Aquarist Atlas
Aquarist Atlas · the bonytongue fishes

Arowanas & Oddballs

The order Osteoglossiformes — the bonytongues, named for the toothed bone on the tongue — is one of the oldest living branches of the teleost fishes, a scattered handful of survivors from deep time. It gathers the aquarium’s great oddballs: the surface-cruising, jumping, mouthbrooding arowanas; the two- to three-metre arapaima, one of the largest freshwater fish on Earth; the leaf-mimic African butterflyfish; the blade-bodied featherback knifefishes; and the weakly electric elephantnoses and mormyrids, which read the water with an electric field and carry an outsized brain. Several breathe air, several jump, several are sold small and grow enormous. Built to the same standard as the cichlid, cory and pleco pages — researched, cited, no Wikipedia, no invented facts.

© Adobe Stock

We cover the bonytongue fishes (order Osteoglossiformes) as a whole — the arowanas and arapaima (Osteoglossidae and Arapaimidae), the African butterflyfish (Pantodontidae), the featherback knifefishes (Notopteridae), and the elephantnoses, mormyrids and the aba (Mormyridae and Gymnarchidae). The Old-World featherbacks live here, not with the New-World electric knifefishes (the Gymnotiformes), which they only resemble by convergence. Following the Catalog of Fishes for valid names, we group the order by a friendly ecology axis rather than by clade. A few honest themes run through the accounts. The size trap: the arapaima and clown knife are sold small and grow into pond- or public-aquarium fish. Jumping & air-breathing: arowanas and the butterflyfish leap, and the arapaima, Heterotis and Gymnarchus must breathe air — a tight lid and surface access matter. Weak electricity: the mormyrids and Gymnarchus electrolocate, need soft water and groups, and are sensitive to medication. And the Asian arowana is listed on CITES Appendix I — we state that plainly and give no trade or legal advice. Read each account before you buy.

A silver arowana cruising just under the surface, its long ribbon-like body and upturned mouth catching the light
A silver arowana (Osteoglossum bicirrhosum) — the “water monkey” of the Amazon. It cruises the surface with an upturned, drawbridge mouth and leaps clear of the water to snatch insects and even small birds from overhanging branches, so a heavy, tight-fitting lid is not optional. It reaches close to a metre, and the male is a paternal mouthbrooder that carries the large young in his mouth. A magnificent fish that needs a very large tank — the size is stated plainly in every account.
An arapaima — a huge, dark, armour-scaled fish with a coppery-red tail — rising toward the surface
An arapaima or pirarucu (Arapaima) — one of the largest freshwater fish on Earth, reaching two to three metres. It is an obligate air-breather that must rise to gulp at the surface, so it drowns without access to air. This is the size trap in its purest form: a public-aquarium and food-farm fish, never a home-tank animal. The recent split of Arapaima into several species means the valid name is treated carefully in each account.
A Peters' elephantnose fish — dark, with a downward-curving chin appendage — over a soft sand bed
A Peters’ elephantnose (Gnathonemus petersii) — a weakly electricfish that reads its dark, tannin-stained world by an electric-organ discharge, navigating and “seeing” by field distortions rather than eyesight. Its trunk-like chin, the Schnauzenorgan, sweeps the sand for worms. Famous for one of the highest brain-to-body ratios of any fish, it needs soft water, dim light, a group of its own kind and gentle, copper-free medication.
A clown knifefish — a long, blade-shaped silver fish with black ocelli along its lower flank — in dark water
A clown knifefish (Chitala ornata) — a featherback, named for the long ribbon-fin that runs the length of its blade-shaped body and drives its ghostly, undulating swim. A nocturnal ambush predator, it grows to about a metre and is a genuine size trap. Its small African cousins in the genus Xenomystus are the community-friendly exception. (These are the Old-World featherbacks — not the New-World electric knifefishes, which have their own section.)
An African butterflyfish resting at the surface, its broad, wing-like pectoral fins spread like a floating leaf
An African butterflyfish (Pantodon buchholzi) — a “living leaf” that hangs motionless at the surface, its broad wing-like pectoral fins spread flat, waiting to leap for insects. The sole member of its family and little changed for tens of millions of years, it is an upward-looking ambush predator that keeps to the top of the tank — and, like the arowanas, it jumps, so the lid stays shut.

Photos © Adobe Stock

201 species in the atlas

Grouped by a friendly ecology axis — the arowanas & arapaima, the African butterflyfish, the featherback knifefishes, and the elephantnoses & mormyrids — rather than by the shifting family boundaries, though each account names its current family and authority. Osteoglossiformes is a small order in famous fish but a large one in obscure African mormyrids; following the Catalog of Fishes and FishBase, the roster runs from a handful of aquarium legends to some two hundred little-collected single-river species, and new accounts are added as they are researched.

Species

201 of 201 species

Arowanas & arapaima · 11

Osteoglossidae and Arapaimidae — the great bonytongues. The arowanas (silver and black Osteoglossum in South America, the Asian Scleropages formosus, the Australasian S. jardinii and S. leichardti, and the African Heterotis) are surface-cruising predators that LEAP clear of the water for prey — a tight, heavy lid is mandatory — and the Scleropages and Osteoglossum are paternal MOUTHBROODERS that carry the young in the mouth. The arapaima or pirarucu (Arapaima) is one of the largest freshwater fish on Earth at two to three metres and an obligate AIR-BREATHER: a public-aquarium fish, never a home-tank one. The Asian arowana is listed on CITES Appendix I — legally protected, farmed under certificate and microchip; we state this factually and give no trade or legal advice.

African butterflyfish · 1

Pantodontidae — a single species, Pantodon buchholzi, of the soft, warm forest waters of West and Central Africa. A 'living leaf' that hangs at the surface with broad, wing-like pectoral fins spread flat, it is an upward-looking ambush predator that leaps to snatch insects and, like the arowanas, JUMPS — keep the lid shut. It stays around 10–13 cm and keeps to the top of the tank; it eats anything small enough to fit the mouth and is best with calm, surface-sharing tankmates. Little changed for tens of millions of years, it is one of the aquarium's true living fossils.

Featherback knifefishes · 11

Notopteridae — the Old-World featherbacks: the Asian clown knife (Chitala ornata), the giant Chitala lopis, Notopterus, and the African Papyrocranus and Xenomystus. Named for the long ribbon-like anal fin that runs the body and drives a ghostly, undulating swim, they are nocturnal ambush predators that gulp air from a modified swimbladder. The clown knife is a SIZE TRAP — sold small, it reaches about a metre — while the little African brown knifefish Xenomystus (~20 cm) is the community-friendly exception. NOT the New-World electric knifefishes (Gymnotiformes), which they resemble only by convergence and which have their own section.

Elephantnoses & mormyrids · 178

Mormyridae — the ~230 African elephantnoses and mormyrids (Gnathonemus, Campylomormyrus, Mormyrus, Marcusenius, Petrocephalus, Brienomyrus and kin) — plus the aba, Gymnarchus niloticus (Gymnarchidae). These are WEAKLY ELECTRIC fish: they generate an electric-organ discharge to electrolocate and to communicate, reading a dark, tannin-stained world by field distortions — a lineage entirely separate from, but convergent with, the New-World knifefishes. They are famous for one of the highest brain-to-body ratios of any fish. Keep them in soft, dim water, in groups where the species allows, with copper-free, gentle medication; they are essentially never bred in captivity. Gymnarchus is a large, aggressive, air-breathing, electric nest-builder.