Eels are elongated, snake-like fish that belong to the order Anguilliformes β a group of over 800 species found in oceans, rivers, and estuaries worldwide. Despite their appearance, eels are true fish with fins, gills, and scales (though those scales are so small they’re nearly invisible). Some species migrate thousands of miles across open ocean to spawn. Others can breathe through their skin. A few produce enough electricity to stun a horse. Eels are stranger than most people realize.
What Is an Eel? Fish or Snake?
Eels are fish β members of the order Anguilliformes β not reptiles. The confusion with snakes is understandable given their elongated, limbless bodies, but eels have gills (not lungs), lateral lines for detecting vibrations in water, and the defining vertebrate fish characteristic of a notochord. Most eels also retain pectoral fins, though these are small and often reduced in more serpentine species.
Electric eels, despite the name, are not true eels at all β they are knifefish, belonging to the order Gymnotiformes, more closely related to catfish than to true eels. This is a common point of confusion.
Major Types of Eels
Moray Eels (Family Muraenidae)
Moray eels are the most familiar reef eels β large, muscular, and frequently photographed with their mouths open in a threatening posture (which is actually breathing, not aggression). There are about 200 species, ranging from the giant moray (Gymnothorax javanicus), which can exceed 10 feet and 66 pounds, to small species barely a foot long.
Morays have a remarkable adaptation: a second set of jaws inside their throat, called pharyngeal jaws, that shoot forward into the mouth to grab prey and drag it backward when the outer jaws cannot generate enough suction. This two-jaw system, first described scientifically in 2007, allows morays to hunt prey that suction-feeding fish cannot catch, including in tight reef crevices.
Moray eels are not aggressive toward humans unprovoked, but they have poor eyesight and strong bite reflexes β divers feeding morays by hand have suffered serious injuries. Their saliva contains mild toxins in some species, and flesh is ciguatoxic (causes ciguatera poisoning) in large reef-dwelling species.
Conger Eels (Family Congridae)
Conger eels are among the largest eels in the world. The European conger (Conger conger) reaches over 10 feet and 240 pounds β the heaviest eel species by mass. Congers are found in both shallow coastal waters and the deep sea, hiding in wrecks and rocky crevices during the day and hunting fish, squid, and crustaceans at night.
Like many eels, congers are semelparous β they breed once and die. Before spawning, females undergo dramatic physiological changes: their eyes enlarge, their digestive systems degenerate (they stop eating), and they migrate to deep water to release their eggs. The larvae drift in ocean currents for up to two years before metamorphosing.
European and American Eels (Genus Anguilla)
The European eel (Anguilla anguilla) and the American eel (Anguilla rostrata) are among the most extraordinary migrators in the animal kingdom. Both species spawn in the Sargasso Sea β a region of the western Atlantic bounded by ocean currents β and their larvae drift thousands of miles back to rivers and streams in Europe and eastern North America, where they mature over 6β20 years before making the return journey to spawn.
The full lifecycle of these eels remained a mystery for most of recorded history β Aristotle believed eels spontaneously generated from mud, and no one had ever observed them spawning in the wild until very recently. The European eel is now critically endangered due to habitat loss, barriers to migration (dams), overfishing, and the parasite Anguillicola crassus.
Garden Eels (Subfamily Heterocongrinae)
Garden eels are among the most visually striking eels β colonies of hundreds or thousands of small eels that live in sandy seafloor burrows, each emerging to feed on passing zooplankton. From above, a garden eel colony sways in the current like a living garden of thin grass. They retreat into their burrows when approached, giving the appearance of the “garden” vanishing.
Snake Eels and Worm Eels
Snake eels (family Ophichthidae) are masters of substrate-burrowing. Many species enter sand or mud tail-first using a hardened tail tip as a drill. Some are remarkably snake-like in appearance and behavior, and some are mimics of venomous sea snakes β gaining protection through resemblance to toxic species without being venomous themselves.
Eel Reproduction: One of Nature’s Great Mysteries
Eel reproduction is unusual even by fish standards. Many species are catadromous β meaning they live in freshwater and migrate to the ocean to breed, the opposite of salmon. The European and American eels represent the most dramatic example of this strategy.
After years or even decades in rivers, freshwater eels undergo a dramatic metamorphosis called “silvering”: their eyes enlarge, their skin darkens on the back and turns silver on the belly, their digestive systems degenerate, and their gonads develop. They stop feeding and begin migrating downstream and out to sea. European eels are believed to travel 4,000β5,000 miles to the Sargasso Sea β a journey no adult has ever been tracked completing, because none have been recovered there. The adults presumably die after spawning; no adult has ever been seen returning.
