Taxonomy & naming
Leiarius arekaima was described by the Scottish naturalist Sir William Jardine in 1841, based on material collected during Robert Schomburgk's survey of British Guiana and published in Schomburgk's "Fishes of Guiana" (Edinburgh, 1841). It belongs to the family Pimelodidae, the long-whiskered catfishes of tropical South America, a family that has been heavily reshuffled by modern taxonomy as older, catch-all genera were split apart. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) retains Leiarius arekaima as the valid combination for this species.
The genus Leiarius is small, containing only a couple of recognised species alongside the more frequently traded Leiarius pictus (previously often confused with, or marketed under, arekaima or under older generic placements typical of nineteenth-century pimelodid taxonomy). The epithet arekaima appears to reference a Guianese place or indigenous name tied to the type locality rather than a Latin descriptive term; as with many South American catfishes of this era, the original description predates modern molecular taxonomy, and the genus placement has been stable under Leiarius in recent authoritative catalogs.
Morphology
Leiarius arekaima has the classic pimelodid silhouette: a broad, flattened head, a long, scaleless body tapering to a forked tail, and three pairs of long, sensory barbels — a prominent maxillary pair reaching well past the head and shorter mandibular barbels — used to feel out prey in dark or turbid water. The mouth is wide and terminal, built for engulfing prey whole rather than for grazing or grinding.
FishBase records a maximum size of 26 in standard length for this species — genuinely large for a home aquarium fish, and easily overlooked when the animal is purchased as a 2–3 in juvenile in a shop tank. Like other pimelodids it is entirely scaleless, with smooth, naked skin, and it carries stiff, serrated, locking spines at the leading edge of the dorsal and pectoral fins. These spines readily snag in an aquarium net and can jab the handler, so this fish should always be moved in a container rather than netted.
Habitat
The species is native to the Amazon and Guiana Shield drainages of Guyana and Brazil, where it occupies benthopelagic habitat — living near the bottom but ranging up into open water — of tropical river systems. Guiana Shield rivers are typically warm, softwater habitats draining ancient crystalline terrain, though FishBase does not publish confirmed temperature or pH figures specifically for L. arekaima.
As with many large, benthic South American catfishes, this species is a river-dwelling, nocturnal predator that likely shelters by day among submerged wood, undercut banks, or deep pools and becomes active as light fades. Data on the specific chemistry of its home waters is sparse; keepers should default to the warm, moderately soft-to-neutral conditions typical of Amazonian and Guianese lowland rivers rather than assuming blackwater extremes without direct confirmation for this species.
Feeding
Leiarius arekaima is a large nocturnal piscivore. In the wild it hunts fish and invertebrates from ambush or by active night-time foraging, using its long barbels to locate prey in low light, and it is capable of swallowing anything that fits inside its considerable gape.
In captivity it takes whole fish, prawn, and other meaty foods readily, along with large sinking pellets once acclimated. Any tankmate must be too large for this fish to physically swallow — a functional rule of thumb rather than a matter of temperament, since a mid-sized or small companion fish will simply be eaten regardless of how "peaceful" the catfish otherwise behaves toward larger fish.
Mating
There is no documented account of natural courtship or pairing behaviour for Leiarius arekaima in the aquarium literature. Large pimelodid catfishes of this kind are not known to spawn spontaneously in home aquaria; where this genus and its relatives are propagated at all, it is done commercially using hormone injection to induce spawning in mature broodstock, following the pattern used across large Neotropical catfish aquaculture.
Breeding
This is a large commercial catfish, and reproduction in this species is essentially confined to farm- or hormone-induced spawning rather than natural pairing in captivity. Its adult size of around 26 in, its nocturnal benthic habits, and the lack of any published account of spontaneous home spawning all point the same way: this is not a fish that home aquarists breed. Any offspring reaching the trade are the product of induced spawning at aquaculture or research facilities rather than hobbyist tanks, and there is no meaningful parental care associated with that process beyond the artificial handling of eggs and fry post-stripping.
In the aquarium
Leiarius arekaima is a serious size-trap species: it is often sold small and cheap, but a fish reaching 26 in needs housing on the scale of a large pond, a very large custom aquarium (many hundreds of litres, ideally 6 feet or longer at minimum once even half-grown), or a public-aquarium-style system — not a standard home tank. Prospective keepers should plan for the adult fish from day one, not the juvenile in the shop tank.
Because it is scaleless, it is medication-sensitive: copper-based treatments and many standard medications should be dosed conservatively, often at reduced strength, as is standard practice for naked-skinned catfishes. Its dorsal and pectoral spines are stiff, serrated, and lock into place, so it should be moved in a container rather than netted to avoid tangling and injury to both fish and handler. As a nocturnal predator it appreciates a dim retreat by day (a large cave, pipe, or shaded area) and becomes most active after lights-out. Tankmates must be robust and, above all, too large to be swallowed — this fish will eat any companion that fits in its mouth.
Conservation
Leiarius arekaima has not been evaluated by the IUCN Red List, so no formal conservation status exists for the species. It occupies river systems in the Amazon and Guiana Shield drainages of Guyana and Brazil that are not, on current information, subject to any documented species-specific pressure, but the broader region does face habitat pressures from mining, deforestation, and river modification that affect many co-occurring fishes. In the absence of an assessment, its true status in the wild is unknown, and the aquarium trade appears to rely on captive or farmed stock rather than heavy targeted wild collection of this particular species.