Aquarist Atlas
Aquarist Atlas · the electric knifefishes

Knifefishes

The knifefishes of the New World — the order Gymnotiformes — are fish that navigate by electricity. Nearly all of them generate a weak electric organ discharge, a private field around the body that they use to feel their way and to talk to one another in the murky, lightless water they live in — each species with its own signature “voice”. They swim without visible effort, rippling a single long fin, and hunt at night: the matte-black ghost knifefish, the see-through glass knifefish, the barred banded knifefish, and — the one that startles everyone — the electric eel, not an eel at all but a two-metre knifefish that can throw a 600-volt shock. This section is built to the same standard as the cichlid, cory and pleco pages: researched, cited accounts with no Wikipedia and no invented facts.

© Adobe Stock

These are electric fish. Every gymnotiform but one is weakly electric: an electric organ running down the body emits a continuous wave or a train of pulses — the electric organ discharge (EOD) — and sensors in the skin read how nearby objects distort it, so the fish “sees” in the dark by electrolocation and signals rivals and mates with its EOD. The electric eel (Electrophorus) is the exception: it is strongly electric, discharging hundreds of volts to stun prey and deter attackers — genuinely dangerous, and never a casual home fish. Two things follow for keeping them: nearly all are nocturnal, murky-water predators that want a dim, well-planted, cave-filled tank and will eat bite-sized tankmates; and, being scaleless, they are sensitive to medications (no copper, half doses). Note two common mix-ups. The “clown” or “featherback” knifefishes of Asia (Chitala, Notopterus) are not in this family — they are Old-World bonytongues, covered elsewhere. And the electric eel lives here, among the knifefishes, not with the true eels.

A matte-black ghost knifefish threading through bright green aquarium plants, its long ribbon-like anal fin rippling beneath it and a pale band near the tail tip
The black ghost knifefish (Apteronotus albifrons) is the section’s star — and its classic trap. Sold at 8 cm, it reaches 40–50 cm, hunts by night, and reads its lightless world through a weak electric field rather than its eyes.
A near-transparent glass knifefish, its spine and internal organs visible through the body, swimming by rippling a single long fin
The glass knifefish (Eigenmannia) is translucent enough to show its own backbone. Like most of the order it “hums” — a steady electric wave it uses to feel its way and to talk to its neighbours in the dark.
An electric eel filling the frame head-on — its broad, pore-studded grey-brown head and small eyes in dim water
The electric eel (Electrophorus) is not an eel at all but a two-metre knifefish — the one member of the order that is strongly electric, stunning prey with shocks past 600 volts. An obligate air-breather, it is a public-aquarium fish, never a casual home one.

Photos © Adobe Stock

279 species in the atlas

We cover the order Gymnotiformes — the New-World electric knifefishes — grouped by kind: the ghost knifefishes (Apteronotidae), the glass & rattail knifefishes (Sternopygidae), the banded knifefishes & the electric eel (Gymnotidae), and the bluntnose & sand knifefishes (Hypopomidae and Rhamphichthyidae). Sizes are given as total length. Following the Catalog of Fishes and FishBase, the valid set is being written up as each species is researched; the Asian featherback “knifefishes” are a different order and are left out.

Species

279 of 279 species

Ghost knifefishes · 105

Apteronotidae — Apteronotus, Sternarchella, Sternarchorhynchus and kin. Blade-bodied fish that glide by rippling one long anal fin, hunt at night, and carry the order's most refined electric sense — a fast, wave-type discharge from an organ built of modified nerve cells rather than muscle. The matte-black ghost (Apteronotus albifrons) and the brown ghost (A. leptorhynchus) are the aquarium stars; most of the ~90 species are deep-channel Amazonian fish rarely seen in the trade.

Glass & rattail knifefishes · 61

Sternopygidae — Eigenmannia, Sternopygus, Rhabdolichops, Distocyclus. Slender, long-tailed fish of open, flowing water; the glass knifefish (Eigenmannia) is translucent enough to show its spine, while the rattail Sternopygus is among the largest gymnotiforms. Continuous 'hummers' whose electric organ discharge is a steady wave rather than a pulse.

Banded knifefishes & electric eel · 49

Gymnotidae — the banded knifefishes (Gymnotus) and the electric eel (Electrophorus). Gymnotus are stout, boldly barred, air-breathing predators of swamps and floodplains across the Neotropics. The electric eel — not an eel at all but a giant knifefish — reaches about two metres and is the only member of the order that is strongly electric, discharging 600 volts and more to stun prey. Both are obligate air-breathers.

Bluntnose & sand knifefishes · 64

Hypopomidae & Rhamphichthyidae — the bluntnose knifefishes (Brachyhypopomus, Hypopomus) and the sand or tubesnout knifefishes (Rhamphichthys, Gymnorhamphichthys). Small, cryptic, nocturnal fish of leaf litter and sandy shallows; the sand knifefishes have a long, probing snout for sifting invertebrates from the substrate. Pulse-type electric fish, each species with its own signature discharge.

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