The larvae (leptocephali) that emerge are transparent, leaf-shaped creatures that bear almost no resemblance to adult eels. They drift in the Gulf Stream for 1β3 years before metamorphosing into “glass eels” β transparent juveniles that begin ascending rivers.
Eel Adaptations
Cutaneous Respiration
Some eels, particularly the European eel, can absorb a significant amount of oxygen through their skin β enabling brief overland travel through wet grass, which they use to access isolated ponds and streams not connected to the river system. This ability is enhanced by their mucus-covered skin, which stays moist and permeable to gas exchange.
Pharyngeal Jaws (Morays)
As mentioned above, moray eels have evolved a second set of functional jaws in their throat β an adaptation unique among vertebrates that has been extensively studied since its formal description in 2007. The pharyngeal jaws act like a second mouth, extending forward to grip prey that cannot be subdued by suction alone.
Body Shape
The elongated eel body is a powerful adaptation for life in tight spaces. Eels can pursue prey into rock crevices, coral tunnels, and buried burrows that other predators cannot access. Their flexible bodies allow them to knot themselves around prey to gain leverage while biting β a behavior observed in morays when dealing with difficult prey.
Are Eels Dangerous?
Most eels are not dangerous to humans unless provoked or mishandled. The main risks:
- Moray eels have powerful jaws and sharp, backward-pointing teeth that can cause deep lacerating wounds if they bite. They are not aggressive but have strong bite reflexes and poor eyesight.
- Conger eels are large and powerful. Anglers who catch large congers have been injured by their thrashing and biting.
- Ciguatera poisoning from eating large reef eels (particularly morays) is a genuine health risk in tropical regions. Ciguatoxins bioaccumulate in large reef predators and cause neurological symptoms that can last months or years.
- Electric eels (not true eels) produce shocks of up to 860 volts, capable of stunning large animals and causing drowning if a human is incapacitated in the water.
Eels in Human Culture and Cuisine
Eels have been eaten by humans for millennia. European eels were a staple of medieval diets, particularly in England (jellied eels remain a traditional London dish). In Japan, unagi (freshwater eel) and anago (conger eel) are central to Japanese cuisine β unagi kabayaki (grilled eel basted with sweet soy sauce) is considered a summer stamina food and commands high prices.
The cultural significance of eels extends to MΔori mythology in New Zealand, where the giant longfin eel (Anguilla dieffenbachii) is revered as an ancestor figure. In ancient Rome, wealthy households kept moray eels in private piscinae (fish ponds), reportedly feeding them and giving them names.
Conservation Status of Eels
Several eel species face significant conservation pressure. The European eel has declined by over 90% since the 1980s and is listed as Critically Endangered. The American eel and New Zealand longfin eel face similar pressures. Primary threats include:
- Dams and barriers preventing migration between rivers and ocean
- Overfishing of glass eels (juveniles) for aquaculture, particularly in Asia
- Habitat degradation of rivers and estuaries
- The parasite Anguillicola crassus, introduced from Asia, which damages swimbladders and impairs migration
- Climate change altering ocean current patterns that eel larvae depend on
Frequently Asked Questions
Are eels fish or reptiles?
Eels are fish β elongated ray-finned fish belonging to the order Anguilliformes. They breathe through gills, not lungs, and are cold-blooded vertebrates with fish anatomy. Their resemblance to snakes is convergent evolution, not a sign of reptile ancestry.
How long can eels live?
Freshwater eels are among the longest-lived fish. European eels can live 15β85 years in rivers before migrating to spawn, and some specimens in captivity have exceeded 80 years. Marine moray eels typically live 10β30 years.
Do eels have bones?
Yes, eels have a full bony skeleton, including a spine and numerous small bones. Eating eels β particularly freshwater species β requires care because of small intermuscular bones throughout the flesh, which is why traditional eel dishes often involve long cooking times or specific preparation to soften them.
Where do eels lay their eggs?
Catadromous eels (like European and American eels) migrate to the deep ocean to spawn β European and American eels specifically target the Sargasso Sea. Marine eels typically spawn in open water or on reefs. No catadromous eel has ever been directly observed spawning in the wild.
Can eels breathe out of water?
Some eels, particularly the European eel, can survive brief periods out of water by absorbing oxygen through their moist skin. This allows them to travel overland through wet grass to access isolated bodies of water. They cannot breathe air the way amphibians do but can manage short excursions when conditions are sufficiently wet.
What do eels eat?
Most eels are carnivorous predators. Moray eels eat fish, octopus, squid, and crustaceans. Freshwater eels eat insects, worms, fish, frogs, and crustaceans. Some deep-sea eels are filter feeders or scavengers. Diet varies significantly by species and habitat.