Taxonomy & naming
Anguilla anguilla was described by Linnaeus in 1758 and is the type species of the genus Anguilla, the freshwater eels, family Anguillidae, order Anguilliformes. This is what separates it from almost everything else in this section: it is a true eel, a member of the great marine eel order that also contains the morays and conger eels, and not one of the spiny or swamp "eels" that merely share its shape. The genus Anguilla contains roughly nineteen species worldwide, all of them catadromous — growing in fresh water but spawning at sea.
The European eel is one of three closely related North Atlantic and East Asian species — alongside the American eel (Anguilla rostrata) and the Japanese eel (A. japonica) — that are so similar as larvae and glass eels that they can be confused, and the European and American eels even share the same spawning region in the Sargasso Sea. This cryptic similarity within Anguilla is exactly why the genus is a textbook case for DNA barcoding, which is now routinely needed to tell glass eels and traded eel products apart.
Morphology
The European eel has the classic true-eel form: a long, cylindrical, snake-like body that becomes laterally compressed toward the tail, with a single low fin running continuously around the rear third of the body from the dorsal, around the tail tip, to the anal fin. The pectoral fins are present but small and rounded, and there are no pelvic fins. The skin is thick and slimy and the scales are tiny and embedded, set in a basket-weave pattern, so the fish appears scaleless. The lower jaw projects slightly beyond the upper.
Colour and form change dramatically through the life cycle. The oceanic larva is a transparent, leaf-shaped leptocephalus; the glass eel that arrives at the coast is a colourless, transparent miniature; the growing yellow eel of fresh water is olive-brown to greenish above and yellowish below; and the migrating silver eel develops a dark back, silver-white belly, enlarged eyes and a more pointed snout as it readies for the marine journey. Females grow much larger than males, reaching well over a metre, while males stay smaller.
Habitat
Across its continental phase the European eel is astonishingly catholic in its habitat. After arriving as a glass eel it penetrates almost every kind of accessible fresh and brackish water across Europe and North Africa — rivers and their headwaters, lakes, ponds, canals, estuaries and coastal lagoons — from Scandinavia and the Baltic south through the British Isles and the whole Atlantic seaboard to the Mediterranean and the rivers draining into it. It can cross damp ground to reach landlocked waters and climbs obstacles as a young eel.
It tolerates a very wide range of temperature and salinity, from cold temperate rivers to warm brackish lagoons, and it is most active at night, sheltering by day in burrows, among roots and under stones. The crucial point is that none of this fresh and brackish water is its breeding habitat: the entire continental range is a feeding and growth ground, and reproduction happens only after a one-way migration far out into the Atlantic.
Feeding
The yellow-eel growth phase is a nocturnal generalist carnivore. European eels eat a broad diet of invertebrates and fish — insect larvae, crustaceans, molluscs and worms when small, and increasingly fish, crayfish and amphibians as they grow — hunting by smell and touch in darkness and in murky water. Larger eels are capable predators that take a significant role in the food webs of the waters they colonise.
The most extraordinary feeding fact, however, is the absence of it at the end of life. As an eel matures into a silver eel and begins its spawning migration, its gut degenerates and it stops feeding entirely, living on stored fat reserves for the months-long, several-thousand-kilometre swim to the Sargasso Sea. It spawns once on those reserves and dies; it never feeds again. This is why the species cannot simply be "grown on" to breeding in captivity the way a pond fish might be.
Mating
The European eel spawns in the Sargasso Sea, a region of the western North Atlantic, and nowhere else. The details remain one of the great mysteries of fish biology — no adult European eel has ever been reliably observed spawning in the wild — but the broad picture, pieced together from the distribution of the smallest larvae, is clear: silver eels migrate out of Europe's rivers, cross the ocean, and reproduce in deep, warm oceanic water far from any coast.
Spawning is semelparous: each eel breeds a single time and then dies. There is no parental care of any kind. The fertilised eggs hatch into leptocephalus larvae that are carried back toward Europe over a journey lasting many months to a few years, drifting on the Gulf Stream and North Atlantic currents before transforming into glass eels at the continental shelf. This obligatory marine, oceanic spawning is the single reason the species — unlike the spiny and swamp "eels" — has never been bred in an aquarium or a farm.
Breeding
The European eel cannot be bred in captivity. Its reproduction requires the deep-ocean spawning migration and the oceanic conditions of the Sargasso Sea, which no aquarium or farm can reproduce, and the fish does not mature sexually under normal captive conditions at all. Eel "farming" is therefore not breeding: the entire aquaculture industry depends on catching wild glass eels and growing them on, because no hatchery can supply juveniles.
Decades of research have managed to induce sexual maturation and obtain fertilised eggs and short-lived larvae experimentally, using hormone treatments, but rearing those larvae through the long leptocephalus stage to a glass eel has proven extraordinarily difficult, and a closed life cycle is not yet available at any practical scale. The consequence is stark for conservation: because the species cannot be bred to relieve pressure on wild stock, every farmed and traded eel ultimately begins as a wild-caught juvenile, and the collapse of wild recruitment is felt directly throughout the trade.
In the aquarium
The European eel is not an aquarium fish in the ordinary sense, and it is not part of the tropical hobby. It is a large, cold-temperate, extremely long-lived predator that can spend decades in fresh water, and it is now a protected, Critically Endangered species whose capture, keeping and trade are tightly regulated under European law and the CITES convention. It is encountered far more often in fisheries science, in public-aquarium and conservation displays, and in restocking and "trap-and-transport" programmes than in private tanks.
Where it is held legally for such purposes it shows the family's hallmarks writ large: it is a determined escape artist that will leave any tank that is not sealed and can travel over damp ground; it is scaleless and sensitive; and it needs cool, well-filtered water and a great deal of space. For the home aquarist the practical message is simple — this is a true eel, a Critically Endangered one, and a fish to read about and help protect rather than to keep.
Conservation
Anguilla anguilla is assessed as Critically Endangered on the IUCN Red List, and its decline is one of the most severe of any widespread European animal. Recruitment of glass eels to Europe's rivers fell by an order of magnitude — by most measures more than 90% — from the levels of the late twentieth century, and the species has been the subject of an EU recovery regulation and a CITES Appendix II listing intended to control its trade.
The causes are multiple and compounding: overfishing of both glass eels and adults; the blocking of rivers by dams, weirs and hydropower turbines that kill migrating silver eels and bar young eels from upstream habitat; habitat loss and pollution; the introduced swimbladder parasite Anguillicola crassus; shifts in ocean currents affecting larval survival; and a large illegal trade in glass eels, which are smuggled in great numbers to East Asian aquaculture. Because the eel cannot be bred in captivity to take the pressure off, protecting the wild population — restoring river connectivity, curbing fishing and stamping out the glass-eel black market — is the only route to its recovery.