolivia. Despite the shared common name "knifefish", Gymnotiformes are entirely unrelated to the Asian featherback and clown knifefishes (Chitala, Notopterus) — the resemblance is convergent body shape, not kinship. The Catalog of Fishes is the authority for the valid name.
Morphology
Brachyhypopomus arrayae is a small, soft-bodied fish reaching around 5 in total length in the largest known specimen, a paratype female. Like other bluntnose knifefishes it has a blunt, rounded snout rather than the elongated probing snout of the sand knifefishes (Rhamphichthys, Gymnorhamphichthys), and — as in all Gymnotiformes — it lacks a dorsal fin and a proper tail fin, swimming instead by rippling a single long anal fin that runs for most of the length of its underside. This allows it to move forwards and backwards with equal ease, useful for a fish that spends its life navigating tangled leaf litter and root mats.
The species can be distinguished from its close relatives by fin-ray counts: 175–212 rays in the anal fin and 16–19 (typically 17) in the pectoral fins. Females are reported as darker than males, a rare hint of external dimorphism in a group where the sexes are usually very difficult to separate visually. As with all gymnotiforms, the body is scaleless, and an electric organ runs along the trunk paired with sensory pores across the skin that read the field it produces.
Habitat
The species is known from the río Beni drainage of Bolivia, part of the upper Madeira system that ultimately feeds the Amazon. The holotype was collected from a stream near Puerto Hamburgo on the río Beni (approximately 11°01'52"S, 66°05'39"W). It inhabits high-conductivity whitewater floodplain systems, with recorded water conductivity around 150 µS/cm — sediment-laden, nutrient-rich water typical of Andean-fed Amazon tributaries rather than the blackwater habitats favoured by some other gymnotiforms.
It occurs in apparent sympatry with its sister species B. alberti, the two seemingly separated by fine differences in microhabitat rather than by geography. Specific temperature and pH data for the type locality have not been documented in the available literature; as with related Brachyhypopomus, it is reasonable to expect warm, soft, mineral-rich floodplain water, but this should be treated as inference rather than a measured record for this species.
Feeding
No species-specific diet study exists for Brachyhypopomus arrayae. Across the genus, Brachyhypopomus are cryptically pigmented, nocturnal predators of small aquatic invertebrates, hunting after dark in lentic and slow-flowing habitats such as streams, swamps and seasonal floodplains. Like other gymnotiforms they generate a pulsed electric organ discharge — in this genus typically 0.5–5 milliseconds in duration — which they use to electrolocate prey hidden in sediment or leaf litter where eyesight is of little use.
In the absence of direct observation of B. arrayae, its diet is inferred to be similar to congeners: small worms, insect larvae and other benthic invertebrates picked out of the floodplain substrate at night. This is a reasonable extrapolation from the genus rather than a documented fact for this species.
Mating
As in other gymnotiforms, courtship in Brachyhypopomus is thought to be mediated at least in part by the pulsed electric organ discharge, which carries information about a fish's identity and readiness to spawn and which rival or prospective mates read and respond to in the dark. No courtship behaviour has been directly documented for B. arrayae itself; this account follows what is known more generally of pulse-type Hypopomidae, where electric signalling accompanies physical approach rather than a documented, choreographed display.
Breeding
Breeding biology for Brachyhypopomus arrayae is undocumented — this is a fish known essentially only from museum type material and a handful of field collections, not from sustained observation in the wild or in captivity. Across Hypopomidae generally, where breeding has been recorded at all, these are egg-layers that scatter or deposit eggs among leaf litter, roots or sand, typically timed to the wet season, with little or no parental care afterward.
There is no aquarium breeding record for this species, and none should be expected without further study. Data sparse is the honest summary; nothing here should be read as an established protocol.
In the aquarium
Brachyhypopomus arrayae is not established in the aquarium trade and there is no husbandry record for it. What can be said follows from its family and its habitat: bluntnose knifefishes are peaceful, delicate, nocturnal fish that do best in soft, dimly lit tanks with a soft sand substrate and ample cover — leaf litter, roots and fine plants — to mimic the floodplain leaf litter and root mats they shelter in by day. Being scaleless, gymnotiforms as a group are sensitive to ammonia and nitrate and to copper-based or otherwise harsh medications, which should be avoided or used at reduced dose.
Given this species' small recorded size (around 5 in) and its known conductivity range from a high-conductivity floodplain system, water should be kept clean, warm and stable, with gentle flow and minimal disturbance. Any fish reaching hobbyist hands would almost certainly be a byproduct of scientific collection rather than the ornamental trade, and should be treated as a delicate, poorly known species rather than a standard community fish.
Conservation
Brachyhypopomus arrayae has not been formally assessed by the IUCN Red List; its status is Not Evaluated. This is typical for recently described gymnotiforms — the species was named only in 2017 as part of a fifteen-species revision of the genus, and formal conservation assessments have not caught up with the taxonomy.
It is known only from the río Beni drainage in Bolivia, part of the upper Madeira system, making it a narrow-range endemic by the evidence currently available. Species restricted to a single river system are inherently more vulnerable to habitat change, damming and pollution than widespread relatives, even absent a formal assessment; without further survey work, its true conservation status remains unknown.