Taxonomy & naming
Arapaima leptosoma was described by the American ichthyologist Donald Stewart in 2013, in a revision of the genus Arapaima that split what had long been treated as a single wide-ranging species, A. gigas, into several distinct species scattered across the Amazon basin — among them A. gigas, A. agassizii, A. mapae and A. leptosoma. It sits in family Arapaimidae (sometimes still placed as a subfamily within Osteoglossidae) in the order Osteoglossiformes, the bonytongues — one of the oldest surviving branches of the teleost fish tree, so called because a toothed bone on the floor of the mouth works against a toothed plate on the roof of the mouth to grip and crush prey. The type locality is the confluence of the Solimões and Purus Rivers in Amazonas state, Brazil, and the species is known so far from a single holotype specimen; the fine-grained taxonomy of the wider Arapaima gigas species complex remains debated in the literature, and this account follows Stewart's original description while treating the genus-level picture as still unsettled.
The epithet leptosoma comes from Greek leptos, "slender", and soma, "body", describing the comparatively slender body form that Stewart used to help distinguish the new species from its relatives even at the juvenile stage of the holotype. The genus name Arapaima is generally traced to an indigenous Amazonian name for the fish, later adopted directly into scientific nomenclature; the widely used Brazilian name pirarucu, applied across the genus, is usually explained as combining Tupi words for "fish" and "red", after the coppery-red tail seen in mature arapaima.
Morphology
Because the only formally documented specimen of Arapaima leptosoma is a juvenile or subadult, its morphology is known largely from that single fish: a maximum recorded length of about 30.6 in standard length. That is a fraction of the size the genus is famous for — full-grown Arapaima gigas can approach 2 to 3 metres — and almost certainly reflects the age of the holotype rather than any adult ceiling for the species, but true maximum size for A. leptosoma is presently undocumented; more than that would be guessing.
What has been described is consistent with the rest of the genus: a heavily built, torpedo-shaped body, a broad and somewhat flattened head, and fins set well back toward a rounded tail. Distinguishing leptosoma from its Amazon-basin relatives rests on the comparatively slender body proportions in Stewart's description — the source of the epithet — rather than on any single obvious field mark, which is one reason live arapaima seen in local markets or the aquarium trade are so often mislabelled at the species level. As in the whole family, it carries the genus's defining trait: a lung-like, heavily vascularised swim bladder that functions as an accessory air-breathing organ, letting the fish survive in water too oxygen-poor for gill respiration alone.
Habitat
Arapaima leptosoma is endemic to Brazil, documented specifically from the Solimões River — the name given to the upper Amazon within Brazil — in Amazonas state, at and near its confluence with the Purus River. Like other members of the genus it is presumed to be a fish of the Amazon floodplain: slow-moving river channels, oxbow lakes, and seasonally flooded forest (várzea) that expand with the annual flood pulse and can become warm and severely oxygen-depleted as water levels fall, conditions its air-breathing physiology is built to tolerate.
Detailed water chemistry has not been published for this species specifically; based on the wider genus and the Amazon floodplain habitats it occupies, water is presumed warm, roughly 75–86 °F. Its full distribution within Brazil beyond the type locality is not well documented, and that gap in basic range and habitat data is itself part of why the IUCN currently lists the species Data Deficient.
Feeding
No diet has been documented for Arapaima leptosoma specifically — the species is known from a single preserved holotype rather than from field observation of live, feeding fish. Based on the well-studied biology of its congeners, adult arapaima are opportunistic carnivores and largely piscivorous ambush predators, cruising slowly before striking at fish near the surface, while juveniles are thought to take more insects and crustaceans before shifting toward a fish-based diet as they grow.
Until field or aquaculture observations of A. leptosoma itself are published, its feeding ecology should be treated as inferred from the genus rather than confirmed for this species — a case where saying less is more honest than extrapolating a full diet profile from a single juvenile specimen.
Mating
No wild spawning of Arapaima leptosoma has been documented. What breeding information exists comes from general accounts of the genus and from scattered hobby and aquaculture reports of Arapaima species, which describe a substrate nest-builder: a shallow depression roughly 20.1 in across and 5.9 in deep is excavated in sandy bottom, typically during the low-water season, and the female's eggs are fertilised by the male within it — the same low-water nesting strategy documented in the better-known Arapaima gigas.
Whether A. leptosoma follows this pattern exactly, on the same seasonal timing, has not been confirmed by direct observation; it is presumed by analogy with its close relatives rather than established for this species in particular.
Breeding
Successful breeding of Arapaima leptosoma, in the wild or in captivity, is essentially undocumented. General reports on Arapaima describe eggs and fry guarded by both parents after hatching, and aquarium and aquaculture attempts across the genus are noted to have low success rates given the space and expertise a large-bodied, air-breathing nest-builder demands — a constraint that applies at least as strongly to a species this poorly known. Unlike Arapaima gigas, which is farmed at real scale for food across Brazil and Peru, A. leptosoma has no documented role in aquaculture.
What can be said with more confidence, again by analogy with the wider genus, is that Arapaima parental care is substantial rather than minimal: eggs and fry are guarded rather than abandoned, and young are reported to shoal tightly near a guarding parent. Confirming that this species follows the same pattern awaits further study.
In the aquarium
Arapaima leptosoma is occasionally seen in the aquarium trade, but it is typically mislabelled as Arapaima gigas — unsurprising for a species split out only in 2013, with no reliable field mark to separate small individuals at a glance, and with genuine uncertainty about how many valid species the genus contains. Whatever the label, any young arapaima sold in the trade should be treated as a giant in waiting: even the holotype size on record, 30.6 in, is already well beyond what a home aquarium can sensibly house for a fast-growing, deep-bodied fish, and its close relatives reach 2 to 3 metres. Keeping any Arapaima requires tanks on the order of thousands of litres — public aquarium and commercial aquaculture territory, not the home hobby.
Any system holding an arapaima, of any species, must guarantee unobstructed surface access at all times, since the fish is an obligate air-breather that will drown if it cannot reach the surface to gulp air. In practice, Arapaima leptosoma belongs to institutions equipped to house a large, air-breathing predator responsibly, not to a private tank.
Conservation
The IUCN Red List currently lists Arapaima leptosoma as Data Deficient, an assessment made in 2020 that reflects a straightforward lack of information on the species' population, range and threats rather than any finding of safety or decline — the species has been little studied since its 2013 description, and no dedicated population survey has been published.
At the wider genus level, Arapaima species face significant fishing pressure across the Amazon, since air-breathing fish that must surface regularly to breathe are unusually easy to locate and harvest compared with fish that never leave deep water; this pressure is well documented for Arapaima gigas in particular. Whether it applies to A. leptosoma at its type locality has not been separately assessed, and no species-specific threats have been documented — a case where the honest conservation picture is genuinely "unknown" rather than reassuring or alarming.