Aquarist Atlas
Aquarist Atlas · the freshwater “eels”

Eels

Almost every fish sold as a “freshwater eel” is not an eel at all. The aquarium trade lumps three unrelated families under one snake-shaped name: the spiny eels (Mastacembelidae) — the fire eel, the tire-track and peacock eels, and the astonishing radiation of Lake Tanganyika; the air-breathing swamp eels (Synbranchidae) — burrowers that gulp air, survive drought in mud, and change sex as they grow; and the only real eels of the lot, the true eels (Anguilla) — catadromous wanderers that grow in fresh water but must return to the open ocean to spawn, which is why no one has ever bred them in a tank, and why the European eel is now Critically Endangered. 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 — and every account says plainly whether the fish is a true eel.

© Adobe Stock

“Eel” here is a hobby word, not a clade. We group these fish honestly by what they actually are: spiny eels and swamp eels belong to the order Synbranchiformes and are no more eels than a snake is a worm, while the true Anguilla eels are the genuine article (order Anguilliformes). We follow the Catalog of Fishes for valid names, so the African spiny eels appear under their current genus Mastacembelus even where older books still say Aethiomastacembelus or Caecomastacembelus. We leave out the look-alikes that get sold as eels but are something else entirely — the kuhli loach (a loach), the electric eel and other knifefishes, the marine morays, and the “rubber eel,” which is an amphibian. Breeding is not uniform here: the true eels cannot be bred at all (they spawn at sea), the spiny eels are substrate egg-scatterers, and the swamp eels are hermaphrodites whose males guard a bubble-nest. Nearly all are escape artists, nocturnal, scaleless and sensitive to copper — read each account before you buy.

A fire eel, jet-black with vivid red-orange stripes and spots along its body, over rounded river pebbles
The fire eel (Mastacembelus erythrotaenia) is the spiny-eel everyone wants — but it is not a true eel; it is a member of the Mastacembelidae, related to nothing in the sea. It reaches most of a metre, burrows in sand by day, and climbs out of an open tank at night.
The head of a peacock eel, sandy-brown and finely mottled, with the drawn-out, fleshy snout typical of the spiny eels
A peacock eel (Macrognathus) — the small, peaceful end of the spiny-eel family, named for the gold-ringed eyespots near the tail. A few peacock eels stay under 20 cm, but the famous aquarium “eels” mostly do not — and none of them are eels.
A true eel, genus Anguilla — bronze and snake-like with a blunt head and a continuous low fin — against a backdrop of green water plants
A true eel (Anguilla) — the only real eels in this section. They are catadromous: they grow in fresh water but spawn far out at sea, so they cannot be bred in captivity. The European eel is now Critically Endangered, a collapse driven by overfishing, dams and glass-eel poaching.

Photos © Adobe Stock

133 species in the atlas

We keep the three families together because the hobby does — but we label each one honestly, grouped as spiny eels (Mastacembelidae), swamp eels (Synbranchidae) and the true Anguilla eels (Anguillidae). The look-alikes sold as “eels” — kuhli loaches, the electric eel and other knifefishes, morays and the amphibian “rubber eel” — are deliberately left out. Following the Catalog of Fishes and FishBase, the valid set is large and still being written up; new accounts are added as they are researched.

Species

133 of 133 species

Spiny eels · 91

Family Mastacembelidae (order Synbranchiformes) — NOT true eels, but the aquarium core: the fire eel (Mastacembelus erythrotaenia), the tire-track and peacock eels, and the spectacular rift radiation of Lake Tanganyika, plus African river species (Mastacembelus, Macrognathus, Sinobdella). Elongate, scaleless burrowers with a row of spines along the back, found from West Africa across to Indochina and Borneo. Substrate egg-scatterers; most are rarely bred in the aquarium.

Swamp eels · 25

Family Synbranchidae (Synbranchiformes) — also not true eels: the air-breathing swamp eels (Monopterus, Synbranchus, Ophisternon and kin) of the paddies, swamps, ditches and caves of the tropical Old and New World. Eel-shaped, near-finless and almost scaleless, they gulp air, survive drought buried in mud, and many are protogynous hermaphrodites — born female, later turning male. The Asian swamp eel is a notorious invader of the Florida Everglades.

True eels · 17

Family Anguillidae, genus Anguilla (order Anguilliformes) — the ONLY true eels here. Catadromous: they grow for years or decades in fresh water, then migrate thousands of kilometres out to sea to spawn once and die, so they CANNOT be bred in aquaria. The European eel is Critically Endangered and the American and Japanese eels are threatened — a real conservation story of overfishing, dams and glass-eel poaching.

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